Virtual Atlas of Opaque and Ore Minerals
Cassiterite, zircon and thorite. Greisenized granite. Ririwai, Nigeria
Cassiterite crystals (light grey, right) show bireflectance along twin planes (top centre). Enclosed zircons (medium grey, centre and bottom right) show strong zoning with a lower reflectance zone about a higher reflectance core. Discrete euhedral zircons are also strongly zoned (bottom left). Thorite crystals (centre, centre bottom) have lower reflectance than the zircon cores and are characterized by faint red internal reflections due to very fine-grained haematite inclusions. They occur about the main cassiterite aggregate. Brass (yellow, high reflectance, centre) and polishing pits (black) are present, some of which carry angular grinding grit (top left).
Polished thin section, plane polarized light, x 160, air