Natural Iron Silicides: A Systematic Review

Abstract

This review systematically presents all finds of geogenic, impact-induced, and extraterrestrial iron silicide minerals known at the end of 2021. The respective morphological characteristics, composition, proven or reasonably suspected genesis, and possible correlations of different geneses are listed and supported by the available literature (2021). Artificially produced iron silicides are only dealt with insofar as the question of differentiation from natural minerals is concerned, especially regarding dating to pre-industrial and pretechnogenic times.

1. Introduction

With the industrial production of iron silicides in the 20th century and parallel intensified research of versatile Fe-Si systems for high-tech applications, attention has been focused on rare natural iron silicides. In the last 20 years, more and more localities with different compositions of natural iron silicides have been discovered. The conditions and routes leading to the formation of mineral forms of these unusual compounds are numerous, e.g., the formation in the mantle and core of the Earth and the other terrestrial planets as well as of the Moon, through serpentinization, by impacts and lightning, during the entry of meteoroids into the atmosphere, during the recondensation of ejecta vapor, by space weathering, during the formation period of the solar planetary system, in the envelopes of S-stars, Luminous Blue Variables, classical novae, type II supernovae, and in certain nebulae. The compilation presented in this paper is intended to provide an overview of all natural iron silicide finds, as known to the author at the end of 2021. Such work has been lacking until now. The focus is on identifying the framework conditions and the different ways in which the formation, differentiation, mixing, persistence and decomposition of these rare mineral forms, as well as their association with paragenetic minerals, may be triggered. This synopsis can serve as a basis for further studies aimed at better distinguishing the formation of natural (geogenic-aerodynamic, exoplanetary, and cosmic) iron silicides from technogenic ones. Various disciplines are involved in this study, from geosciences to cosmochemistry, depending on each case. First, a brief outline of the early history of the artificial production of iron silicides is given. For some find situations, it is important to be aware of the history of the artificial synthesis of iron silicides in order to make an informed distinction between natural formation and artificial production. Iron silicides as minerals are the subject of the following section. Here, information on the mineralogical, chemical, and physical properties of iron silicides is given in the form of references to databases on the internet and some authoritative presentations in technical papers. This is followed by chapters on iron silicides as components of fulgurites, in planetary mantles and cores, in the core-mantle boundaries of Mercury, the Moon and super-Earths, in interplanetary dust, in meteorites, on the Moon, in association with a ureilite parent body (UPB), in rocks of unconfirmed status derived from extraterrestrial dust, as recondensation of ejecta vapor, in association with craters, as a component of circumstellar envelopes (CSE) and in interstellar matter (ISM), and in association with novae and supernovae. The conclusion consists of a brief synopsis.

2. Artificial Production of Iron Silicides

The first intentional artificial production of an iron silicide alloy with a silicon content of 9% was undertaken by Jöns Jakob Berzelius in 1810 [1,2]. In 1811, Friedrich Stromeyer (1776–1835) successfully repeated the experiment, which produced alloys with a silicon content of 2.2–9.3% [1]. In 1872, Valten reached 10–12% silicon content, and later, 20% [2]. In 1875, Alexandre Pourcel succeeded in producing iron silicide (10–18% silicon) in a blast furnace [1,2]. From this time onward, the production of iron silicides in blast furnaces became common. Starting in 1890, Ferdinand Frederic Henri Moissan also produced ferrosilicon with an electric arc furnace [2]. In 1899, Guillaume de Chalmot also used this technique to produce iron silicide (25–50%) [1,2]. This process became established worldwide on a large industrial scale for iron silicides with more than 20% silicon content. The blast furnace can be used only for iron silicides with a silicon content of 10–20% [2]. Usually, blast furnaces produce an iron silicide alloy with Si contents ranging from 10 to 15% [2]. The commercial and industrial production of ferrosilicon, based on the use of electric furnaces, started in the USA in 1898 and in Europe (France) in 1899 [2].

FeSi was isolated and described by H. Hahn in 1864 [3], and later synthetically prepared by Edmond Frémy [2]. Artificially produced (Fe,Mn)₃Si, corresponding to Fe₃Si, was first documented in 1898 by Marie Adolphe Carnot and E. Goutal [2]. They also detected Fe₂Si, as did F. Osmond, Lebeau, Hahn, and Moissan between 1891 and 1912. In 1914, Fe₃Si₂ was found. FeSi₂ was studied by Hahn and Lebeau, and FeSi₃ by Carl Naske [2]. Artificially-produced Fe_xSi_y phases, with the exception of Fe₅Si₃, were described before 1934 [4,5]. However, the existence of FeSi₂, Fe₂Si₅, Fe₃Si, and Fe₃Si₂ was still uncertain at that time [6]. Fe₅Si₃ was identified in 1943 by A. R. Weill [7]. In 1939, as far as is known, the first Fe₂Si (denoted as SiFe₂), an unclear polymorph, was produced [8]. Since 1973/1974, trigonal (P3-m1), tetragonal (Pm3-m; CsCl), and hexagonal Fe₂Si (P3-m1; Ni2In) have been described [9,10,11,12,13].

Research shows that in blast furnaces, to a small extent, iron silicide phases (mainly Fe₃Si, but also Fe₅Si₃), together with SiC, form in the tuyere coke zone [14,15,16,17]. There are different types of deposits, i.e., droplets (spherules and semispherules) and irregular forms [15,16]. Carbon coats the droplets in a thin layer, and graphite crystals and flakes are inside them. Iron, silicon, and the coke matrix interact in varying degrees of penetration, saturation, and cooling. This process is reflected in the composition and shape of the droplets. It cannot be excluded that iron silicides were also produced as a byproduct, to a small extent, in some blast furnaces 2000–2500 years ago. In ancient China, blast furnaces existed in the Warring States Period (4th–3rd c. BC) [18]. In Jiudian (Xiping county, Henan province, China), a blast furnace was excavated, and its function was modelled. Close to the tuyère, 1800 K could be reached, while in the upper area, the temperature was still 700 K. In the center, the temperature was higher than in the upper and the outer area. In China, in the 9–8th c. BC, chimneys on ceramic furnaces, with which temperatures of up to 1473 K could be achieved, and blast furnaces with tuyères for copper smelting dating from the 7–5th c. BC, were precursors to the more mature technology of blast furnaces which were in use after the 4th–3rd c. BC. Around 300 AD, a process of preheating (up to 873 K) was used to generate temperatures of up to 2073 K in a tuyere-ventilated furnace at Early Iron Age sites at Kemondo Bay on Lake Victoria (Tanzania), in particular, at site KM3 [19,20,21].

3. Irons Silicides as Minerals

According to the Dana Classification, the minerals fersilicite/naquite (FeSi), ferdisilicite/linzhiite (FeSi₂), hapkeite (Fe₂Si), gupeiite Fe₃Si), luobusaite (Fe₀.₈₄Si₂), xifengite (Fe₅Si₃), suessite (Fe,Ni₃Si), mavlyanovite (Mn₅Si₃), and brownleeite (MnSi) make up suessite group silicides [22,23]. The Nickel-Strunz Elements Classification lists TiFeSi₂, mavlyanovite (Mn₅Si₃), suessite ((Fe,Ni)₃Si), perryite ((Ni,Fe)₈(Si,P)₃), fersilicite (FeSi), ferdisilicite (FeSi₂), luobusaite (Fe_0.83Si₂), gupeiite (Fe₃Si), hapkeite (Fe₂Si), and xifengite (Fe₅Si₃) [24,25,26,27,28,29,30,31,32,33,34,35]. Other stoichiometries are known but have not been named. Natural iron silicides generally exist in orders of magnitude from a few µm to several nm [36]. They predominantly formed in extremely reducing environments under the highest temperatures and pressures. Iron silicides had been approved as minerals in the following order (IMA_Master_List_2021_09): suessite (Fe₃Si)—1979-056, xifengite (Fe₅Si₃)—1983-086, gupeiite (Fe₃Si)—1983-087, hapkeite (Fe₂Si)—2003-014, luobusaite (Fe_0.84Si₂)—2005-052a, and linzhiite (FeSi₂)—2010-011 [37]. In 1969, fersilicite (FeSi), described in 1930, was recognized as a mineral, but this was not accepted by the IMA. However, it was approved by the IMA under the new name ‘naquite’—IMA 2010–010 [38,39]. Similarly, ferdisilicite (FeSi₂) was described in 1968, but first approved by the IMA under the new name ‘linzhiite’ (IMA 2010-011) [40,41]. Some studies summarize the physical and chemical data of Fe_xSi_y [42,43,44]. The long-term stability of Fe_xSi_y under the Earth’s oxidizing atmosphere is still an open question. Theoretical considerations show that Fe-Si alloys at 298 K and 1 bar in air and water have a passivation film made of Fe₂O₃ and SiO₂ [45]. However, the resistance of artificial iron silicides in oxidizing atmospheres has been the subject of various experimental and theoretical studies against a background of possible industrial application. It is shown that iron silicides at temperatures between 673 K and 1 273 K are involved in the formation of a very thin protective layer (SiO₂, Fe₂SiO₄) which significantly reduces corrosion [46]. Further studies examining natural iron silicides in this respect are needed.

The natural origin of fersilicites, reported as early as 1909/1910 in Ireland [47], and then in 1911 (Fe: 71.39%, Si: 20.03%, C: 8.14% from a kimberlite pipe’s diamond mine, called Du Toit’s Pan Mine, Kimberley, Sol Plaatje, Frances Baard, Northern Cape, South Africa, 28°45′49″ S, 24°47′28″ E) [48], 1924 (Fe: 65%, Si: 35%, British Guiana, unknown coordinates) [49], and 1926 (Renison Bell, Melba Flats, Tasmania, Australia, 41°48′ S, 145°25′ E) [50], and in Greece (unknown coordinates; FeSi, Fe₃Si, and FeSi₂, probably other phases, too; Ni: 0.10%) [49], which were termed “meteoritic ferrosilicine” (British Museum), has been questioned [47,49]. Now and then, the found material turns out to be of industrial origin [51,52]. However, some finds were hastily classified as terrestrial slag from industry, and a thorough analysis was not carried out. Although it is clear today that natural iron silicides exist terrestrially and extraterrestrially, as can be seen from the many examples below, it is still an important task to provide clear criteria for distinguishing between anthropogenic-industrial and natural origin. In the case of natural sources, it is necessary to improve the methods of determining which genesis is causal.

4. Fe_xSi_y as Components of Fulgurites

Lightning strikes can be powerful, with voltages >100 MV, amperages up to 10⁵ A and more, energy of up to 1–10 GJ in the air, ~1 MJ on the ground, peak (1.5 µs) temperatures of up to ≃32,000 K, and sometimes exceeding 10⁵ K, pressure/shock, according to modelling, of up to >7 Gpa (rock surfaces; focused on an altered area of radius ~9–11 cm) and sometimes >10 Gpa (soil) or even up to 25 GPa (neutron diffraction data) [53,54,55,56,57,58]. The discharge shock front propagates at more than 4 km/s. Temperatures drop quickly (some µs), but remain temporarily at around 15,000 K [59].

As such, lightning strikes produce fossilized traces in the ground, i.e., fulgurites (Figure 1). Fulgurites have been known since at least 1250 AD, e.g., from the Lapidario of King Alfonso X the Wise (1221–1284) [60,61,62]. However, the term ‘fulgurite’ was coined in 1790 by William Withering [63]. Fulgurites have been researched in depth, concerning their morphology, mineralogical composition, and origin [60,61,64,65,66,67,68].

There are also studies on paleo-fulgurites, the oldest of which may be the Permian, i.e., from 250 Ma years ago [69,70,71]. In the Senckenberg Natural History Collections, Dresden, a 4.60-m long fulgurite is about 15 million years old [72]. Fulgurites are found in high mountains, e.g., the Black Forest (Germany) or the Alps [72,73,74,75,76,77]. Droplet fulgurites, produced by lightning strikes, are known from the archaeological site of the Roman settlement of Amallobriga, ca. 3rd c. AD, in Tiedra, province of Valladolid, Castile and León, Spain [45]. Different forms and compositions depend on the target ground. They also carry liquefied material into the air above the impact site, generating exogenic fulgurites [45]. An exogenic fulgurite cools down very quickly, deforms in the air like drops, solidifies, and falls back to the ground as a droplet fulgurite. These are characterised by the double process of ejection into the air and re-entry. There is a morphological classification of fulgurites (I–V) [78,79,80], comprising sand fulgurites (Type I), clay fulgurites (Type II), caliche fulgurites (Type III), rock fulgurites (Type IV), and droplet fulgurites (Type V) [78,80]. The energy of strikes required to produce fulgurites (Type I-IV) is between 1 and 30 MJ/m. The heating of the material is extremely fast, i.e., 1000 K/s. The lightning channel must be about 1 mm thick. The exogenically produced droplets frequently appear vesicular and amorphous, caused by very fast cooling of liquefied material. Due to their low iron oxide content, the droplets are mostly dark green, in contrast to type I–IV fulgurites, which appear orange, reddish or brownish [45] and are coarse and granular; in contrast, type V fulgurites are smooth and glassy.

In any case, high temperatures (>2000 K) and pressures (>10 Gpa) are the most effective producers of fulgurites [81]. The transformation of matter is plasma-induced [82,83]. Shock effects (on Quartz) from 10 to 34 GPa can occasionally be detected in these materials [66,81,84,85], although these seem to be restricted to the outer zones [66]. Toasted quartz (22–30 GPa) and diaplectic glass were discovered in the Greensboro (North Carolina, USA) fulgurite, but no coesite or stishovite were identified, indicating a difference to a hypervelocity impact shock [85]. The fact that lightning strikes cause a change in the target soil’s remanent magnetization is discussed based on findings in archaeological layers [86,87,88].

Analyses have shown that silicides with compositions ranging from Naquite (FeSi), Linzhiite (FeSi₂), Hapkeite (Fe₂Si), Gupeiite (Fe₃Si), Luobusaite (Fe₃Si₇), and Xifengite (Fe₅Si₃), but possibly also Fe₈Si₃ and Fe₇Si₃, as well as FeTiSi₂ or Fe-Si-Al alloys like FeTi(Al,Si)₂, Fe(Al,Si)₃, Fe(Al,Si)₅, or Fe₅AlSi₁₀, exist in fulgurites [62,78,80,89,90,91,92,93,94]. A lightning strike near Houghton Lake, Michigan hit unsorted glacial sediment and impact rocks. It produced a fulgurite (14 cm in diameter) which exhibited round drops (up to ~200 μm) in the vitrification, consisting of intergrown iron silicides naquite (FeSi) and linzhiite (FeSi₂) or xifengite (Fe₅Si₃), in the form of crystals up to 1 μm in size, with lesser amounts of native Si, Fe-Ti-Si, and other Ti-enrichments. Because of a lack of luobusaite (Fe₃Si₇), a low-temperature phase of Fe_xSi_y, which would indicate a very rapid cooling of the target soil, an artificial origin was excluded, and a lightning strike was assumed to be the cause [91]. Moreover, it was suggested (and contested) that the reduced mineral composition of some ophiolites may be the result of lightning strikes [82,83,95].

Thus, new studies have shown that lightning strikes, the conditions in the mantles and cores of terrestrial planets, and high-velocity impacts (in space and on Earth) produce convergent results in materials within a specific range. Certain shock features and highly reduced minerals, including various iron silicide phases (Fe_xSi_y), are present in fulgurites, ophiolites, meteorites, impact ejecta, and impact rocks [66,85,90,96,97,98,99]. However, it is essential to keep in mind the scale of the effects and the area, as well as the material penetration. Discussion is ongoing as to which criteria, taken together, allow distinctions to be made between terrestrial or impact-induced processes based only on the material in each case [47].

