Abstract
Eocene shales metamorphosed by a naturally ignited coal seam in the Powder River Basin, Wyoming record a continuum of mineralogic and textural changes from relatively unaltered shale to melt developed during pyrometamorphism. Samples collected along a section 2 m in length, corresponding to a temperature range of approximately 1300°C, were examined optically and by XRD, SEM, and STEM. The low temperature samples are comprised primarily of silt-sized quartz, K-feldspar, and minor amounts of other detrital minerals in a continuous matrix of illite/smectite (I/S). Delamination of phyllosilicates due to dehydroxylation occurs early in the sequence with curling of individual layers from rim to core. Within one-half meter of melted areas, phyllosilicates have undergone an essentially isochemical reconstitution with nucleation and growth of mullite crystals with maximum diameters of 50 nm, randomly distributed within a non-crystalline phase that replaces I/S. Large detrital grains remain for the most part unaffected except for the inversion of quartz to tridymite/cristobalite. Within 1 mm of the solid/melt interface, the mullitebearing clay mineral matrix is essentially homogeneous in composition with obscure grain boundaries, caused by apparent homogenization of poorly crystalline material. This material is similar in composition to parent clays and acts as a matrix to angular, remnant tridymite/cristobalite grains. Rounded, smaller silica grains have reaction rims with the non-crystalline matrix; K-feldspar is no longer present (apparently reacted with the matrix) and the matrix contains abundant pore space due to shrinkage upon dehydroxylation. As isolated pods of paralava (glass) or fractures are approached, Fe−Ti−Al oxides become abundant. Vesicular glass is separated from clinker by a well-defined interface and contains numerous phenocrysts. XRF analyses and reduced area rastering using EDS imply enrichment of the melt phase in Fe, Ca, Mg and Mn, apparently due to vapor transport from other layers lower in the sedimentary sequence.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Allen JA (1874) Metamorphism produced by the burning of lignite beds in Dakota and Montana Territorics. Boston Soc Nat Hist Proc 16:246–262
Baker G (1953) Naturally fused coal ash from Leigh Creek, South Australia. Trans R Soc Aust 76:1–20
Bastin ES (1905) Note on baked clay and natural slags in eastern Wyoming. J Geol 13:408–412
Bentor YK (1984) Combustion — metamorphic glasses. J Non-Cryst Solids 67:433–448
Bentor YK, Kastner M, Yellin Y (1981) Combustion metamorphism of bituminous sediments and the formation of melts of granitic and sedimentary composition. Geochim Cosmochim Acta 45:2229–2255
Brcarley AJ (1986) An electron optical study of muscovite breakdown in pelitic xenoliths during pyrometamorphism. Mineral Mag 50:385–397
Brearley AJ (1987) A natural example of the disequilibrium breakdown of biotite at high temperature: TEM observations and comparison with experimental kinetic data. Mineral Mag 51:93–106
Brearley AJ, Rubie DC (1990) Effects of H2O on the disequilibrium breakdown of muscovite+quartz. J Petrol 31:925–956
Brindley GW, Nakahira M (1959) The kaolinite-mullite reaction series: II, meta kaolin. J Am Ceram Soc 42[7]:314–318
Busch W, Schneider G, Mehnert KR (1974) Initial melting at grain boundaries: intermediate composition rocks. Neues Jahrb Mineral Monatsh 8:345–370
Cameron WE (1977) Composition and cell dimensions of mullite. Ceram Bull 56[1]:1003–1011
Comer JJ (1960) Electron microscope studies of mullite development in fired kaolinites. J Am Ceram Soc 43[7]:375–384
Cosca MA, Essene EJ (1985) Paralava chemistry and conditions of formation, Powder River Basin, Wyoming. EOS-Trans Am Geophys Union 66:396
Cosca MA, Essene EJ, Geissman JW, Simmons WB, Coates DA (1989) Pyrometamorphic rocks associated with naturally burned coal beds, Powder River Basin, Wyoming. Am Mineral 74:85–100
