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The structural role of Fe 3+ in biotites from the Euganean Hills, Italy

Published online by Cambridge University Press:  05 July 2018

Renzo De Pieri*
Affiliation:
Istituto di Mineralogia e Petrologia dell' Università, Corso Garibaldi 37, 35100 Padova, Italy

Summary

Plots of partition coefficient (mineral/matrix) against ionic radius of the elements in seventeen biotites from trachytes and rhyolites of the Euganean Hills do not show differences in the type of diagram between the biotites with tetrahedral sites completely filled by Si and Al, and those in which filling is only possible with the intervention of Fe3+ (or Ti4+) since these sites are incompletely filled by Si and Al. This behaviour seems to exclude, for the latter, the presence of Fe3+ (or Ti4+ in the tetrahedral site. Similar indications are given by structural analysis. The possibility of existence of [(OH)4]4− groups or, more probably, vacancies in these sites is therefore suggested. The consequences that these results would have on the possibility of correct calculation of the crystallo-chemical formula are discussed.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1980

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