Fulgurites, impacts, and nuclear explosions belong to a common category regarding high temperatures, pressures, shock effects, extremely short-term heating and subsequent prolonged cooling, vitrification, and a highly reducing environment, although the differences cannot be overlooked. The nuclear explosions, e.g., at the Trinity site (near Socorro, New Mexico, 16 July 1945), exceeded 1773 K and reached pressures of at least 8 Gpa, causing planar deformation features (PDF) [100], but nonetheless being at the lower end of the value scale for fulgurites and high-velocity impacts [81,101].

5. Iron Silicides within Planetary Mantles and Cores

The Earth’s lowermost mantle and core conditions are characterized by high temperatures and pressures as well as an extremely reducing environment. The entire mantle’s oxygen fugacity (fO2) is discussed intensively in [102,103,104,105,106,107,108,109]. These publications show that heterogeneity and the development of the mantle across eras play a significant role. Some studies see Fe_xSi_y as an indicator for the mineral composition of the Earth’s mantle [110]. From the decrease in seismic velocity, it is inferred that a small amount of B2-type FeSi phase is probably responsible for the ultralow velocity zone at the base of the lower mantle, that is, the core-mantle boundary zone (CMB) [18]. This is because only B2-FeSi is stable at the very high temperatures that prevail there, in contrast to other iron silicides [111]. With the high pressures and temperatures which prevail in the CMB, the silicate mantle, consisting mainly of perovskite (Mg,Fe)SiO₃, reacts with the liquid iron in the core [112,113]. This leads to the formation of FeO and naquite (FeSi) at 140 GPa, as well as the silicates stishovite (SiO₂) and perovskite (MgSiO₃) that make up the “D” layer, the lowermost, very mixed 200–300 km of the Earth’s mantle. These processes, accumulating a large quantity of B2-FeSi, which is stable at high pressures, explain the unusually high electrical conductivity and can influence the terrestrial magnetic field on the surface [114,115,116]. Some iron silicides are related to the Earth’s mantle (Figure 2).

Kimberlites come from the Earth’s upper mantle. They form there at depths of 150–450 km. They rise very quickly to the earth’s surface in the form of violent vertical eruptions (kimberlite pipes), or horizontally as tabular sheet intrusions (sills) or magmatic flow filling up fractures in existing rock (dykes). While α-SiC and β-SiC crystals can be embedded in diamonds, SiC grains themselves often contain inclusions of Fe_xSi_y, SiO and nanodiamonds [112,117,118,119].

Ferdisilicite (Fe,Ti,Mn,Cr,Ni)Si₂, corresponding to FeSi₂, is found trapped in natural moissanite (SiC) grains from the Mir kimberlite pipe sites in Yakutia (Udachnaya, Daldyn, Mirninsky District, Sakha Republic, Russia, 66°25′59″ N, 112°19′0″ E) and limestones in north-western Bulgaria, attributed to the Triassic (251.902 ± 0.024–201.36 ± 0.17 mya) [120,121]. The grains have sizes of between 1 mm (NW Bulgaria) and 100 microns (Mir, Yakutia). In one SiC grain of Udachnaya, an iron silicide was detected which was close to (Fe,Ti)Si₃ [120]. In the FeSi₂ inclusions, light rare elements (LREE) are significantly more enriched than heavy rare earths (HREE) [120], i.e., there are negative anomalies in Eu, Sm and Yb, as well as a substantial Zr content.

Iron silicide (Fe₃Si₇) was found to be embedded in 0.5–1 mm-sized SiC crystals (hexagonal α-SiC 6H, trigonal 33R and 15R, cubic β-SiC polytypes) from the Sakha Republic (Yakutia), Russia, kimberlite pipes, Mir (Mirny, Mirninsky District, Sakha Republic, Russia 62°31′45.92″ N, 113°59′36.74″ E), nine crystals, and from Aikhal (65°59′56″ N, 111°13′57″ E), 14 crystals [117,122,123,124]. Si, Fe-Ti silicides, Rare Earth Elements silicate, sinoite (Si₂N₂₀), Fe, and traces of Al, Ca, V, Cr, and Mn were admixed [117]. Ferdisilicite (FeSi₂) was also found [120,124]. Several hypotheses about the origins of these compounds have been put forward, but from detailed research, it was concluded that the SiC and the embedded iron silicides originated by the metamorphism of reduced carbonaceous sediments due to subduction. Irons silicides in moissanite are also present in the Sytykanskaya (Alakit field, Yakutia, Russia) kimberlite pipe [122,125].

Peridotite xenoliths collected at the still active Avachinksy stratovolcano (Kamchatka Peninsula, Russia; 53°15′18″ N, 158°49′48″ E; altitude: 2741 m) were attributed to upper mantle material. Native Ni, Fe, and possible Ti, as well as Fe silicides (Fe-Si-Ti, Fe-Ni-Si, Fe₅₅Si₄₀Ti₆, luobusaite/FeSi, Fe₃Si₇) and Fe-Ni-Ti, were found [126]. It is suggested that H₂O (supercritical) fluids, which are carried by the subducting plate (oceanic lithosphere) moving through the peridotite layer within the mantle wedge, which is situated below the overriding plate (continental lithosphere) with the volcanic front to the cool forearc, reacted with it (serpentinization). The volcanic arc source depth is between 70 and 170 km [113]. H₂- and CH₄- fluids are produced and rise from the subducting plate to the overriding mantle wedge. H₂ reduces the peridotite, thereby triggering the precipitation of metals, including iron silicides.

Xenocrysts of corundum (Al₂O₃), moissanite (SiC) crystals (≤4.14 mm), native iron (Fe), and some other minerals, as well as partially extremely reduced matter, including iron silicides (Fe_xSi_y), were found in pyroclastic ejecta of Late Cretaceous (Cenomanian/Turonian stage, 98–94 mya) volcanoes, Mount Carmel (northern Israel, southern Galilee, Haifa District, Israel, 32°44’ N, 35°3’ E); these are associated with alluvial deposits in the Kishon River area [114,115]. The so-called ‘Carmel Sapphire’ nongem corundum contains melting nests which may be differentiated into four types (S, A, N, and DF). Type A melts consist of Fe-Ti-Si-C-P alloys which have crystallized to phases including gupeiite (Fe₃Si), FeTi(Si,P), FeTiSi, FeTi, and TiC (khamrabaevite). The iron silicides are gupeiite (Fe₃Si), xifengite (Fe₅Si₃), naquite (FeSi), hapkeite (Fe₂Si), and (unnamed) Fe₅Si, either pure or in combination with Ti and P [114,116,127,128]. However, these findings are disputed and the materials in question are considered by some to be industry-generated (abrasives) [129,130]. Nonetheless, there are also good arguments for a natural origin [115,131,132].

At a location in the Yizre’el Valley of the Kishon River (Israel; 32°40′20″ N, 35°17′22″ E), close to Mt. Carmel, moissanite (SiC) crystals (3–4 mm) with metal-silicide and silicon inclusions in situ from tuff, dated to the Miocene (23.03–5.332 Ma), were discovered [112]. Zangboite (FeSi₂Ti) and ferdisilicite/linzhiite (FeSi₂) could be identified based on the crystal structure. The analysis and comparison with findings at other locations suggest that high temperature and highly reduced conditions, but not necessarily high pressure, were needed for their formation [116,133]. It was suggested that the matter formed at a depth of 60–100 km at ca. 2 Gpa and at ≥1273–1423 K to ≤1683–1933 K.

At the old platinum mining area at Is River (Nizhnyaya Tura, Sverdlovsk Oblast, Russia; 58°46′59″ N, 59°40′0″ E), naquite/fersilicite (FeSi), gupeiite (Fe₃Si), and xifengite (Fe₅Si₃) were discovered [25,26,38,134].

During a collision of the oceanic lithosphere (especially of an atypical ocean crust) and, partially, the underlying upper mantle with the continental lithosphere, components, especially basic and ultrabasic rocks, rise and are emplaced (obduction) onto the continental crust [135]. Thus, ophiolites are parts of the oceanic crust and old continental margins and the Earth’s lithospheric upper mantle, whose typification depends on the extent of partial melting [135]. Ophiolites occur mostly in sutures; they are associated with the suprasubduction zone (SSZ), which is caused by a very rapid expansion of the fore-arc crust at the beginning of subduction. In any case, ophiolites provide insights into the asthenosphere, lithosphere and melting and cooling processes, as well as into continental drift [135] (Figure 3).

Fe_xSi_y were detected in chromitites of the Luobusa ultramafic massif (southern Tibet, China), belonging to the Indus-Yarlung Zangbo suture [136]. They were attributed to a midocean ridge which probably existed ca. 177 mya ago and, around 126 mya, was changed by mafic extrusive rock melts (Boninite), extending over an intraoceanic subduction [137,138]. In the Luobusha mining district (Qusum Co. [Qusong Co.], Shannan Prefecture [Lhokha Prefecture; Lhoka Prefecture], Tibet, China; 29°13′52″ N, 92°11′25″ E), fersilicite/naquite (FeSi) [38,138,139], ferdisilicite/linzhiite (FeSi₂; Fe_0.84Si_2.00) [41,139,140,141,142,143], β-FeSi₂ [139] irregular grains (0.1–0.2 mm) of Luobusaite (Fe₃Si₇; empirical Fe_0.83Si₂) [33,142,144], Fe₇Si₃, Fe₆Si₄, and Fe₄Ti₃Si₂P [143] as well as zangboite (TiFeSi₂) [145] or zhiqinite (TiSi₂) [132,146] were verified. The Fe_xSi_y in Luobusa ophiolites are likely to have been created in an extremely reducing environment at high pressures [135,138,139,140,143,147]. The evidence of diamonds and coesite suggests that the material was formed at a depth of more than 300 km and under more than 10 Gpa pressure, and thus, must be assigned to the lower part of the upper mantle or even the lower mantle [147,148]. From the deeper zones, this material was transported to the lower ones by a rising plume and then embedded as xenocrysts [140,142,149]. However, this analysis is not without controversy. Instead of being attributed to the Earth’s mantle, some have associated these materials with fulgurites, triggered by plasmas, i.e., by lightning strikes [82,95,150]. Native Fe_xSi_y was detected in ultramafic and metamorphic rocks and granite at other places in China [151].

At Dalihu (Inner Mongolia, China, 48°17″18.5′ N,106°57″12.8′ E) within Neogene (23.03 ± 0.3–2.588 ± 0.04 mya) basalt, situated in the inner Mongolia Daxing’anling Orogenic Belt (Daxing’anling Prefecture), carbonaceous xenoliths found in situ mostly contained SiC (20–50 mm) with TiC, Si, Fe, Ni, Fe-Cr, and Fe_xSi_y [152,153]. Polytopes of SiC occur in descending order of degree: β-SiC (3C), 4H, 15R, and 6H (C: cubic; H: hexagonal; R: rhombohedral). The iron silicides comprised samples DLH06112-3 (Fe,Ni)₂Si (empirical: Fe_1.56Ni_0.49Si_1.00), corresponding to hapkeite (Fe₂Si), DLH0601-2 (Fe,Ni)₃Si (empirical: Fe_2.92Ni_0.02Cr_0.01Si_1.08) corresponding to suessite, and DLH06112-5 (Fe,Ti,Ni)₃Si₇ (empirical: Fe_2.86Ti_0.19Ni_0.16Al_0.11Si_7.00). Native carbon in the xenoliths exists as graphite (10–70 nm) and diamond (around 20 nm). These minerals are considered typical examples of upper mantle material recycling. Volcanic activity began about 15 mya ago and continued until 0.16–0.19 mya.

The sandstone beds of the Manitanyrd Ridge of the Ray-Iz massif, Polar Urals (Russia, 66°55′ N, 65°25′ E), contain moissanite (SiC) in two different forms (crystals, 0.2–0.4 mm and grains), together with native Si and Fe_xSi_y [137,154,155]. The grains contain minute (3–100 μm) [156] inclusions of (unnamed) Fe₃Si₇ (empirical: Fe_2.3–3.7Si_6.3–7.7) and Fe₃Si₄, Fe₆Si₄ (empirical: Fe_6.4Si_3.6), as well as Si and SiO₂. The Ray-Iz material, mainly peridotite and chromitites, belongs to the Voikar-Syninsk ophiolite belt and probably originates from the upper mantle (top of the transition zone [410–660 km depth]: >300 km depth) [137]. Besides SiC, diamonds were also found in this material [154]. Indeed, more than 60 minerals were discovered in these samples [154]. The chronology is early Ordovician (Tremadocian: 485.4 ± 1.9–477.7 ± 1.4 to 485.4 ± 1.9 mya) to late Cambrian (Furongian:497 ± 1.0 to 485.4 ± 1.7 mya) [137].

On a beach on the Turkish coast of the Mediterranean Sea, ca. 150 km north-west of Izmir, a special rock was found [119]. Though it was a pick-up find, analysis favors a natural source. Possibly, it was formed through tertiary (66.0–2.6 mya) volcanism. It was shown to contain Fe_xSi_y, especially the (unnamed) phase Fe₃Si₇, as exsolution areas within the Si or showing up at the Si-/SiC- boundaries. An origin from the Earth’s mantle was assumed.

Naquite (FeSi) was discovered in kimberlites dykes from the Yimeng Mountains (Yimeng Shan, Linyi, Shandong, China; 35°40′ N, 117°47′ E) [157]. There are several locations in China where ferdisilicite/linzhiite (FeSi₂) have been found: Tibet, Xinjiang, Anhui, Liaoing, Jiangsu, Hebei, and Zhejiang [158,159,160]. The rocks containing iron silicides were magmatic, sedimentary, or metamorphic (mafic, ultramafic) and chronically belong to a time of Archean (4000–2500 mya) until today.

Some Fe_xSi_y minerals are thought to have originated in the Earth’s mantle (kimberlites of Tieling and Fu county, Liaoning province or Mengyin county, Shandong) [158]. Ferdisilicite/linzhiite (FeSi₂) may be found in ultrabasic rocks and chromite of the northern Jiangsu province (Donghai Co., Lianyungang, China; 34° N, 118° E) [159]. Its formation is probably related to the collision of oceanic crust and underlying oceanic upper mantle with the continental crust and the underlying upper mantle, as well as the obduction of the former with the latter two, as evidenced by harzburgite and lherzolite rocks. Ferdisilicite/linzhiite (FeSi₂), together with natural silica in gneisses connected with gold ores, was found in the Shandong Gold Province (eastern Laizhou Bay, Shandong Peninsula, Shandong Province, China; 37.47° N, 119.43° E) [161]. The chronology is the Early Cretaceous to Pleistocene (123 ± 4.2 mya to 0.3 mya).

At Longquan (Lishui, Zhejiang, China; 28°04′ N 119°08′ E), there are naquite (FeSi), linzhiite (FeSi₂), xifengite (Fe₅Si₃) as well as Fe₂Si₃ and Fe₅Si₂ as irregular grains (1.5 mm) in rocks from the Proterozoic (2500–541.0 ± 1.0 mya) [162]. The grains contain trace elements, i.e., Mn, Al, Ce, Eu, Ti. It has been suggested that the fersilicites found widely in the southern Zhejiang metamorphic area may be related to a sizeable Archean meteorite event, which is thought to have triggered the recondensation of ejecta vapor. Though such meteorite falls of the Archean are, in general, topics of recent research work and discussion [163,164,165,166], the justifications (formation conditions of xifengite, secondary enrichment) are not very convincing and leave the genesis open.

