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Chemistry of dark zinnwaldite from bom futuro tin mine, Rondônia, Brazil

Published online by Cambridge University Press:  05 July 2018

G. R. Lowell  and
M. Ahl
G. R. Lowell*
Affiliation:
Department of Geosciences, Southeast Missouri State University, Cape Girardeau, Missouri, USA
M. Ahl
Affiliation:
Department of Geology and Geochemistry, Stockholm University, S-106 91 Stockholm, Sweden
*
*E-mail: glowell@semovm.semo.edu
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Abstract

Physical and chemical properties of lustrous black mica from greisen veins at the Bom Futuro tin mine indicate that it is a relatively Fe-F-rich, Li-poor zinnwaldite with a composition near the midpoint of the polylithionite-siderophyllite join. On the basis of 22 oxygens, the mica contains 1.28–1.73 atoms of Li with an average octahedral occupancy of 5.61 and a filled interlayer site. Compositional variation reflects the net effects of alteration, coupled tetrahedral-octahedral substitutions, and petrogenetic factors related to vein fluid chemistry. Geological constraints and compositional data suggest the Bom Futuro zinnwaldite crystallized at T ≈ 500°, P ≈ 1–500 bar, and fO2 ≈ 10−23. High Mn-Zn concentrations in the Bom Futuro mica appear to be a feature of Fe-Li micas from evolved anorogenic granitic systems not shared by orogenic counterparts.

Keywords

micazinnwalditegreisenSn mineralizationBom Futuro mineBrazil
Type
Research Article
Information
Mineralogical Magazine , Volume 64 , Issue 4 , August 2000 , pp. 699 - 709
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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