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Mineralogical Magazine; August 2000; v. 64; no. 4; p. 699-709
© 2000 Mineralogical Society of Great Britain and Ireland
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Chemistry of dark zinnwaldite from Bom Futuro tin mine, Rondônia, Brazil

G. R. Lowell1,* and M. Ahl2

1 Department of Geosciences, Southeast Missouri State University, Cape Girardeau, Missouri, USA
2 Department of Geology and Geochemistry, Stockholm University, S-106 91 Stockholm, Sweden

* E-mail: glowell{at}semovm.semo.edu

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 {approx} 500°, P {approx} 1–500 bar, and fO2 {approx} 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: mica, zinnwaldite, greisen, Sn mineralization, Bom Futuro mine, Brazil




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G. Tischendorf, H.-J. Forster, and B. Gottesmann
Minor- and trace-element composition of trioctahedral micas: a review
Mineralogical Magazine, April 1, 2001; 65(2): 249 - 276.
[Abstract] [Full Text] [PDF]


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