The Ir-Tash Stream Basin (Arashan Mountain, Chatkal-Kuraminskii Range, Tashkent, Uzbekistan; 41°23′59′ N, 70°30′0″ E). Amygdules of basaltic porphyrite (≈100 μm) contains calcite (CaCO₃), khamrabaevite ((Ti,V,Fe)C, type locality) [167,168], suessite (Fe,Ni)₃Si), gupeiite (Fe₃Si; empirical: Fe 84.3 wt.%, Si 13.8 wt%; Fe_3.02Si_0.98) and is covered with thin graphite (C) rims. The suessite accumulates in ellipsoidal and mostly spherical shapes, with a minimum of 0.25 mm but a maximum of up to 4 cm in diameter. Within the suessite enrichments, there are graphite and cubic crystals of TiC, preferentially at the rims. Moreover, there are tiny inclusions of feldspar (K, Na), lepidocrocite, quartz, and octahedral mica. The chronology is assigned to be the Early Permian (298.9 ±0.15 to 254.14 ±0.07 mya).

Suessite-like mineral (Cr,Fe)₃Si and native chromium (Cr 92.2 wt%, Fe 5.7 wt%, Ti 1.4 wt%, Ni 0.2 wt%, Co 0.1 wt%, V 0.1 wt%) were found in dykes of gabbro diabase in the Kurama Ridge (Gava-sai; Chatkal-Kurama ridge, Middle Tien Shan, Uzbekistan, 40°50′00.0″ N, 71°07′00.0″ E) volcanic area [168,169,170]. The mineral is included in the core of amygdales (0.05–0.8 mm, 0.2 mm on average), which are embedded alkali basalt porphyry. The globules are polymineralic and zoned. Chromium containing suessite surrounds the nucleus and is also found in the interstices of the matrix. The outer shell of the tiny globes consists of cohenite. In between, there is (Cr, Fe, Ti)₃Si with the empirical formula Fe 21.8, Cr 55.2, Ti 7.1, Ni 0.2, Cu 0.1, Si 15.2 intermediate. The mineral is assigned to the Early Triassic (251.902 ± 0.024–247.2 mya).

From the Targhasa reef massif (Eastern Sayan mountains, Krasnoyarsk Krai, Russia; 55°56′40.7″ N, 92°51′11.4″ E), along the river Kaltat (Bazaikha), at a depth of 330 m, limestones were collected in a sequence of bedrock exposures [171,172,173,174,175]. The repeated detection of iron silicides in different layers provides evidence for a nonanthropogenic origin. The first sample was taken from red sandy limestones in the lower part of the section and the second from reef limestones in the upper part. The carbonate rocks were assigned to the Early Cambrian (541 ± 1.0 to 509 ± 1.7 mya). There are iron silicides grains (≤1 mm) in large amounts, i.e., naquite (FeSi), ferdisilicite/linzhiite (FeSi₂), hapkeite (Fe₂Si), xifengite (Fe₅Si₃), (unnamed) FeSi₉, (unnamed) Fe₃Si₂, and (unnamed) Fe₄Si₉ were found in two samples (371-1 and 373-16) from exposures opposite the mouth of the Kaltat [176]. Glass accompanies the iron silicides. There is much moissanite (α-SiC) and occasional native Si in the fine intergrowths of the Fe_xSi_y. A principal-component factor analysis was made concerning the distribution of trace elements in meteorites of different types, terrestrial rocks, and the samples of the river Kaltat [173]. The composition of the material from different phases of iron silicides and moissanite (α-SiC, β-SiC), the quantities of Cr, Co, Ni, Cu, Ga, As, Au, W, and the presence of fused particles resembling cosmic dust suggest a proximity to the iron and stony meteorites and ultrabasic rocks, as well as the influence of high temperatures and pressures [173].

Naquite (FeSi) and ferdisilicite/linzhiite (FeSi₂), intertwined with grains up to 120 µm, isolated grains (≥100 µm) luobusaite (Fe_0.84Si₂), and zangboite (TiFeSi₂) as grains (≤0.15 mm) associated with linzhiite, as well as single grains of SiO₂ were discovered in Late Sarmatian limestones of Crimea (Kamenolomnya, Eupatoria, Saksky district, 45°14′52″ N, 33°25′3″ E) at a depth of 64 m [177,178]. Admixed were moissanite (SiC) with inclusions of iron silicides and native Si, cohenite (Fe₃C) and other minerals, still not identified. The chronology is the Lower Miocene (23.03 ± 0.05 mya to 15.97 ± 0.05 mya). The present researchers propose the following hypothesis to explain the origin of iron silicides: The source is associated with the rapid upwelling and intrusion of superdeep and super-compressed high-temperature fluids from the asthenosphere. Local areas were created, in which mineral-forming solutions could have a highly reducing effect, possibly with present-day methane-processing through the involvement of bacteria, thus triggering the formation of iron silicides.

Suessite (Fe,Ni)₃Si with empirical formula (Fe_3.05Mn_0.02)_3.07Si_0.97 and (Fe_3.13Mn_0.02)_3.15Si_0.84 was discovered in the ore-bearing perlite rhyolite deposits of the submarine paleo volcano Chinarsay (Hissar range, Tian Shan mountain ranges, Uzbekistan, 38°55′ N 68°15′ E) [179]. Accompanying minerals were ferrian kinds and a new mineral Cr₃Si, which is an analogue to gupeiite. The suessite concentrates in oval accumulations (≤65 × 100 μm). It is thought that the outflow of acidic lava of the submarine volcano produced a reducing environment. A rapid decrease in temperature and pressure led to the formation of suessite, which formed with manganese-wüstite (FeO with Mn 11.2 wt%) and spherular Fe₃C. The chronology is the Mississippium (358.9–323.2 mya).

Iron silicides Fe_xSi_y are known from Paleogene (66.0–23.03 mya) sediments of the Carpathian mountains (47°00′ N, 25°30′ E) too [180].

In rhyolites from the Hodruša Intrusive Complex, Banská Štiavnica (Banská Štiavnica district, Banská Bystrica region, Slovakia, 48°27′29″ N 18°53′47″ E), the heavy fraction exhibited FeSi grey spherules (0.09–0.12 mm) [181]. The Hodruša Intrusive Complex is a subvolcanic intrusion in the middle of the Banská Štiavnica stratovolcano, belonging to the Central Slovakian volcanic field. The chronology is the Neogene (14.5–11.5 mya or 12.7 ± 0.4 to 11.4 ± 1.2 mya) [182]. The Khodrus intrusion, Klotilda vein, comprises rocks bearing iron silicides. There are different spherules in terms of size, shape, color, and composition. Grey spherules consist of FeSi, and blackish brown ones of Fe. The crust of the grey spherules shows a polygonal fragmentation which can be attributed to the fact that FeSi was heated to about 12,000 K and then rapidly cooled by water. This was associated with an immediate volume compression and a pressure drop. The impact of extraterrestrial bodies or terrestrial phenomena, e.g., volcanism, could have been responsible for such a process. Significant in this context are also spherules (90–120 μm) from the granitoid-metamorphic rock of the Spiš-Gemer Ore Mountains (Hnilec, Spišská Nová Ves district, Košice region, Carpathian Mountains, Slovakia, 48°50′30″ N, 20°30′10″ E), which contain Fe_xSi_y [183,184]. The chronology is Mesozoic (251.902 ± 0.024–66.0 mya).

Iron silicides, fersilicite/naquite (FeSi) and ferdisilicite/linzhiite (FeSi₂), as well as probably α-Fe, have been discovered in quartzite-sandstone deposits on the left bank of the Chusovaiâ river, Kamen Omut (Omutnaya) cliff in the Middle Urals (Visimo-Utkinsk, Sverdlovsk Oblast, Russia, Perm Krai, Russia, 57°39′50″ N, 58°55′32″ E) [172,174]. The chronology could be Devonian (419.2 ± 3.2–358.9 ± 0.4 mya).

Iron silicides were also found in pyroclastic sediments, with diamondiferous intrusions, located at different sites in north-western Ural (Russia), i.e., the Krasnovishersky district (Perm oblast, 60°43′1.2″ N, 55°45′21.6″ E), the Cherdynskiy district (Perm oblast, 60°49′48″ N, 56°28′58.8″ E), the Gornozavodsky district (Perm oblast, 58°35′2.4″ N, 57°32″42″ E), and the Alexandrovsky district (Tomsk oblast, 60°26′0″ N, 77°54′0″ E) [185]. The chronology is the Palaeozoic (541.0 ± 1.0–251.902 ± 0.024 mya). Many minerals have been detected which formed in a high explosive eruption of quickly decompressed and boiled magma which was very enriched with water. This process occurred under high heat (1473–2773 K). Planar defects in xenocrystic quartz grains evidence shock pressure. The evaporation and condensation of the matter led to the separation and formation of various materials. Iron silicides formed from the gas phase at 1473–1673 K. In the rocks, moissanite (SiC) and corundum (Al₂O₃) grains had inclusions of native silicon (Si), fersilicite/naquite (FeSi), ferdisilicite (FeSi₂), gupeiite (Fe₃Si), and other unnamed Fe_xSi_y phases (FeSi₅, Fe₅Si), Fe-Ni-silicides, (Fe,Ti)Si. palladium silicide (Pd₃Si). Khamrabaevite ((Ti,V,Fe)C) was also present.

The occurrences of Fe_xSi_y in the Tolbachik volcanic complex (Kamchatka Peninsula, Russia, 55°49′51″ N, 160°19′33″ E) [186,187]) are doubted [129,188]. Instead, because of the comparability of a few indicators in the micro-inclusions of natural with industrially-produced corundum grains, anindustrial genesis (e.g., abrasive materials, steel production) is suggested [188]. According to the authors, the questionable mineral inclusions include carmeltazite (ZrAl₂Ti₄O₁₁), tistarite (Ti₂O₃), titanium nitride (TiN), titanium carbide (TiC), iron silicides (Fe_xSi_y), Fe-Si-Ti alloys, hibonite ((Ca,Ce)(Al,Ti,Mg)₁₂O₁₉), grossite (CaAl₄O₇), anorthite (CaAl₂Si₂O₈), residual feldspathic ((Ba,Ca,Na,K,NH₄)(Al,B,Si)₄O₈) glass, and even diamonds. The researchers, questioning the findings so far, call for criteria to distinguish between natural and industrial generation. However, there are arguments in favor of natural origin concerning the apparent differences between the natural minerals and artificial substances [132,135,189].

Xifengite (Fe₅Si₃), empirical Fe (68–69%), Si (24–25%), Ni (5.2%), and Cr (0.7–0.8%), was found alongside around 80 other rare minerals, e.g., moissanite (SiC), as well as native elements, metallic alloys, etc., associated with the Bobruisk Ring Structure (Babruysk District, Mogilev Region, Belarus; 52°58′56″ N, 28°59′49″ E) [190], which is a multiringed basin. The inner ring, which shows significant anomalies, measures 28–32 km across. The outer ring has a diameter of 40–48 km. The structure is estimated to belong to the Paleoproterozoic (1.8–1.9 Ga) and to have formed by mantle plumes fluids.

Iron silicides were detected either completely or partially in material found at different locations in Ukraine, e.g., aluminum-bearing deposits near the ultrabasic intrusion of Kazachyn (Kirovogradskaya region, Golovanivsky district, Ukraine, 48°16′27″ N, 29°56′37″ E), Putritsi (Khmelnytskyi region, Shepetivsky district, Ukraine, 50°08′58″ N, 26°49′14″ E), Ternava, Dobromil region, Sambirskiy district, Ukraine, 49°34′14″ N, 22°47′22″ E), as well as the diamondiferous sediments of the Bilokorovitsky structure (2–6 km width, 22 km length), 1.98–1.80 bya old (Ukrainian Shield). In material from the Bilokorovitsky structure, there were fersilicite/naquite (FeSi) and ferdisilicite/linzhiite (FeSi₂), diamond (C), moissanite (SiC), coesite (SiO₂), cogenite (Fe₃C), kusongite (WC), kyanite (Al₂[O|SiO₄],), pyrope (Mg₃Al₂[SiO₄]₃), pyro ilmenite (FeTiO₃), chromium spinels, gold (Au), native metals (Zn, Sn, As, Sb, Fe, Pb, Cu, Ni, Cr, Ag), and their intermetallic alloys, e.g., metal alloys of the Cu-Zn system. In grains of corundum (Al₂O₃), inclusions of Fe_xSi_y, native Fe and amorphous oxide and metal phases of the solid solution of the Zr-Ti-Al-Fe-Sc-TR system phases of various metals (Zr, Ti, Sc, Mg, Ca, Al, Si) were found. These high-pressure minerals attest to an origin in the Earth’s mantle.

A metallic piece (65 mm × 22 mm × 10 mm), embedded in a mineral carbonate silicide rock, was discovered at a depth of 1 m on the property of the Ancient Chernihiv National Architectural and Historical Reserve in Chernihiv (Chernihiv district and region, Ukraine, 51°29′20.13″ N, 31°18′22.97″ E) [110,191]. It was found in a layer from sandy loam and pieces of brick, dated to the early 19th century. It is not anthropogenic. The piece has two areas: an outer fine-grained zone composed of graphite (C), khamrabaevite (Ti,V,Fe)C [167] with empirical formula (Ti_0.7V_0.3)C and suessite (Fe,Ni)₃Si), and an inner lamellar-crystalline zone composed of suessite with moissanite (SiC). Moreover, other polymorphs of carbon, probably carbines, seem to be present. There are also detectable admixtures of K, Na, Ca. The piece is thought to have originated in the northern part of the Ukrainian Shield, as material from it was used to construct buildings in the Ancient Chernihiv National Architectural and Historical Reserve site. Its production is probably related to a mantle plume. The caldera of the paleo volcano of Chernihiv (Late Devonian 382.7 ± 2.8–358.9 ± 2.5 mya) is located at a great depth below the site.

Crystalline and microcrystalline inclusions of suessite have been reported in veins of the mineraloid shungite (elementary noncrystalline carbon), which appears as migrated shungite, in Shunga (Zaonezhie peninsula, Lake Putkozero, Karelia, Russia, 62°35′33″ N 34°56′14″ E), as well as in shungite basalts near Lebeshchina (Karelia, Russia, 62°31′55″ N, 35°20′38″ E) [110,192]. These are very similar to meteoritic suessite, and chronology dates them to the Lower Proterozoic.

Likewise, a natural terrestrial origin is assumed for the suessite in the magmatic norite breccias of the Tim-Yastrebovskaya structure, Oskol Ore province, crystalline Voronezh massif/Voronezh Anteclise (Voronezh Oblast, Russia, 51°40′18″ N, 39°12′38 E), as part of the East European Craton, which were unearthed from a depth of 320–446 m (borehole 22) [110,193]. The mineral is found together with native gold, chromium, iron, and bismuth, with alloys of gold-cadmium and natural zinc-copper, graphite, chromium nitride as micro-inclusions in native chromium, tungsten carbide, and moissanite (α-SiC). The chronology (Rb-Sr dating) is 1985 ± 8 and 1972 ± 19 Ma (Orosirian Period of the Paleoproterozoic). The origin of the material is attributed to paleo-volcanism.

In the Fore-Sudetic Monocline, appearing in the Polkowice-Sieroszowice and Rudna copper mines (Gmina Nowa Ruda, Lower Silesian Voivodeship, Kłodzko County, Poland; 50°33′2″ N, 16°31′16″ E), Fe_xSi_y exists in the phases of fersilicite/naquite (FeSi), xifengite (Fe₅Si₃), Fe₂Si₃, and Fe₄Si₉ [178], shaped as spherules with different structures and mixtures. They are mainly composed of Fe₅Si₃ (xifengite), with some P, Ti, Cr, and Mn. Slight traces of very native Si and Ti, rare in nature, were detected. The chronology of the material is the Permian age (red-shale, Rotliegend-Ludwikowice, 301.2–298.9 mya). At present, it is not possible to make a clear statement about the origin of the spherules; they may have formed of ultramafic magnetic material collected from neighboring areas in sediments, or may have originated from extraterrestrial dust, as suggested by the author.

