Cassiterite, thorite and zircon. Greisenized granite. Ririwai, Nigeria
250µm
Cassiterite crystals (light grey) show very faint bireflectance variations due to twinning (top centre). Small zircon crystals (medium grey) are enclosed within, and surround, cassiterite. Discrete zircons (centre, centre top left) show zoning, with thin lower reflectance zones about a higher reflectance core. Thorite (bottom centre) has low reflectance with haematite-rich areas showing slightly higher reflectance. Quartz is the main silicate with some feldspar (bottom right) showing red internal reflections due to very finely disseminated haematite. Much of the mica is poorly polished and shows parallel striations due to microprobe damage. Black areas are polishing pits, often infilled with grinding grit (top right and left). The angular shape, low reflectance that is similar to rutile and restricted size distribution of the grit are all characteristic. Brass (yellow, high reflectance, centre) is present in a triangular-shaped pit.
Polished thin section, plane polarized light, x 80, air
