Chemistry of dark zinnwaldite from Bom Futuro tin mine, Rondonia, Brazil
Chemistry of dark zinnwaldite from Bom Futuro tin mine, Rondonia, Brazil (in Carbonatite thematic set, A. P. Jones)
Mineralogical Magazine (August 2000) 64 (4(425)): 699-709
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 nearly equal 500 degrees , P nearly equal 1-500 bar, and f (sub O2) nearly equal 10 (super -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.