In the suessite group, there are other silicides which are structurally like the iron silicides brownleeite (MnSi), mavlyanovite (Mn₅Si₃), zangboite (TiFeSi₂), or perryite ((Ni,Fe)₈(Si,P)₃)). These are associated with volcanism, but are also in meteorites and even in comets [194,195]. Similar to iron silicides, there is often an association with moissanite (SiC) and graphite (C) [196]. Cubic brownleeite (Mn,Fe,Cr)Si, with the empirical formula (Mn_0.77Fe_0.18Cr_0.05)Si_1.00, was found in the dust stream of short periodic (5 a 38 d) comet 26P/Grigg-Skjellerup (diameter: 2.6 km) in three grains (100, 250, and 600 nm). Comets are the types of locality for this mineral [194,197]. Hexagonal mavlyanovite (Mn₅Si₃) grains (≤1–2 mm) with empirical formula ((Mn_4.66Fe_0.40)_5.06(Si_2.91Ti_0.01P_0.02)_2.94) were detected in a lamproite diatreme (Koshmansay River, Chatkal-Kuraminskii Range, Tashkent, Uzbekistan, ca. 40°45′ N, 70°10′ E) [198]. Mavlyanovite is the analogue of xifengite Fe₅Si₃ with the substitution of Fe by Mn. The accompanying minerals are suessite ((Fe,Ni)₃Si), moissanite (SiC), khamrabaevite ((Ti,V,Fe)C), native Iron (Fe), diamond (C), chromite (Fe²⁺Cr³⁺₂O₄), and alabandite (MnS). The mineral is believed to be from the Upper Mantle. It was also found in the Volnovakha River basin (Eastern Azov area, Azov Sea Region, Donetsk Oblast, Ukraine, ca. 47° N, 37° E) [196]. Here, the accompanying minerals are alabandite, graphite, khamrabaevite, and moissanite, i.e., nearly the same as in the previous example, but with an additional, unnamed mineral which is a manganese-iron silicide (Mn,Fe)₇Si₂ [199]. The type locality of zangboite (TiFeSi₂) is Orebody 31 (Chromite deposit 31) of the Luobusha Mine (see above), Qusum Co. (Shannan Prefecture, Tibet, 29°13′52″ N, 92°11′25″ E). This mineral belongs to the Luobusha ophiolites. Zangboite, together with the iron silicides fersilicite/naquite (FeSi), ferdisilicite/linzhiite (FeSi₂) and luobusaite (Fe_0.84Si₂), also exist in the Sarmatian limestones in Crimea (see above) [141]. Trigonal perryite ((Ni,Fe)₈(Si,P)₃)) has only been found in meteorites to date (as of 2021: 27 meteorites) [30,200]. The type locality is the Horse Creek meteorite (Baca County, Colorado, USA, 37°35′ N, 102°46′ W) [200,201]. Finally, grains (diameter 0.7–39.1 mm) of a palladosilicide (Pd₂Si) were found in the Kabanga and Kapalagulu Intrusion in the mafic and ultramafic layers (Lake Tanganyika, Kigoma Region, Tanzania, 5°53′16″ S, 30°03′51″ E and 5°54′26″ S, 30°05′37″ E) and in the Bushveld Complex (UG-2 Reef, Bojanala Platinum District Municipality, North West, South Africa, ca. 25° S, 27° E) [202].

The Earth’s core is thought to consist of an Fe-Ni alloy and additional components of light elements, because of the measured core density, density variations, and sound speeds [111,203]. The composition of the light elements in the core is important for planetary formation, differentiation, motion conditions, material reactions, temperature course, and changes in the geodynamo in the core [203]. From experimental studies and modelling dedicated to the understanding of the Fe-Si system (phases at high pressure and temperature, melting curves) and focused on iron silicides Fe-16wt%Si, Fe-17wt%Si, Fe-18wt%Si, Fe-5wt%Ni-15wt%Si close to Fe₃Si (gupeiite, suessite), it was determined that a certain amount (~6 wt%?) of Si must be dissolved in the Fe-Ni alloy of the Earth’s core [204,205,206]. However, Fe-Si-H alloys (Fe_0.88Si_0.12H_0.17) were also suggested to fit seismological data for the outer core [207]. A Fe–5wt%Ni–4wt%Si alloy was studied to create a temperature-pressure phase diagram of the Earth’s core [208]. Similar research was done concerning the Fe–S–Si immiscible system [206,209]. Si could be a main alloy component in the Earth’s outer core. Fe_xSi_y can exist at the core-mantle boundary (CMB) at 4000 K, 136 Gpa conditions [210,211,212,213,214,215,216]. Thus, experimental research in the lab, supported by geophysical and geochemical data as well as modelling, was and is important to assess the structural properties, interrelationships of the phases, behavior of iron silicides (FeSi_2, FeSi₃, Fe₂Si, Fe₃Si, Fe₅Si, Fe₅Si₃, Fe₁₁Si₅), chemical equilibria, flows at the highest pressures and temperatures, and to derive conclusions for their existence and the reaction processes at the core-mantle boundary [43,111,203,205,213,217,218,219,220,221,222,223,224]. However, research shows that oxygen in higher amounts, in addition to Fe, Ni, and Si, plays a nonnegligible role in the outer core [208,225]. It has been proposed that the outer core contains a (Fe–Ni)–O–Si system, e.g., Fe-5.8(0.6) wt%Si–0.8(0.6) wt%O [208]. The inner core could consist of a Fe–Ni–Si [214,215] mixture, e.g., Fe–5wt%Ni–4wt%Si (B2 and hexagonal packed [hcp]) [214]. The outer core could have Si: 1.2–3.6% and O. While FeSi could be stable at the lowermost part of the mantle (the D″ layer), stability is discussed for the inner core [219]. It seems, however, that the high-pressure phase of iron silicide FeSi with a CsCl (B2) crystal structure is a stable candidate (the only one) at up to 400 GPa for a phase at the inner core boundary (ICB) at around 5000 K [205,210,211,212,213,214,215,216,220,222,223,226,227,228].

6. Iron Silicides in the Core-Mantle Boundaries of Mercury, the Moon, and Super Earths

The existence and significance of iron silicides in the cores of terrestrial planets is a matter of debate, especially concerning the characteristics of the Fe-S-Si ternary system [216] (see Figure 4). The Fe-Ni-Si system was also studied in detail regarding the possible composition of terrestrial planets [229]. Moreover, some models calculate hydrogenation of FeSi (FeSiH_x) in the innermost region of planetary cores (>10 Gpa) [230].

Iron silicides are thought to be essential in Mercury’s core–mantle boundary (CMB), though NiSi may also be important in that case [216,231]. Mercury could have an extraordinarily Fe-rich inner solid core and a liquid outer core based on measurement data from the Messenger spacecraft, cosmochemistry, and modelling. Above this would be a solid outer core layer of iron sulfide (Fe-S), then a solid mantle and a solid silicate crust [115]. At the core–mantle boundary (CMB) at 1900 K and 5 Gpa, there would be an Fe-Si/S alloy (Si: 8.5–33.5 wt%, S: 0–36.5 wt% on average) [216,232,233]. An Fe-Si alloy (Si: 0–25%) is proposed to exist in both the outer and inner core, or a core consisting of Fe-S-Si, where molten Fe-Si and Fe-S alloys form two layers inside, surrounded by a more solid Fe-S layer [234,235].

From models of the Martian core, a Fe-S core is suggested with a large amount of S (16–20 wt%), little O (<1 wt%) and negligible Si [216,236]. Too little data are available for Venus at present. However, the data and modelling suggest that despite the presumed Earth-like inner structure, there are likely few, if any, light elements, i.e., also Si, in the core [237]. Concerning the interior of more minor planets, NiSi at 13 GPa and 1000 K could be more significant than FeSi [115].

The moon’s (Figure 5) lunar seismic profiles, sound wave velocity measurements, and modelling indicate a division into a liquid outer core and solid inner core. S, C, Ni, Si, and P had been suggested to make up the light elements in the core. However, the P content seems to be negligible. A pure Fe core has also been proposed; however, it is very probable that at the CMB, Fe-Si alloys (Si: 2–17 wt%) exist at 1600 K and 5–7 Gpa [216,238].

For Jupiter’s Ganymede, the largest moon in the solar system, modelling shows that Fe-S alloys predominate at the core-mantle boundary (CMB) and within the inner-core boundary (ICB). There is (probably) no Si present [216].

Recent studies have dealt with the Fe/Si ratios in super-Earth type exoplanets (~1.3 R_⊕ [Earth’s radius]) [239,240,241]. Pressure experiments and modelling, regarding crystal structures and densities, show that the hcp- (Fe-7Si) and bcc-phase (Fe-15Si) are stable up to 1314 Gpa, with the former in a hexagonal and the latter in a cubic structure. Thus, such Fe-Si alloys could be essential components of super-Earth cores of ~3 R_⊕.

7. Iron Silicide in Interplanetary Dust

In an interplanetary dust particle (IDP), L2055I3, with a diameter of ~4 µm, three grains (100, 250, and 600 nm) of Brownleeite (Mn_0.77 Fe _0.18Cr_0.05)Si were discovered [194,242]. The IDP was collected during the Earth’s passage through the dusty tail of the short periodic comet 26P/Grigg-Skjellerup (size: 2.6 km; aphelion: 4.9332 AU; perihelion: 1.1168 AU; eccentricity: 0.6631; orbital period: 5.31 a; orbital velocity at perihelion: 38.48 km/s; Inclination: 22.36°), which triggers the meteor stream pi Puppids. Therefore, an origin from that comet is very probable. However, this could also have been associated with matter condensing in the early solar system, with circumstellar envelopes or supernova remains. Brownleeite (Mn, Fe, Cr)Si belongs to the fersilicite group and, according to the Dana classification, to the Suessite Group of silicides (01.01.23) [195,197].

The Stardust mission to comet 81P/Wild 2 (size: 5.5 km × 4.0 km × 3.3 km; density: 0.6 g/cm³; mass: 2.3 × 10¹³ kg) provided evidence of iron silicides, but these are very likely to be secondarily produced. In aerogel track #44 within three grains (C2004,1,44,1,0, C2004,1,44,2,0, and C2004,1,44,3,0 all approximately 15 μm × 20 μm sized), Fe-Si phases (~100 nm, quenched-melt spheres) Fe₂Si (hapkeite), Fe₃Si (suessite), up to (Fe,Ni,Cr)Si alloy (Fe_3.35Ni_0.13Cr_0.05)(Si)_1.0, corresponding a Fe₇Si₂ (unnamed), were detected [243]. Suessite was also found in 16 grains of track #35 [244]. These iron silicides are thought to have been produced by a hypervelocity impact of (Fe,Ni)-S particles, leading to a temperature above 1400 K, heating the aerogel substrate (containing Si), melting it and mixing matter from the particle and the aerogel (SiO₂), followed by rapid cooling.

In this context, the discovery is significant of a meteorite which was assigned to the meteor shower of the Leonids (Comet of origin is 55P/Tempel-Tuttle). In 1998, a slaggy rock found at El Aybal (near Salta, Department of La Paz, Catamarca Province, Argentina; 24°52′ S, 65°29′ W) contained wüstite and the iron silicides gupeiite (Fe₃Si), xifengite (Fe₅Si₃), fersilicite (FeSi) and ferdisilicite (FeSi₂), in addition to native Cu, Fe und Si [245,246]. The detected spherules were highly magnetic. A connection with the meteor shower of the Leonids (Comet of origin is 55P/Tempel–Tuttle), which occurred on the day of the find (17 November 1998) in the La Puna area, is suspected. The find is recognized as a meteorite and was included in the Catalogue of Meteorites from South America. However, it is not officially listed in the Meteoritical Bulletin Database [246].

In a dust particle (4 μm) associated with the comet Grigg-Skjellerup, free tiny grains (≤600 nm) of Brownleeite (MnSi) were detected [194]. Brownleeite is structuraly similar to fersilicite/naquite (FeSi) and is in the suessite group (see above).

From these research results and data on meteorites, iron silicides in nebulae, as well as the envelops of novae and supernovae type II remains (see below), it is concluded that fersilicite/naquite (FeSi), ferdisilicite/linzhiite (FeSi₂), hapkeite (Fe₂Si), suessite ((Fe,Ni)₃Si), gupeiite (Fe₃Si), luobusaite (Fe_0.84Si₂), xifengite (Fe₅Si₃), and zangboite (TiFeSi₂) may be attributed to the high pressure impact phase during the formation and differentiation process of the planetary system, also caused by impacts, and thus, to the original mineral of the primeval Earth’s Hadean eon surface (~4600–4000 mya) [247,248].

It has been experimentally shown that, under cooling and reduction conditions, iron silicides are produced as grains (10 nm) from kamacite and taenite. Iron silicides fersilicite (FeSi), gupeiite (Fe₃Si) and xifengite (Fe₅Si₃) form in grains with sizes of 30–100 nm, via the melt condensation of high temperature gases [249]. Native Fe and iron sulfides are embedded in amorphous silicate grains with sizes of some hundred nanometres. The experimentally produced matter is very similar to the natural one found in interplanetary dust. It is believed to be part of the building blocks of the solar planetary system. The material could form in the region of a protosolar disk, i.e., in the early stages of planetary systems, or in the envelopes of stars in the final stages of their evolution (AGB stars, supernovae type II-P, i.e., the core collapse supernova of a star with more than eight solar masses).

8. Iron Silicides in Meteorites

In 1859, a phase of iron silicide (Fe: 87.279%, Si: 11.008%, P: 1.312%, C: 0.400%, Mg: traces) was found in a rock (≈8 cm × 7 cm × 5 cm) near Rutherfordton (Rutherford County, NC, USA) by C. U. Shepard, named ‘ferrosilicine’ [2,49,250,251]. However, the assessment of this sample as being of natural origin and meteoritic was questioned by some researchers, and industrial production was hypothesized [49]. Again and again, some finds are initially considered meteoritic, but then a terrestrial or anthropogenic (industrial) origin is found to be more likely upon closer analysis [52,250]. However, there is also clear evidence for the existence of iron silicides in various types of meteorites.

8.1. Iron Silicides on the Moon

Over aeons, a process known as ‘space weathering’ shapes the surface of airless (near-vacuum) bodies (planets, dwarf planets, moons, planetoids, comets) in planetary systems. Significant impacts, the effect of a steady flow of micrometeorites, the solar wind and cosmic rays continuously forms and reshapes the soil and regolith on these bodies if they hold only a sporadically forming atmosphere of trace gases and solid particles, or if they are directly surrounded by the vacuum of space [36,252,253,254,255,256,257,258,259,260,261,262,263,264,265].

