Summary
Anandite has an approximate formula of Ba(Fe3+, Fe2+)3[Si2(Fe3+, Fe2+, Si)2O10−x(OH)x] (S, Cl) (OH), withx=0−1, and belongs to the 2 O brittle mica group. It is orthorhombic; space groupPnmn;a=5.468(9) Å,b=9.489(18)Å,c=19.963(11) Å;Z=4.
The structure was determined from 3dim. Weissenberg-data, starting with an approximate structure in the pseudo space groupCcmm. Least squares refinement resulted inR=0.061 for 409 photometric intensities, andR=0.131 for all 853 observedhkl-reflexions.
The iron of the tetrahedral layer is concentrated in one of the two crystallographically different kinds of tetrahedra. The basal oxygen rings of the tetrahedral layer form approximate hexagons and have not the ditrigonal configuration of the common micas. This peculiarity is considered to be a consequence of the size and charge of the barium ion. The role of OH in the common micas is played partly by S2− and Cl− in anandite.
Zusammenfassung
Anandit hat die ungefähre Formel Ba(Fe3+, Fe2+)3[Si2(Fe3+, Fe2+, Si)2O10−x(OH)x] (S, Cl) (OH) mitx=0–1 und gehört zur 2O Sprödglimmergruppe. Er ist rhombisch; RaumgruppePnmn; a=5,468(9) Å,b=9,489(18) Å,c=19,963(11) Å;Z=4.
Die Struktur wurde aus Weissenberg-Daten bestimmt, wobei mit einer approximativen Struktur in der PseudoraumpruppeCcmm begonnen wurde. Die Verfeinerung nach der Methode der kleinsten Quadrate führte für 409 photometrierte Reflexe aufR=0,061 und für alle 853 beobachtetenhkl-Reflexe aufR=0,131.
Der Eisengehalt der Tetraederschicht ist in einer der beiden kristallographisch verschiedenen Tetraederarten konzentriert. Die basalen Sauerstoffringe der Tetraederschicht bilden annäherungsweise Sechsecke und haben nicht die ditrigonale Konfiguration der gewöhnlichen Glimmer. In Anandit spielen S2− und Cl− teilweise die Rolle der Hydroxylgruppen in den gewöhnlichen Glimmern.
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Giuseppetti, G., Tadini, C. The crystal structure of 2O brittle mica: Anandite. TMPM Tschermaks Petr. Mitt. 18, 169–184 (1972). https://doi.org/10.1007/BF01134206
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DOI: https://doi.org/10.1007/BF01134206