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.