According to several studies, there were tetragonal Fe₂Si, named Hapkeite-1C (c for cubic), naquite (FeSi), linzhiite (FeSi₂), and (Fe_2.9Ni_0.1)₃Si_1.1 close to gupeiite (Fe₃Si)/suessite, (Fe_4.33Cr_0.67)₅(Si_3.04 P_0.07)_3.11 close to xifengite (Fe₅Si₃), and (Fe_2.89 Mn_0.04Cr_0.03Ti _0.04)_3.0Si_7.09 close to (unnamed) Fe₃Si₇, in the lunar meteorite Dhofar 280 (Dh 280; Dhofar, AI Janubiyah Province, Oman, 19°19′36″ N, 54°47′0″ E) [266,267,268,269,270]. Hapkeite exists as a relatively large metallic grain, i.e., ~35 μm. Dhofar-280 is an impact melt breccia of anorthosite type, containing inclusions of Si grains and Fe_xSi_y droplets (2–30 μm) that are oxygen-depleted. There are some minor elements in the ferroan anorthosite matrix [267]: Al, Ca, Co, Cu, Mn, Ni, Mg, P, S as well as trace amounts (if any) of Na, K, T, and Cr. In addition to the iron silicides, there are CrSi₂, TiSi₂, and Fe-Ni phosphides. The melt consists of SiO₂, Al₂O₃, FeO, MgO, CaO, Na₂O, and K₂O and, to a small extent, Ni, N, and Ti. Space weathering by micrometeorite impact melting and vaporizing lunar soil are the favored explanations for these [266,267,269,270]. It is, however, thought that a giant impactor was primarily or secondarily responsible for the very high temperatures (~1500° C), leading to the necessary reducing conditions for the iron silicides [267]. A candidate for that huge impact is the same as that which created the South Pole-Aitken Basin (53°S 169°W; diameter: ca. 2500 km, depth: 6.2–8.2 km) on the Moon’s far side, one of the largest impact craters in the Solar System [271]. There is growing evidence that the basin was caused by a low-velocity projectile (about 200 km) that hit the lunar surface at an angle of about 30° or less [272]. The impact that created the Copernicus crater about 800 Ma years has also been suggested as a possible primary triggering event [271]. In any case, the impact likely happened ≤ 1 Ma ago [271]. Moreover, the meteorite experienced three more impact events [271]. High shock, caused by impacts, is indicated by maskelynite glass [270,273].

Hapkeite (Fe₂Si) was also discovered, together with inclusions of troilite (FeS), in one of the samples from crater ejecta blanket at the Mare Crisium landing site (12°14′16″ N, 62°11′56″ E), returned by the Luna-24 mission (22 August 1976) [274,275]. Luna-24 made a core drilling in the Mare’s lunar soil.

Moreover, an Apollo 16 regolith sample (# 61500, subsample 61501,22; A6–8, A6–7), from Flag Crater (8.97° S, 15.45° E; diameter: ca. 40 m, depth: >20 m) in the Descartes Highlands contained FeSi (Fe₄₄Si₅₆), Suessite (Fe,Ni)₃Si, Linzhiite FeSi₂ (Fe₃₂P0₂Si₆₆), and Luobusaite Fe₃Si₇, (Fe₂₈Si₇₁) as 0.1–2 μm-sized inclusions, as well as Si metal (<1 μm) embedded in a plagioclase matrix [276,277,278,279,280].

8.2. Iron Silicides and an Ureilite Parent Body (UPB)

It is notable that iron silicides occur frequently in urelite meteorites [281] (Figure 6). It is suggested that some iron silicides are the remains of an ureilite parent body (UPB), disrupted by different impactors, which re-accreted, forming the regolith of the UPB, mixed up with the core and the mantle, as evidenced by polymict ureilite meteorites [281,282,283,284,285,286,287,288,289,290,291,292,293,294]. The parent body could have been a protoplanet in the size range between Mercury and Mars [289]. There is still disagreement about whether ureilites originate from different parent bodies or other areas of a single object. It is also unclear whether ureilitic meteorites are the remains of undissolved material from the time when the solar system was formed, or building blocks that were left over after more volatile components had melted away during specific processes of heating. Finally, ureilites could also represent a mixture of carbonaceous chondrite and molten basaltic rocks. There are indications that the inner and outer solar system material was mixed [295].

Suessite (Fe,Ni)₃Si is found in polymict meteorites DaG 1000 (Dar al Gani, Libya; 27°00.81′ N, 16°21.95′ E) [296,297], heavily shocked DaG 1023 (Dar al Gani, Libya; 27°1′ 33″ N, 16°23′16″ E) [296,297], and DaG 999 (Dar al Gani, Libya; 27°1′33″ N, 16°21′57″ E) [296,297].

In the polymict ureilite DaG 319 (Dar al Gani, Libya; 27°1′41″ N, 16°21′31″ E), suessite (Fe₃Si) and perryite (Fe₅Si₂) has been identified. The highly shocked polymict ureilite DaG 1047 (Dar al Gani, Libya; 27°2′9″ N, 16°23′7″ E) has feldspars, olivine, pyroxenes, and troilite. Moreover, it shows suessite (Fe,Ni)₃Si and kamacite (Fe,Ni) with up to 27.6 wt% Ni [298,299,300].

The polymict ureilite DaG 1054 (Dar al Gani, Libya; 27°25′40″ N, 16°9′56″ E) is highly shocked. It consists of mainly olivine (≤30 μm), few orthopyroxenes, rare grains of sulfide (mainly troilite), carbon including tiny diamonds at the grain boundaries, and suessite [(Fe,Ni)₃Si], with fractions of Cr and Ni, grains (≤100 μm) or metal veins [301].

Suessite (Fe₃Si), hapkeite (Fe₂Si), and naquite (FeSi) are present in the barely weathered polymict meteorite, DAG 1066 (Dar al Gani, Libya; 27°9′7″ N, 16°16′12″ E) [302,303,304]. The meteorite shows signs of being highly shocked (S3). The iron silicides indicate a formation under significantly reducing conditions [304]. They are embedded as minute spots in a matrix largely consisting of olivine crystals (0.2–1.5 mm) and, to a lesser extent, pyroxene and feldspar, on the edge of the olivine grains and graphite. Other inclusions are pentlandite (Fe,Ni)₉S₈ and FeNi. An analysis showed that the embeddings are partially ureilitic, carbonaceous chondrites (CC), or chondrules rich of forsterite and enstatite. The meteorite is assumed to be of ureilite origin based on the present investigation.

The polymict ureilite EET 83,309 (Elephant Moraine, Antarctica; 76°18′52″ S, 157°13′11″ E) [297] consists of olivine, pyroxene, and plagioclase in one grain (0.6 mm), as well as suessite (Fe₃Si) and many fragments (>300 μm) of opal (SiO₂ nH₂O), which, as a 10 μm thick skin, completely envelopes the suessite (Fe₃Si) grains [305,306,307]. In EET 87,720 (Elephant Moraine, Antarctica; 76°11′ S, 157°10′ E) [297], the (unconfirmed) detection of a hapkeite (Fe₂Si) grain was reported [291].

In the North Haig polymict ureilite (Sleeper Camp, North Haig, Western Australia, 30°13′ S, 126°13′ E), section B (WAM12,809 ‘B′), mainly suessite (Fe,Ni)₃Si, filling cracks and appearing as blebs (1 μm) or grains (30 μm × 150 μm), and one grain xifengite (Fe₅Si₃), were found, while in section ‘A′, a (hitherto unnamed) phase (Fe,Ni)₄Si was detected [282,308,309]. There is also a magnetic signal indicating suessite [288]. The meteorite gives the type specification for suessite (Fe, Ni)₃Si [309]. Previously, the existence of suessite had been doubted [190]. Probably paired with the North Haig is the polymict ureilite Nilpena (Hundred of Nilpena, County Taunton, Australia; 31°5′ S, 138°18′ E) [282], which also contains suessite (Fe,Ni)₃Si, with a different percentage of Si (North Haig: 10–16.7%, Nilpena: 11.8–17.1% [308,310,311].

Paired meteorites from Frontier Mountain [312], Antarctica (FRO 90,036 [72°57′12″ S, 160°26′22″ E], FRO 90,054 [72°57′21″ S, 160°26′19″ E], FRO 90,168 [72°57′18″ S, 160°26′46″ E], FRO 90,233 [72°57′22″ S, 160°26′18″ E], FRO 90,228 [72°57′ S, 160°26′ E], and FRO 93,008 [72°57′16″ S, 160°26′16″ E], found between 1990 and 1993) [312,313,314,315,316,317,318,319] have been found to contain Fe_xSi_y. In all of them, the unnamed phase (Fe,Ni)₉Si was identified [281]. FRO 90,168 and 93,008 contain suessite (Fe,Ni)₃Si in the veins, produced by high degrees of shock (as mosaic olivine) [291,320]. A grain of Fe₂Si, probably hapkeite, was detected in FRO 90,228 dimict ureilite [291]. Remarkably, suessite (Fe,Ni)₃Si and hapkeite (Fe₂Si) were discovered in the veins of dimict ureilites, indicating their origin as impact-induced melt [291].

The monomict ureilite NWA 1241 (north-western Africa, unknown coordinates) consists of silicates (olivine, pigeonite), carbonaceous material, nanodiamonds, and, as the main abundant metallic phase, suessite [288,297,321,322]. From the mineralogy of NWA 1241, it is assumed that due to an impact on the ureilite parent body (UPB), suessite was produced by olivine and kamacite at ca. 1400–1500 K and very high fugacity of oxygen.

The highly shock-melted and recrystallized ureilite NWA 14,274 (north-western Africa, unknown coordinates) is composed of olivine (a dunite >90 vol.%), pigeonite, orthopyroxene, kamacite, graphite, secondary calcite and gypsum, as well as glasses enriched in Si, NaMgAlSi, and FeSi [323].

In addition to olivine, pigeonite, clinopyroxene, graphite, troilite, pentlandite, sub calcic augite, and kamacite (Ni: 3.4%, Si: 3.8%), the ureilite meteorite NEA 027 (Northeast Africa, unknown coordinates), which was shock recrystallized, also contains suessite [324].

The superbolide 2008 TC3 Almaha Sitta (Nahr an Nil, Nubian Desert, Sudan, 20°44′45″ N, 32°24′46″ E) was the first planetoid whose entry into the Earth′s atmosphere on 7 October 2008 over the Nubian Desert could be accurately predicted. It was 3–4 m in size (Apollo type) and disintegrated at an altitude of about 37 km [325]. This superbolide belongs to the F-type of planetoids [325]. It consists of more than 30 individual components with 20 different compositions, including sections of 18 different amino acids and polycyclic aromatic hydrocarbons (PAHs). It may be associated with the main belt, in particular, the planetoids of the Nysa-Polana family [326]. Recent research suggests a Ceres-sized (ca 640–1800 km) planetoid as the water-rich parent body [327]. All ureilitic fragments of 2008 TC3 showed various mixtures of kamacite with very low nickel content, suessite (Fe₃Si), schreibersite (Fe,Ni)₃P, troilite (FeS), and daubréelite (FeCr₂S₄)–heideite (Fe,Cr)_1.15(Ti,Fe)₂S₄ in grains of olivine close to the veins, which were enriched in C [328,329]. The magnetism was attributed to these components and an extraterrestrial origin was assumed [288,322]. The exact contribution of suessite (Curie-temperature: 823–873 K) to magnetisation is the subject of further investigation [288].

Moreover, the ureilite meteorites Goalpara (Assam, India; 26°10′ N, 90°36′ E) [330], Novo Urei (Respublika Mordoviya, Russia; 54°49′ N, 46°0′ E) [330], and Dingo Pup Donga (Western Australia, Australia; 30°26′ S, 126°6′ E) [330] as well as NWA 766 (Northwest Africa; unknown coordinates) [331], Kenna (New Mexico, USA; 33°54′0″ N, 103°33′12″ W) [330], and Dho 837 (Zufar, Oman; 18°18′21″ N, 54°8′59″ E) [296] all show magnetic signals of suessite [288,332].

8.3. Iron Silicides in other Types of Meteorites

The EH3 chondrite Northwest Africa 8789 (coordinates unknown) contains suessite (Fe,Ni)₃Si (Ni 80.7 wt.%, Si 14.8 wt.%, Fe 4.1 wt.%, Co <0.1 wt.%) as well as several other minerals within chondrules (0.6 ± 0.4 mm, one 2.1 mm, in a red-brown matrix [333,334]. An analysis concluded that the EH and the EL type were once exposed to powerful shocks at their location in the planetary system [333]. In addition, the enstatite chondrites EH3 Qingzhen and MacAlpine Hills (MAC) 88,136 (EU) contain perryite (Ni,Fe)₈(Si,P)₃ [335]. The meteorite NWA 13,108 (unknown coordinates) is classified as an enstatite chondrite (EL6). It consists of many other minerals and iron silicide without Ni, probably naquite (FeSi) [336].

Based on spectral analyses, the Main Belt planetoids (21) Lutetia and (97) Klotho could be associated with the parent body of enstatites (M type; size: 121 ± 1 × 101 ± 1 × 75 ± 13 km; density: 3.4 ± 0.3 g/cm³) [337]. Lutetia seems to consist partially of enstatite material (E-type chondrite) and carbonaceous chondrites of CB, VH, or CR [338]. However, based on the densities of the two classes of meteorites and (21) Lutetia, it can be concluded that a differentiated core exists [338]. (12) Lutetia could be a primordial planetesimal [338,339]. Another planetoid, (97) Klotho (M type; size: 100.717 km; density: 4.16 ± 0.62 g/cm³), is also seen as a possible candidate for the enstatite parent body [337].

The carbonaceous CH3 chondrites SaU (Sayh al Uhaymi) 290 (Al Wusta, Oman; 21°4′32″ N, 57°8′49″ E) [340] and Asuka 881,020 (Antarctica; 72° S, 26° E) [330] contain suessite (Fe,Ni)₃Si, fersilicite (FeSi), and perryite ((Ni,Fe)₈(Si,P)₃), in addition to barringerite, andreyivanovite, troilite, daubreelite, pyrrhotite, pentlandite, and magnetite. There are places that are free of silicon and others where silicon is enriched [341]. Based on spectral analyses, planetoid 21 Lutetia is proposed for the CH3 meteorites, as is the case for the enstatites listed above [342].

Iron silicides occur in grain 126 of the carbonaceous chondrite Khatyrka CV3 meteorite (Listvenovyi stream, Koryak Mountains; Chukotka Autonomous Okrug, Russia; 62°39′11″ N, 174°30′2″ E) [343,344,345,346]. The meteorite is believed to be one of the solar system′s building blocks, created some 4.5 billion years ago. It is rich in Al, Ni, and Cu, e.g., Al_0.97Cu_0.03, Ni_0.91Fe_0.05Cu_0.04, Cu_0.96Fe_0.04, or unnamed Al₇₈Cu₁₅Fe₇ beads of Fe (<10 nm to ~5 μm) were found. These include iron silicides Naquite (FeSi), suessite (Fe₃Si), xifengite (Fe₅Si₃), but less Ni (<0.1 wt%) [345]. There are many other minerals in that meteorite, e.g., hercynite, chromite, magnetite, corundum, iron, taenite, hollisterite (Al₃Fe), kryachkoite (Al,Cu)₆(Fe,Cu), stolperite (AlCu), steinhardtite (Al_0.38Ni_0.32Fe_0.30) [345,347,348,349]; it is an unusual mix. Moreover, icosahedrite (Al₆₃Cu₂₄Fe₁₃ and Al₆₂Cu₃₁Fe₇), as well as decagonite (Al₇₁Ni₂₄Fe₅; ≤60 μm), the first natural quasicrystals [350], were detected [351,352,353,354,355]. It was assumed that these were formed by shock synthesis when planetoids collided [356,357]. Khatyrka experienced at least one high-velocity impact event.

The CV3 carbonaceous chondrite Allende (Pueblito de Allende, Chihuahua, Mexico; 26°58′ N, 105°19′ W) contains many different minerals, CAIs, graphite, nanodiamonds, alkane, amino acids, and hemolithin [358]. Moreover, a xifengite (Fe₅Si₃) grain was detected [359]. Kaitianite, Ti³⁺₂Ti⁴⁺O₅ tistarite (Ti₂O₃), rutile (TiO₂), corundum (Al₂O₃), mullite (Al₆Si₂O₁₃), osbornite (TiN), and a Ti,Al,Zr-oxide were present in this area. The meteorite is thought to be 4.567 billion years old.