Deer WA, Howie RA, Zussman J (1966) An introduction to the rock-forming minerals. Longman, London
Furlong RB (1967) Electron diffraction and micrographic study of the high-temperature changes in illite and montmorillonite under continuous heating conditions. Proc Clays Clay Miner 15th Conf, Pergamon Press, New York, pp 87–101
Grapes RH (1986) Melting and thermal reconstitution of pelitic xenoliths, Wehr Volcano, East Eifel, West Germany. J Petrol 27:343–396
Guggenheim S, Chang Y, Koster van Groos AF (1987) Muscovite dehydroxylation: high-temperature studies. Am Mineral 72:537–550
Heller L (1962) The thermal transformation of pyrophyllite to mullite. Am Mineral 47:156–157
Herring JR (1980) Geochemistry of clinker, naturally burning coal, and mine fires. In: Symposium on the geology of Rocky Mountain coal 80-1: pp 41–44
Hooper RL (1982) Mineralogy of a coal burn near Kemmerer, Wyoming. MS Thesis, Washington State University, Pullman, Washington
Hower J, Eslinger EV, Hower ME, Perry EA (1976) Mechanism of burial metamorphism of argillaceous sediments, 1: mineralogical and chemical evidence. Geol Soc Am Bull 87:725–737
Jackson KC (1981) Clinker genesis and the comparison of naturally occurring clinker to compounds in the system FeO−Al2O3−SiO2. BS Thesis, Beloit College, Beloit, Wisconsin
Lee JH, Ahn JH, Peacor DR (1985) Textures in layered silicates: progressive changes through diagenesis and low-temperature metamorphism. J Sediment Petrol 55:532–540
Matthews A, Gross S (1980) Petrologic evolution of the “Mottled Zone” (Hatrurim) metamorphic complex of Israel. Isr J Earth Sci 29:93–106
Naslund HR (1983) The effect of oxygen fugacity on liquid immiscibility in iron-bearing silicate melts. Am J Sci 283:1034–1059
Norrish K, Hutton JT (1969) An accurate X-ray spectrographic method for the analysis of a wide range of geological samples. Geochim Cosmochim Acta 33:431–453
Raask E (1985) Mineral impurities in coal combustion: behavior, problems and remedial measures. Hemisphere Publishing Corporation, Washington, DC
Rogers GS (1918) Baked shale and slag formed by the burning of coal beds. USGS Prof Pap 108-A:pp 1–10
Schairer JF, Bowen NL (1947) The system leucite-anorthitesilica. Geol Soc Finland Bull 20:67–87
Smith DGW (1969) Pyrometamorphism of phyllites by a dolerite plug. J Petrol 10:20–55
van der Pluijm BA, Lee JH, Peacor DR (1988) Analytical electron microscopy and the problem of potassium diffusion. Clays Clay Miner 36:498–504
Vedder W, Wilkins RWT (1969) Dehydroxylation and rehydroxylation, oxidation and reduction of micas. Am Mineral 54:482–509
Weill DF, Kudo AH (1968) Initial melting in alkali feldspar-plagioclase-quartz systems. Geol Mag 105:325–337
Whitney JA, Hemley JJ, Simon FO (1985)_The concentration of iron in chloride solutions equilibrated with synthetic granitic compositions: the sulfur-free system. Econ Geol 80:444–460
Whitworth HF (1958) The occurrence of some fused sedimentary rocks at Ravensworth, NSW. J Proc R Soc NSW 91–92:204–208
Wirth R (1985) Dehydration and thermal alteration of white micas (phengite) in the contact aureole of the Traversella Intrusion. Neues Jahrb Mineral Abh 152[1]:101–112
Worden RH, Champness PE, Droop GTR (1987) Transmission electron microscopy of the pyrometamorphic breakdown of phengite and chlorite. Mineral Mag 51:107–121
Author information
Authors and Affiliations
Additional information
Contribution No. 490, the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan
Rights and permissions
About this article
Cite this article
Clark, B.H., Peacor, D.R. Pyrometamorphism and partial melting of shales during combustion metamorphism: mineralogical, textural, and chemical effects. Contr. Mineral. and Petrol. 112, 558–568 (1992). https://doi.org/10.1007/BF00310784
Received:
Issue Date:
DOI: https://doi.org/10.1007/BF00310784