Résumé
On a étudié statistiquement les variations de (OH+F) dans 392 micas (muscovites, biotites, micas lithiques). Dans les muscovites et les micas lithiques alumineux l'écart du nombre de (OH+F) à la valeur théorique apparait dû à des erreurs de dosage de H2O+. Dans les micas lithiques ferreux on observe une moyenne de 4,62 (OH+F)/maille. Cet excès en (OH+F), exprimé en oxonium H3O+, correspond au déficit en cations XII. Dans les micas ferromagnésiens (biotites, phlogopites) on observe un déficit en (OH+F) (x = 3,59). On rend compte de ce déficit en supposant que Fe3+ présent dans la structure est dû à une oxydation in situ par déprotonation.
Les conséquences quant aux erreurs introduites sur le nombre de cations lors du calcul de la formule structurale sont discutées.
Summary
A statistical study of the variations of (OH+F) content of 392 micas (muscovites, biotites, lithium micas) has been undertaken. In muscovites and lithium-aluminium micas, the fluctuation of (OH+F) content can be explain by analytical errors in determination of H2O+ and F. In ferrous lithium micas the mean content of (OH+F) is 4.62 at. per unit cell. Converted to oxonium H3O+, this excess is equal to the K+ deficiency. Biotites and phlogopites are hydroxyl deficient (x = 3,59). This can be explained if it is assumed that all Fe3+ present is due to auto-oxydation of Fe2+ with loss of H+ to maintain charge neutrality.
A discussion of the cationic variations introduced in the calculation of the structural formulae is given.
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Levillain, C. Etude statistique des variations de la teneur en OH et F dans les micas. TMPM Tschermaks Petr. Mitt. 27, 209–223 (1980). https://doi.org/10.1007/BF01082370
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DOI: https://doi.org/10.1007/BF01082370