The achondrite NWA 8014 (Northwest Africa; unknown coordinates) also contains suessite (Fe,Ni)₃Si.

A special find, that was identified as meteoritic, was made around the Kyker and Zelenoye Ozero villages (Tungokochenskii district, Transbaikalia Krai, Zabaikalsky region, Russia; 53°19′ N, 116°19′ E) which was named after the nearby Ilekta creek river [51,360]. The high nonuniform magnetic boulder (15 cm × 9 cm × 1–8 cm; 12 kg) is rounded, appears shiny metallic with an oxidized surface. On it and parts of the interior, traces of melting and boiling are visible. The stone shows plastic deformations and brittleness, depending on the heating and cooling it experienced. Moreover, the boulder is dotted with sprinkles in the holes. The surface is covered on one side with an oxide crust (1–4 mm thick) caused by the prolonged period of time it spent on the bottom of a river terrace, where the surface was exposed on one side. Basically, however, the block is unoxidized due to the high content of iron silicide. The boulder consists mainly of gupeiite (Fe₃Si; empirical formula: Fe 81.60–86.87%, Si 14.63–15.54%, Mn 1.06–1.23%, T: 1.13–1.25%, P 0.30–0.42%), with trapezoidal crystalline inclusions of TiC (Ti 47.48–68.5%) and schreibersite ((Fe,Ni)₃P; empirical formula: Fe 73.53–75.65%) P 13.79–15.27%, Mn 5.00–5.28, Si 6.62–7.29%, Ti 0.58–1.79%, Cr 0.36–0.51%), with the latter shaped as needle-like crystals. It is assumed that the block is extraterrestrial, as there is no evidence of anthropogenic production; there has never been any industrialisation in this area, and other findings support this. A terrestrial origin has not been evidenced so far.

A conspicuous lump of magnetic rock (16 cm × 12 cm × 8 cm, 5934 kg) with a molten surface was found in a suburb of the city of Chernivtsi (Ukraine; 48°17.4′ N; 25°52′ E) at a depth of about 16.0 m [361]. The chronology is Serravalian (13.82–11.63 mya; Dashavian Miocene, N1ds). Two samples were taken for examination. The matrix (> 90%) consisted of (Fe_3.04Mn_0.03Cr_0.01)_3.08(Si_0.9Ti_0.02V_0.01)_0.93, corresponding to gupeiite (Fe₃Si). In the second sample, there was a small amount of Ni (0.01 vol%). There were rectangular, trapezoidal, tandem, isometric, and irregular within inclusions (0.01–0.5 mm) of titanium carbide (TiC; empirical formula: (Ti_0,98V_0,05)_1,03C_0,96). An extraterrestrial origin is suggested.

9. Iron Silicides Deriving from Extraterrestrial Dust

The irregularly shaped particle L 2009 I14 (33 μm × 22 μm) was collected during a flight with NASA ER-2 in several successive ascents to 20 km altitude in the stratosphere. If the system had been near equilibrium, it would have contained SiO₂, pyroxene (enstatite), cordierite, pentlandite (Fe_0.65Ni_0.35)₉S₈ and iron silicide hapkeite (Fe₂Si). There were also more minor traces of Mg, S and Ni and much smaller indications of Al and Cr. NASA classified the particle as cosmic dust (type “C”) [362]. On Earth, iron silicides can be found in so-called extraterrestrial dust (Figure 7).

In the Ferghana Valley (Isfara river, Uzbekistan, 40°44′24″ N, 72°37′48″ E), naquite (FeSi), gupeiite (Fe₃Si) and xifengite (Fe₅Si₃) spherules (<2 mm), together with moissanite (SiC) spheric nodules, were discovered. The moissanite globules consisted of SiC in the nucleus, a mantle of C, and an envelope of Fe₃C. Two of the Ferghana samples had Si 10–15wt%, Fe > 30 wt%, Ca 3 wt%, Na 1 wt%, Ni 0.1–0.2 wt%, Mo up to 0.1 wt%. The dispersed iron silicides area is like a strewn field, roughly 50 km width. The chronology, ~145.0–66.0 mya, is derived from the Cretaceous sedimentary of the Isfara river [156,172,363].

Cubic fersilicite/naquite cubic (FeSi), tetragonal ferdisilicite/linzhiite (FeSi₂), Fe₂Si₃, gupeiite (Fe₃Si), xifengite (Fe₅Si₃), intertwined, were found in placers and drill-core sandstones at the Poltava series (Zachativsk station, near Vysoke, Donetsk, Donetsk Oblast, Ukraine; 48° N, 38° E), Konksko-Yalynskaya depression, North Azov [156,173,176,364,365,366,367,368]. Initially, the minerals FeSi and FeSi₂ were not approved by the CNMMN, but in 2012, they were approved and named naquite and linzhiite [38,41,138,140]. Besides iron silicides, wüstite (FeO) together with silicate glass, was also found. Native Si, magnetite (Fe₃O₄), kamacite (α-(Fe,Ni)), moissanite (α-SiC), barringerite ((Fe,Ni)₂P), schreibersite (Fe₃P) and cohenite ((Fe,Ni,Co)₃C)) were identified. Magnetic rims also showed up in the material. Grains ranged from 0.05 to 7 mm, with those over 0.5 mm predominating. A pronounced groove can be seen on some, especially on the large grains, of the iron silicides. The ferrosilicon is irregularly formed (tetrahedral, prismatic, lanceolate, toothed, dendritic, globular, etc.). An opaque, dark, grey coloured film covers the grains. Only spalling and fractures show the characteristic strong lustre of the mineral. The heavy and bright mineral is found in quantities averaging 3–5 kg/m³, and, in some places up to 250 kg/m³, in an elliptical field (4 km × 1.8 km), 5–90 m deep. The amount of material decreases toward the periphery of the elliptical field. The Poltava series was dated to the Oligocene-Miocene (28.1–11.62 mya). The origin of the material is considered to be anthropogenic, tectonic, or meteoritic, but it may also be meteoritic. Mineralogical, morphological, and structural features indicate a cosmogenic origin for all these minerals, i.e., either cometary material or another source [173]. Barringerite so far is usually (14 times) discovered in meteorites, and at only nine terrestrial sites [369].

Gupeiite (Fe₃Si; empirical formula: Fe_2.971Ni_0.027Mn_0.023Si_0.979) and xifengite (Fe₅Si₃; empirical formula: Fe _4.905Ni_0.018Mn_0.015Si_3.062) inclusions were discovered in the cores of spherules (0.1–0.5 mm) embedded in in placers in the Yan Mountains (Yan Shan, Chengde prefecture, Hebei province, China, 41° N, 117° E) [370,371,372]. The placers have chromite, copper, gold, graphite, titanium carbide, ilmenite. It is the type of locality for gupeiite and xifengite. The spherules consist of magnetite, wüstite, and maghemite, forming a mantle. Then follows an inner shell made up of kamacite and taenite, and inside, there is a core of gupeiite and xifengite. TiC is often in the nucleus, too. The surface characteristics, which indicate flight through the atmosphere, and the minerals present, indicate the spheres to be extraterrestrial in origin.

Hapkeite (Fe₂Si) was reported for magnetic spherules (100–200 μm) from Üveghuta, Lower Carboniferous Mórágy Granite Complex (Tolna County, Southern Transdanubia, Hungary, 46°11′51.9″ N, 18°35′25.7″ E) and from the Mesozoic Gemeric Granite Complex, too. This could be related to extraterrestrial dust or a meteorite impact [183,373]. Besides hapkeite, moissanite (SiC), metallic (Fe) silicate spherules, and minor amounts of Al, Si, P (few wt%), S, K, Ca, Ti, V, Cr, Mn, and Ni were also discovered. Spherules and lamellae produce magnetism. Shocked metamorphic quartz was also identified [183], indicating the effects of high pressure, initially high temperatures (>1473 K) and then very rapid cooling.

In the late Danian (66.0–61.6 mya)/early Selandian (61.6–59.2 mya), an impact ejecta layer was found at two locations on the Isle of Sky (An Carnach and Broadford, Strathaird Peninsula, Scotland, UK; 57°15′ N, 5°55′ W) Fe(Si) [374]. The age is between 61.54 ± 0.42 and 60.00 ± 0.23 mya. At location 1, the impact ejecta layer (≥95 vol% matrix, 47 wt% SiO₂, high Al₂O₃ and FeO) is 0.9 m; at location 2, it is 2.1 m thick. The analysis of a Fe(Si) grain from site 2 shows the inclusion of vanadium-rich osbornite (TiVN), barringerite [(Fe,Ni)₂P], and native Fe(Si) metal. A spherule (≈107 μm) from the exact location shows a ferro silicate glass with vesicles which encloses a spherical core (≈45 μm) of highly reduced native Fe(Si). The material contains unmolten vanadium-rich osbornite (TiVN), niobium-rich osbornite (TiNbN), high-pressure zircon polymorph reidite (ZrSiO₄), barringerite [(Fe,Ni)₂P], baddeleyite (ZrO₂), alabandite (MnS), carbon-bearing native iron (Fe) spherules, graphite (C) trails, FeO, and Si, Ni, and Cu in traces. The crystallisation of the spherules occurred extremely quickly. Reidite in single zircons mostly shows distinct shock lamellae. PDF and diaplectic glass in quartz imply pressures ≥30 Gpa at both locations. All this evidence together makes it clear that an impact created the deposits. TiVN and TiNbN are thought to be the remains of an extraterrestrial body.

Some 200 rock pieces (in total 200 kg) were discovered, conspicuously scattered and embedded in an area of ca. 10,000 m² of a partially eroded colluvial fan, about 50 km north of Belo Horizonte (Minas Gerais, Brazil, 19°55′0″ S, 43°56′0″ W) [375,376]. Whether industrial, natural (terrestrial or even extraterrestrial), the assignment of the locality remains unclear. Most of the pieces show strong magnetism. Less than 5% are slightly to moderately oxidised. There are different mineralogical groupings: the main group (>90%) consists mostly of corundum (Al₂O₃) and a minor spinel (MgAl₂O₄) matrix that contains internally highly crystalline structured polymetallic nodules. Most are spherules (<1 mm), some of which are perfectly rounded. However, oval-shaped nodules (>10 cm) were also observed. Minerals of the Melilite group ((Ca,Na)₂(Al,Mg,Fe₂₊)[(Al,Si)SiO₇]), e.g., gehlenite ((Ca₂Al[AlSiO₇])) are present. The material is unusually enriched in Zr and depleted in Al. Grossite (CaAl₄O₇) was also identified. Cu-Ni-Sn, Fe-Ni-Cu, Fe-Ni-Cu-Sn, Fe-Si-Ti, Fe-Si-Cr, Fe-Si-Ti-Al alloys are present in small quantities in all nodules. Kamacite (a(Fe,Ni)], xifengite (Fe₅Si₃), gupeiite (Fe₃Si)] and spherulite (Zn,Fe)S were also detected, as well as sulfides and phosphides. W, Pb, Zr exist in trace amounts. Some findings seem to indicate a meteoritic origin of the material, i.e., certain meteoritic shapes, regmaglypts, fusion crusts, Ca-Al-rich inclusions, enigmatic triangular crystallization structure like Widmanstätten pattern, and Grossite. The latter is known from only four locations on Earth (Hatrurim Formation, Israel; Kishon Mid Reach zone 2, Kishon river, Israel; Rakefet magmatic complex, Israel; Dellagiustaite type locality, Sierra de Comechingones, San Luis Province, Argentina), but from 17 meteorites. Thus, in principle, the material could be terrestrial, including industrial production, but is more likely meteoritic. In any case, extremely reducing conditions needed to be in place in either case.

Unusual glassy melt spherules (≤700 mm in diameter) were found at Peter’s Pond (Blacksville SC, Carolina Bay, South Carolina, USA, 41°41′30.82″ N, 70°28′49.08″ E) [377]. Additionally, highly heated and partially melted clay clast was observed. The material showed numerous vesicles. There were three types of spherules. Mullite (Al₆Si₂O₁₃), corundum (Al₂O₃), and aluminosilicate were evidenced in glasses and spherules. The mullite cores were partially surrounded by incompletely dissolved kaolin clay balls, which had been slightly heated. Needles of mullite had grown through the boundary layer between clay and glass. Spherules with Mullite of the most common type typically contain a few corundum crystals. Interspersed spherules of gupeiite (Fe₃Si) occurred between the mullite-enriched glass and the layer of heat-decomposed black clay. It is suggested that the Fe₃Si was produced by the reduction of FeO in the glass. This was caused by the carbon content in the black clay, which was exposed to very high temperatures. Cohenite (Fe₃C) globules (10–20 μm), which have very small inclusions of steadite (90.71% Fe + 6.89% P + 2.4% C at 950 °C, and 1.10 wt% Ni, 0.78 wt% Co), were found in the reduced regions of the spherules. These also occur in irregular areas (10–20 μm) within highly oxidised aluminium hematite. The delineation of the fusion exhibits crystals of aluminous hematite quenched in both rich and poor Fe glasses. Moreover, Ir (15 ± 7 ppb), Ni (395 ± 40 ppm), Cr (574 ± 57 ppm) was detected. It is suggested that the melt glasses originated from the kaolinite clays, which contained smaller quantities of iron oxides and illite. The unusual melting material requires high temperatures and extreme reducing processes. The genesis of iron silicides is unclear.

Different spherule types (wüstite, magnetite, iron silicide, chondritic silicate), which show characteristic textures, were discovered at 7 m depth in the chalk of Hogden Lane (North Downs, Ranmore Common, Surrey, UK, 51°15′18.4″ N, 0°23′32.0″ W). The iron silicide type (50–150 μm) contained Fe₃Si (empirical formula: Fe_2.89Si_1.11), without any Ni and with small amounts of O₂ (<1.21 wt%,), Cr (<1 wt%), and Mn (0.3–0.7 wt%) [378]. The chronology is Coniacian (87 ± 1 mya). The iron silicide spherules were identified as original cosmic ones of the iron-type (I-type) that had been diagenetically modified.

10. Iron Silicides as Recondensation of Ejecta Vapour

It has been suggested that many of the spherules thought to be of extraterrestrial origin are, in fact, the result of processing through the recondensation of ejecta vapour [156]. (Figure 8).

A greyish-black rock block (60 cm × 80 cm × 13–15 cm) from Koshava gypsum deposit (Moesia region, Danube River, Northwest Bulgaria; 44°3″8.748′ N, 22°59″35.311′ E), discovered at a depth of 290 m, contained gupeiite (Fe₃Si), hapkeite (Fe₂Si) and probably naquite (or fersilicite) FeSi embedded in petrified organic matter (showing a wooden texture) [379,380,381]. There were also other stoichiometries of Fe_xSi_y (Si: 3.57 wt% to 9.08 wt%). One grain of Suessite (Fe, Ni)₃Si was also verifiable. At first, the find was identified as a meteorite, but since then, it has been been reclassified as impact ejecta. Evidence of extensive high-temperature melting is given by melted quartz rosettes, ballen structures in quartz, melted wood relicts, regmaglypts on the surface crust, lechatelierite (SiO₂), iron silicides (Fe_xSi_y), moissanite (SiC), melted spherules. High shock impact is indicated by coesite (>2.5–3 Gpa and >973 K), mineraloid lechatelierite (SiO₂; >1973 K, 85 GPa), strongly broken quartz, deformation lamellae, and high δ¹⁸O values of quartz. It is suggested that the Ries-Steinheim impact event (Middle Miocene, Langhian, 14.37 ± 0.30 [0.32] mya [2σ]) ejected the block [382]. However, the Koshava gypsum deposit is located ~1100 km to the southeast from that impact site. Such a distance weakens this hypothesis significantly.

