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Evidence for superstructuring in ankerite

Überstrukturen in Ankerit

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Summary

Ankerite from the Radmer-Buchegg and Erzberg mines, Austria, were studied by high resolution electron microscope imaging and selected area diffraction. Electron diffraction data suggest the presence of a periodic antiphase structure. The unit cell of the periodic antiphase structure of ankerite consists probably of three units: CaMg(CO3)2 as a basal unit, CaFe(CO3)2 and CaCO3. The last CaCO3 unit is connected with excess of Ca in ankerite. The upper size limit of CaFe(CO3)2 domains is 20–40 nm according to dark-field image study.

Other rhombohedral dicarbonates (Zn-dolomites, Fe-poor ankerites) have probably also domain structure. The size of domains seems to be controlled by the value of misfit of domain lattice to the host lattice and by the value of octahedral distortion of units of the periodic antiphase structure.

Zusammenfassung

Ankerite der Lagerstätten Radmer-Buchegg und Erzberg, Österreich wurden mittels hochauflösender Elektronenmikroskopie und Elektronenbeugung untersucht. Die Beugungsbilder lassen auf eine periodische Antiphasen-Struktur schließen, die sich aus drei Einheiten aufbaut: CaMg(CO3)2 als Basiseinheit, CaFe(CO3)2 und CaCO3. CaCO3 ist an einen Ca-Überschuß in Ankeriten gebunden. Dunkelfeldaufnahmen entsprechend beträgt die Obergrenze der CaFe(CO3)2 Domänengröße 20–40 nm.

Andere rhomboedrische Doppelkarbonate (Zn-Dolomite, Fe-arme Ankerite) haben wahrscheinlich ebenfalls Domänenstruktur. Die Größe der Domänen scheint durch das “Zusammenpassen” des Domänengitters mit dem Gitter des “Wirtskristalls” sowie durch den Grad der oktaedrischen Verzerrung von Baueinheiten der Antiphasen-Struktur bestimmt zu werden.

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Author notes

  1. H. Kucha

    Present address: Institute of Geology and Mineral Deposits, Av. Mickiewicza 30, PL-30-059, Krakow, Poland

  2. A. Wieczorek

    Present address: Institute of Metallurgy, Av. Mickiewicza 30, PL-30-059, Krakow, Poland

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    1. H. Kucha
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    2. A. Wieczorek
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    Kucha, H., Wieczorek, A. Evidence for superstructuring in ankerite. TMPM Tschermaks Petr. Mitt. 32, 247–258 (1984). https://doi.org/10.1007/BF01081616

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