In Blackville (South Carolina, USA, 33°21′25″ N, 81°16′22″ W), suessite (Fe,Ni)₃Si was found in molten siliceous glass (420–2700 μm) situated in layers at a depth of 1.75–1.9 m, [383]. Moreover, glassy spherules (15–1940 μm) were also present. The suessite appeared together with globules of native Fe, quenched grains of corundum (Al₂O₃) and different stoichiometric mullite (2Al₂O₃·SiO₂ and 3Al₂O₃·2SiO₂), Fe₃C spherules including ferro phosphorus (Fe₂P, Fe₃P). The glass also contains the mineraloid lechatelierite (SiO₂). At Melrose (Pennsylvania), molten siliceous glass spherules (2–5 mm) were found which contained the same minerals and mineraloids as well as molten magnetite [383,384]. The presence of mullite, suessite and lechatelierite in particular, testifies to high temperatures and pressures [383,384]. Suessite forms at 2273–2573 K [383,384]. Lechatelierite in granite needs 85 Gpa to emerge [385]. Thus, it is suggested that the material formed during an impact which melted the silicates. The chronology is the onset of the Younger Dryas, which is believed by research teams to have occurred 12,800 ± 300 cal BP with 2σ (95%) probability (calibrated BP: calendar years before 1950) [383,386]. This dating, however, remains contestable [387,388,389,390]. The onset of the Younger Dryas Event also corresponds to the date of a third site with iron silicides, which is an archaeological settlement, i.e., Tell Abu Hureyra 1 (now submerged in the Lake Assad, Raqqa Governorate, Syria, 35°51′57.6″ N, 38°24′0″ E). The Epipaleolithic or Natufian settlement was founded c. 13,500 years ago, was inhabited for approximately 1000 years, and then abandoned around 12,200 years ago [391]. Excavations have revealed layers rich in charcoal, with molten glass, molten Fe, Si and C spherules, magnetic spherules, nanodiamonds, platinum, lechatelierite, and plant imprints [392]. Molten glass on the surfaces of spherules and in vesicles reveals the enrichment of Fe, FeO and iron silicides [392]. Globules (1–22 μm) of fersilicite/naquite (FeSi), hapkeite (Fe₂Si) with empirical formula ((Fe_1.9 Ni_0.8Cr_0.2)₂ (Si_0.9P_0.1)), and gupeiite (Fe₃Si) were found in the inner walls of vesicles. There, and on the outer surfaces of the glass, native Si was embedded. Temperatures between 1773–2473 K (e.g., suessite, molten magnetite), shock pressure (e.g., lechatelierite), and the effects (e.g., iron silicides, native Fe and Si) of extreme reducing environments only suggest a high-energy event. This could have been the impact of an extraterrestrial body, i.e., a planetoid or a comet, in parts or en bloc, which struck the atmosphere at Mach 8.8, triggering an airburst and cascading into fragments which subsequently hit the ground. The rapidly ignited biomass and the fused soil, both evaporating, account for the reducing carbon. Rapid cooling followed. Because the Younger Dryas event was associated by the research teams with a global effective impact, a cascading impactor (multiple impacts) was also assumed. There are a number of criticisms of the assessment of an impact with global implications and their relation to the origin of the Younger Dryas period. The anomalies could be attributed to volcanism, the Plinian eruption of the Laacher See volcano (Ahrweiler, Rhineland-Palatinate Germany, 50°25′ N, 7°16′ E), the supernova Vela XYZ, a super-sized solar proton event, or a massive melting of the ice sheets during the preceding warm period of the Allerød oscillation (c.13,900–12,900 BP, uncalibrated), resulting in considerable disturbance of the North Atlantic Current and associated cooling (Heinrich Event H0) due to heavy snowfall [390,393,394,395,396,397,398,399]. A combination of different causes cannot be ruled out either.

From an area between the Alatau and Kalu ranges, close to the abandoned settlement of Utar-Yurt (Southern Urals, Ishimbayskiy rayon, Republic of Bashkortostan, Russia, 53°46′0″ N, 56°51′0″ E), natural silicides (0.05–8 mm, irregular to spherical shaped ≤2.7 cm) have been reported in the streambed of a tributary to the Sheshenyak Minor river [156,400,401]. The spherules (5.1 kg) were found scattered or clustered (1 m × 1 m to 2 m × 5 m, partially washed up) over an area of 300 m radius in the streambed, in outcrops on the bank (depth 0.5–1.5 m). The area is difficult to access and is uninhabited. An industrial origin of the iron silicides has been excluded. Most silicides (>98%) appear to be ferromagnetic or paramagnetic. The chronology is Pleistocene (from sediments). The main matrix components are intertwined iron silicides like gupeiite (Fe₃Si) and xifengite (Fe₅Si₃), partially fersilicite/naquite (FeSi) and hapkeite (Fe₂Si), and other phases (Fe₇Si₂, Fe₃Si₂) or (Fe,Ti)₄Si₅, (Fe,Ca,Ti)₅Si₄, and (Fe,Ca,Ti)₄Si₇. There are inclusions (3–10 μm) of fersilicite/naquite (FeSi), linzhiite (FeSi₂), hapkeite (Fe₂Si), titanium carbide (TiC), moissanite (SiC), cohenite (Fe₃C), khamrabaevite (T,V,Fe)C), graphite (C), magnetite (Fe₃O₄) and wüstite (FeO) typical on rims (1–3 μm), zirconium silicide carbide (Zr-Si-C), and unnamed (Fe,Al,U)Si, (Fe,U,Al)₂Si, (Fe,U,Zr)₅(SiP)₂, and (Fe,U)₄(SiP),(Fe,U)₅Si, as well as (Fe,U,Al,Zr)₅Si₃. Smaller areas (<30 μm) contain C, Al, Ti enrichments. Depending on the size of the spherules, the composition and proportion of different iron silicide phases vary. Limonite nodules (20–30 mm) are paragenetic and contain tiny spherules of iron silicides (Fe_xSi_y), spinel (MgAl₂O₄), clay and silicate glass. The matrix of limonite shows signs of shock metamorphism. An isotopic (⁸⁷Rb/⁸⁶Sr, ⁸⁷Sr/⁸⁶Sr, ³He/⁴He, ⁴⁰Ar/³⁶Ar, ¹⁴³Nd/¹⁴⁴Nd, ¹⁴⁷Sm/¹⁴⁴Nd) analysis showed certain deviations from terrestrial values. There was no cosmogenic ³He, ²¹Ne, and ³⁸Al or radiogenic ⁴⁰Al. The elemental ratios of AI, Kr, and Xe evidence terrestrial sediments. Ni is below 0.1%. There is probably no Ir or Os. All this supports sediments of the upper crust of the Alatau and Kalu ranges as the origin of the material. Their chronology is Chibanian/Middle Pleistocene (314–121 ka).

There are similar findings of iron silicides in terms of morphology and composition distributed in an area of 25 m × 5 m, 2.5 m deep, at Laurel Hills, Holmdel (NJ, USA, 39°56′56″ N, 74°54′1″ W) [156]. The 75 magnetic pieces are slightly smaller (0.05–4 mm). They were discovered in the Wenonah formation. The chronology is Upper Cretaceous (100.5–66 mya). From the findings, it was deduced that the material was terrestrial. It is thought that the iron silicide spherules were produced secondarily from terrestrial material following an impact [156]. The morphology of the spherules at both sites gives clear evidence that they were intensely heated (>3700° C) during flight through the atmosphere, before quickly slowing and cooling down in the process. They show characteristics of aerodynamic stress, ablation, quenching, and extremely high temperatures >2100 K (SiC TiC, Fe_xSi_y) during two heating events, as well as shock. A mixing with primary extraterrestrial material cannot be excluded [156]. It is thought that the spherules were produced as ejecta by an impact event and the subsequent hypervelocity crash back to the ground. The researchers argue that the uranium iron silicides and aluminium and zirconium carbides, within the iron silicides, were not formed in any technological process.

The closest and only third hitherto known association of uranium and fersilicites, moissanite, titanium carbide, graphite, and the special khamrabaevite, was found in the uraniferous iron silicides of the Chiemgau Impact site (see below).

11. Iron Silicides Associated with Craters

In a few cases, iron silicides may be associated with individual craters or crater fields (Figure 9). The Haughton impact crater (Devon Island, Territory of Nunavut, Canadian Arctic, 75°22′ N, 89°41′ W), 23–24 km in diameter, contains iron silicides together with moissanite (SiC, native Si, and other silicides of Al, Ni, Ba, Ti, and V (here VSi₂). The hexagonal crystals comprise vanadium silicide (VSi₂) with minor Ti and Ba substitutions for V within silicate glass produced by the impact event [402,403]. The impact is dated to the Eocene, 39 mya ago.

A comprehensive relation of iron silicides in craters in an extensive strewn field may be undertaken using material from the Chiemgau Impact site. The crater strewn field of the “Chiemgau impact” is evidence of a large meteorite impact that occurred in prehistoric times in the foothills of the Bavarian Alps [404,405,406]. The area extends roughly elliptically over an area of about 60 km × 30 km (c. 1800 km², 47.8°–48.4° N, 12.3°–13.0° E) between Altötting, Lake Chiemsee and the Alps, Bavaria, Germany. Nearly 80 craters have been documented. The impactor that caused the event is likely to have been a relatively porous object consisting of various components that broke apart in the atmosphere. The analysis of the composition of an impact rock showing the shock metamorphoses typical for an impact and, at the same time, fusing with the metallic components (high lead bronze and iron) of artefacts from the archaeological layer, makes it possible to date the Chiemgau impact to ca. 900–600 BC [407,408]. The published research results evidence an impact event based on the relevant criteria and methodology required in the scientific community. However, the relationship of the geological and archaeological structures and material findings to an impact event has been questioned [409,410,411,412,413] and debated [404,408,414,415,416,417,418,419,420,421,422].

In the crater strewn field, a total of 2–3 kg of particles, hardly corroded or not corroded at all and showing a metallic sheen, were found distributed over hundreds of square kilometres. They were often are shaped in aerodynamic forms such as ellipsoids, spheres, buttons and drops, but also as splinters and pieces (from 1 mm up to 6 cm and 167 g), or even an 8 kg lump in the subsoil down to the substratum (≈ 30–40 cm) in a glacially formed layer [404,405]. A smoothed convex face and a flat irregularly shaped reverse were frequently observed. The material is tough and magnetic [414,423]. Some specimens show a remaglyptic surface. There is also accretionary lapilli with magnetic xifengite cores. Iron silicide splinters also occurred in foamy-porous carbonate matrices, presumably recrystallised carbonate melt. Big sparkling crystals (moissanite) protruding from the metallic matrix are visible to the naked eye. Fersilicite/naquite (FeSi), ferdisilicite/linzhiite (FeSi₂), hapkeite (Fe₂Si) as cubic (hapkeite-1C) and trigonal (hapkeite-1T), gupeiite (Fe₃Si), suessite (Fe,Ni)₃Si, xifengite (Fe₅Si₃), and in traces suessite (Fe,Ni)₃Si were detected [404,424,425,426,427]. Fe_xSi_y appeared as irregular, round blebs (5–40 μm) and pyramid-shaped formations (≈600 μm) in the microstructure. The intergrown iron silicides formed a matrix for various mineral inclusions. Among them were cubic moissanite ([β]3C-SiC) and titanium carbide (TiC) crystals (≈ 40 μm × 80 μm) of extreme purity, as well as TiC_0.63. Khamrabaevite ((Ti,V,Fe)C) was frequently present. There was zirconium carbide (ZrC), possibly baddeleyite (ZrO₂) and uranium carbide (UC). Zircon Zr[SiO₄] crystals (3–10 μm) and uranium (U) as caps were recognisable. Sometimes, SiC appeared peppered with U blobs. Moreover, calcium-aluminium-rich matter, like the calcium aluminate/krotite (CaAl₂O₄) and dicalcium dialuminate (Ca₂Al₂O₅) [426], was identified in the material. There were also graphite and nanodiamonds (C). Ni (≈ 0.8 wt%) was present in the suessite (Fe,N)₃Si. The amount of Cr was ≈ 0.5 wt%. In addition to the main component, i.e., Fe_xSi_y, more than 40 other chemical elements, including uranium and REE (e.g., Y, Ce, La, Pr, Nd, Gd, Yb) have been detected so far. In one sample Th was marginally detectable, and in another, a trace of Po was found. Lead was completely absent. Previous individual findings of a different nature could not be confirmed [409,410]. Although uranium was present in spectra in clear quantities, there was no evidence of daughter nuclides, grandchild nuclides, etc.

The microstructure of the material showed clear signs of very intense mechanical overload, which, in principle, could have been caused by high shock effects (pressure, dynamic spallation, and thermal). This caused deformation lamellae and various crack features, e.g., tensile open fractures and groups of subparallel open fissures in Fe_xSi_y, TiC crystal, and multiple sets of planar features (PF), kink bands, planar deformation features (PDF) in SiC crystal. The Fe_xSi_y matrix was littered with rimmed microcraters (10–20 μm), sometimes showing “ring walls”, probably from the impacts of microparticles. The fersilicites regularly occurred near rimmed nanometre craters. Detailed images showed that zircon crystals struck the plastically deformed or even liquefied matrix of iron silicides. It is assumed that disturbance waves ran through the material and suddenly stopped, so that the matrix froze.

The mixture of minerals in the iron silicide matrix was unusual; they were distributed in it with low/high pressure and/or low/high temperature. There was monoclinic high temperature (>1773 K), low-pressure dimorph of CaAl₂O₄ [419,426], known as krotite. As a natural mineral, it has been identified in meteorites NWA 1934 [428] and in the basic/ultrabasic basaltic volcano complex of Mt. Carmel (Rakefet magmatic complex, Mount Carmel, Haifa District, Israel, 32°43′59″ N, 35°2′59″ E; see above), dated to the Late Cretaceous (96.7 ± 0.5 Ma) and assigned to kimberlites [429]. At the latter site, orthorhombic dicalcium dialuminate (Ca₂Al₂O₅), was found, i.e., unnamed UM1977-08-O:AlCaH [430], a high-pressure phase (>2.5 Gpa) [431] with the brownmillerite-type structure. This was also identified in the iron silicide matrix of the Chiemgau impact [419,426]. That phase can also be produced at ambient pressure but under quite high temperatures [431]. Moreover, in the large area of the Hatrurim Formation (Israel, 31° N, 35° E), where the rocks, consisting of chalk, limestones, marl, enriched with bituminous compounds, have been intensely heated and metamorphosed, Ca₂Al₂O₅ was also detected [432,433]. The chronology there is Late Cretaceous/Early Eocene (66.0–47.8 mya). Ca₂Al₂O₅ was also detected in the xenoliths of the Ettringer Bellerberg volcanic system (Ettringen, Mayen-Koblenz, Rhineland-Palatinate, Germany, 50°21′0.88″ N, 7°13′41.65″ E), dated c. 0.215 ± 0.004 to 0.190 ± 0.004 mya [434]. In addition, the iron silicide suessite (Fe,Ni)₃Si formed from the matrix at more than 2000 K, and cubic moissanite ([β]3C-SiC) as well as nanodiamonds indicated high shock pressure [243]. Xifengite (Fe₅Si₃) and carbon spherules within amorphous carbon were found in the glazed enamel skin of a pebble from crater #004 in the field. High temperatures (thermal shock), >1773 K and pressures, as well as a magnetic anomaly, have been documented for the rocks in that crater [417,435]. Finally, an iron silicide lump (c. 16 cm × 11 cm × 5 cm, 8 kg), found approximately 30 years ago near Grabenstätt at Lake Chiemsee, is reported to contain cubic hapkeite (Fe₂Si, cubic and trigonal polymorph), gupeiite (Fe₃Si), xifengite (Fe₅Si₃), titanium carbide (TiC)/khamrabaevite ((Ti,V,Fe)C), moissanite (cubic SiC), zirconium carbide (ZrC), graphite and graphene [424,426]. When writing this review, the block is the largest known example containing natural cubic and trigonal Fe₂Si.

Collectively, the iron silicides hapkeite (Fe₂Si), suessite (Fe,Ni)₃Si) and xifengite (Fe₅Si₃) in the matrix, the mixture of mineral inclusions, which prove the effects of high but also low temperatures and pressures, the large-scale distribution, the association with craters in a strewn field, the finds in proven old layers of the Middle Ages from below a medieval hoard of coins and a castle, in peat mires and on the heights (>1000 m) of the neighbouring Alps exclude an anthropogenic-industrial origin (including bombing) [410] of these materials [404,405,414,435]. A geogenic source is also not plausible [414,435]. A primary extraterrestrial, including perhaps already a mixture in space or a secondary terrestrial (ejecta) source, is suggested [404,405,406]. The high degree of similarity among the finds from the Chiemgau impact with those from the Alatau and Kalu ranges (Southern Urals, Ishimbayskiy rayon, Republic of Bashkortostan, Russia Ural, Russia) and Laurel Hills, Holmdel (New Jersey, USA) is striking (see above). The findings on the association of uranium and fersilicites, moissanite, titanium carbide, graphite, and the special khamrabaevite are particularly significant. Thus, the iron silicides of the Chiemgau impact can, in principle, also be classified as (distal) impact ejecta. However, in contrast to, and as an extension of, the Alatau and Kalu as well as the Laurel Hills findings, there is a vast crater-strewn field which is genetically associated with the iron silicides, and within the iron silicide matrix are rare krotite (CaAl₂O₄) and dicalcium dialuminate (Ca₂Al₂O₅). Although Fe_xSi_y can be anthropogenic in origin, it is usually not comparable to the iron silicides and associated material found in the Chiemgau strewn field. Given that the known occurrences of Fe_xSi_y include several examples of extraterrestrial origin, such an origin is plausible unless a separate, nonimpact origin for Fe_xSi_y can be clearly demonstrated.

An additional, still unknown process or a mixture with the extraterrestrial material of the impactor is assumed here.

12. Other Iron Silicide Findings

There are very few iron silicides with other formation geneses (Figure 10). Amorphous Fe-Si mineral in a turbidite was dragged up from the Nares’s abyssal plain (western North Atlantic; 23°30′ N 63°0′ W) [436]. It is thought to have been produced as precipitate by the mixture of diffusing dissolved iron and silica. A piece from ferromanganese crust (7.5 cm thick layer) from a guyot in 2486 m depth, Mid-Pacific Rise, Pacific Ocean (19°37′59″ N, 175°48′0″ W) revealed various minerals like apatite, goethite, barite, and rustenburgite [437,438]. Grains (<3 μm) of Cu-silicides (Cu,Pt)₄Si, (Cu,Pt)₅Si, Fe-silicides (Fe₂Si), (Fe₃Si), and Fe₅Si₃ as well as platinum group elements (PGE) were found [437,438]. The formation of metal silicides in the ocean floor, which was almost completely sediment-free, is here attributed to highly reducing fluids in the context of basalt formation. Still, an artificial origin cannot be excluded concerning the tetracopper silicide (Cu₄Si).

In a heap of the dormant Piast coal mine in Nowa Ruda (Kłodzko County, Lower Silesian Voivodeship, Poland; 50°35′3″ N, 16°31′6″ E) native iron (Fe), possible gupeiite (Fe₃Si)/schreibersite ((Fe,Ni)₃P), (unnamed) Fe₇(P,Si)3 and (unnamed) Fe₅(P,Si)₃ has been found [439]. The natural process of self-heating, set in motion anthropogenically by waste rock piles, induced pyrometamorphism. Among them, iron silicides may form during the spontaneous combustion of coal [439,440]. In comparable burning heaps at other locations, barringerite (Fe,Ni)₂P [369], cohenite (Fe₃C), and oldhamite (Ca,Mg)S [441] have been identified [439]. This moves the generation form of these pyrometamorphic rocks into a certain proximity to meteoritic material [442]. In the latter case, however, there would also have been considerable changes due to shock effects, and many other general conditions would have to be considered.

13. Iron Silicides as a Component of Circumstellar Envelops (CSE) and in Interstellar Matter (ISM)

Iron silicides/Iron-Nickel silicides, typically with sizes of tens of nm and with an abundance of >> 1 ppm, evidenced only as inclusions in other presolar phases, are among the presolar grain types associated with AGB stars and supernovae (SNe) [443].

Theoretical considerations and modelling [444,445,446,447,448] show that in circumstellar envelops (CSE) of certain so-called AGB (Asymptotic Giant Branch) stars, during the final phase of their evolution, if C/O > 1, phases of iron silicide (Fe_xSi_y) can condense into small grains. These seem to be a component of the dust envelops related to S-type stars and Luminous Blue Variables (LBVs). Depending on the proportion, C/O S-type stars with C/O ≈ 1 (±0.25) are classified between carbon stars with C/O > 1 (spectrum dominated by TiO bands) and M-type AGB stars (normal giants) with C/O < 1 (spectrum dominated by ZrO bands) [449,450]. A stellar superwind forms the circumstellar envelopes (gas, dust) during the last phase of the star’s existence, slowly enriching the interstellar matter. There is not enough oxygen in S stars to form oxides and too little carbon to form soot (C/O~1). S-stars mark the transition from M-stars (main sequence stars) to C-stars (carbon stars) on the Asymptotic Giant Branch (AGB) (Figure 11). As such, unusual native Fe (α-Fe) and rare iron silicides FeSi are expected in the dust, along with other silicides and nitrides [445,447,448]. FeSi can be more stable than Fe at a critical carbon content limit. In the dust envelopes of S-stars, SiC (moissanite), TiC (titanium carbide), and Mg₂[SiO₄] (forsterite) also condensate.

Luminous Blue Variables (LBVs; S Doradus variables or Hubble-Sandage variables) are extremely variable supergiants or even hypergiants, which, besides some periodic outbursts, undergo powerful eruptions. LBVs can have between 10 and 100 M_☉. Their luminosity is between 0.25 and 1 × 10⁶ L_☉ [451]. LBVs represent the final stage in the evolution of such massive star types. Random violent mass ejections lead to the shedding of shells with a mass loss of 7 × 10⁻⁷ to 6.6 × 10⁻⁴ M_☉/yr⁻¹ [452]. According to modelling [448] for a sample LBV star (parameters: T_eff = 7500 K, outburst mass loss rate: 0.3 M_☉/yr⁻¹referring to η Car, mass: 100 M_☉, luminance: 2 × 10⁷ L_☉), the LBVs dust ejecta are dominated by metallic Fe, FeSi, forsterite (Mg₂SiO₄). Sic is also produced. It has been proposed that TiC is a supporting grain for dust building.

There is some evidence for iron disilicide (β FeSi₂) in the so-called “Iris Nebula” (LBN487) in the Cepheus constellation [453,454] (Figure 12). It is a reflection nebula (Figure x), ca. 1300 ly away. It measures six light-years in diameter. SAO 19,158, also known as HD 200,775 (Herbig Ae/Be star), is a very bright and hot star (spectral class B2Ve), having ten times the sun’s mass. It is in a formative stage of evolution and illuminates the dust surrounding it. In the case of LBN487, a specific strong emission at 1.5 µm in the north-western, but not in the southern filament, indicates the existence of low-temperature, amorphous FeSi₂ particles of ca. 100 nm size.

There is spectral evidence of iron silicides (FeSi) in the expanding (≈30–40 km s⁻¹) dust shell (≈diameter 8″/2 × 10¹² km) around the spectroscopic binary star, AFGL 4106 [444,455,456], 10.764 ly away. The 47.5 µm and somewhat less clear 45 µm emission band is interpreted as indicating the presence of FeSi [444]. The findings suggest that there may be grains with a radius of 0.1µm and a temperature of 120 K. The expansion age of the envelope is 3.7 × 10³ yr. Both agglutinated dust and ionized gas form the shell. The binary star consists of an F- classed post red supergiant (≈7250 ± 250 K) evolving towards a Wolf Rayet (WR) star and an M-classed member (≈3750 ± 250 K), a red supergiant, of nearly equal luminosity (7.4 × 10⁴ L_☉ and 1.3 × 10⁵ L_☉), both having masses 15–20 times that of the sun (M_☉) [455,456,457]. The stars are separated from each other by 0.3″.

14. Iron Silicides Related to Novae and Supernovae

A classical nova is a close binary star system in which one component has evolved from a progenitor red giant to a white dwarf. The other component may be a main sequence, subgiant, or red giant star [458]. The white dwarf, due to its extraordinary gravitational pull, accretes material from the other orbiting approaching component. It may also be that the other component expands beyond its Roche limit and starts the process. The matter from that component, mostly hydrogen, produces a dense but thin atmosphere around the white dwarf. The thermal heating of that accretion disk by the white dwarf ignites a very powerful thermonuclear runaway that appears as a Nova outburst. It may be potent (i.e., very short-lived temperatures of up to 2–3 × 10⁸ K, expansion speeds of up to some thousand km, 50,000–100,000 times the solar luminosity, gammy rays >100 MeV [459]), triggered by a thermonuclear runaway. That causes them to eject shells of matter (10⁻⁴–10⁻⁵ M_⊙) into the interstellar medium.

For type II supernovae, modelling predicted the condensation of iron silicides in the C-He, Si-S, and Fe-Ni zones (Figure 13). In the Fe-Ni zone, Ti₅Si₃ (an analogue to Fe₅Si₃) condensates at 1622 K, (Fe,Ni)Si [fersilicite/naquite] at 1528 K, (Fe,Ni)₃Si [suessite] at 1561 K. In the Si-S zone, Ti₅Si₃ condensate at 1641 K, TiSi at 1609 K, (Fe, Ni)Si at 1546, and (Fe,Ni)₃Si at 1461 K. In the C-He zone, (Fe,Ni)₃Si condensates at 1176 K [460]. Modelling a supernova with 21 M_⊙ shows that FeSi condenses at the bottom of the O/Si zone at 1060 K, in the Si/S zone between 1405 K and 1200 K, and at the top of the Ni zone at 1400 K [461].

According to another model [462], condensation occurs in zone M6, counted from the outer edge of the star towards the core [463], which is enriched in S and Si, dominated by a 50/50 Fe-Si alloy, condensing at 1670 K, with trace amounts of TiC appearing at 1560 K and TiN at 1510 K. The stable alloy is likely to be iron silicide (FeSi), for which we do not have thermodynamic data. In zone M7 (enriched in Si, Ca, Ti, Cr, Fe, Co, Ni), TiC appears at 1570 K, and TiN at 1540 K, containing all the C and N, respectively. Abundant metallic Ti appears at 1460 K, and Fe_0.65Si_0.35 alloy appears at 1430 K, incorporating 30% Co by 1330 K.

The presolar SiC grain M2-A1-G674 contains FeSi within subgrains 6 and 8 [464]. Subgrains (11 to 45 nm) of presolar SiC X grain KJG-N2-129-1 from the Murchison CM2 meteorite, a carbonaceous chondrite, selected for the grain type (X), contain iron, nickel silicides (Fe,Ni)_xSi_y [465,466]. However, these research results are not yet recognised as sufficient by some scholars [467]. The findings could be explained by the iron silicides (Fe,Ni)₃Si, (Fe,Ni)₂Si. X grains originate from the supernova type II stage of massive stars (8 M_☉ ≥ M <40–50 M_☉). The Murchison CM2 meteorite is approximately 7 × 10⁹ years old [468]. From modelling, it was derived that FeSi alloy condensed at 1670 K in the M6 zone of the supernova shell, and that Fe_0.65Si_0.35 alloy was generated in the M7 zone at 1430 K [462]. However, a secondary formation at the interfaces between SiC domains appears to be more probable than direct condensation from the gas [468].

An analysis of a very tiny piece (7 µm diameter) of graphite (OR1d3m-18) of the Orgueil meteorite, for which the origin from a supernova (SN) could be determined (C, O, N, and Si isotopes), showed grains with unusual enrichments of Fe_xNi_ySi_z (Fe₆₄Ni₁₄Si_21, Fe₆₇Ni₇Si₂₇ and Fe₆₈Ni₉Si₂₃), sometimes associated with TiC. However, notably, there were also spots of cubic Fe₂Si [469]. From the composition analysis and the correlation of the components, the pathway of the condensation of the stellar material can be deduced: TiC→Ni₂Si→α-(Fe,Ni), enriched in Si/Fe₂Si→SiC→C (graphite). This is consistent with the modelling, except that graphite should condense at the beginning. There is more Si enrichment from the Si/S zone of the star.

15. Discussion

The findings to date lead to some fundamental conclusions for the differentiation and methodological determination of the genesis and origin of natural iron silicides, as well as their differentiation from technogenically-synthesized ones.

Natural iron silicides in general form in highly reducing environments and at high temperatures. These conditions exist during lightning strikes, in the mantle and core of the Earth and in other terrestrial planets as well as the Moon, in the primordial period of the solar planetary system during the formation of protoplanets and proto-moons from impacts of planetesimals and cometesimals, during impacts on the Earth or terrestrial planets, during the entry of meteoroids into the atmosphere, during recondensation of ejecta vapour, and nuclear detonations. Similar conditions are given for the condensation of iron silicides in the envelopes of S-stars, Luminous Blue Variables, classical novae, supernovae of type II, and in certain nebulae. In some cases, permanent high pressures are significant (e.g., in the cores of terrestrial planets). High shock pressures are relevant in the rapid formation of iron silicides, e.g., during impact events.

There are a few examples of the generation of iron silicides as pyrometamorphic rocks without thermal shock or shock pressure, e.g., in the natural process of self-heating in coal heaps, mostly anthropogenically generated or in turbidites of the abyssal ocean floor, by highly fluids during basalt formation. In addition, apart from significant impacts, iron silicides on the surfaces of airless bodies (planets, dwarf planets, moons, planetoids, comets) in the planetary system are produced by ‘space weathering’, that is by the effect of a steady flow of micrometeorites, solar wind and cosmic rays.

According to their origin, a classification of iron silicides has been proposed on various occasions [156,179,361]. However, the following typification from the above examples seems appropriate: technogenic, geogenic, aerodynamic, exoplanetary, and cosmic. The clearest possible allocation to the respective categories must be based on a sufficient variety of methods. The exact specification of all setting conditions is of the utmost importance, to distinguish technogenic from geogenic-aerodynamic samples, and these, in turn, from exoplanetary and cosmic natural iron silicides.

There are some more detailed considerations od the chemical-physical processes of the genesis of natural iron silicides and the transitional stages, as well as the end products formed by their thermal decomposition. However, some aspects are not yet fully understood [96,156,172,174,185,191,260,270,378,439,442,446,447,461,462,470,471,472,473].

Finally, the analysis of paragenetic minerals, focusing on unusual conditions of formation, special mixing ratios, contradictions of associations, and signatures of origin, is essential.

Research into natural iron silicides has aroused great interest for decades, and remains an important and exciting task for the future.