Cassiterite, zircon and thorite. Greisenized granite. Ririwai, Nigeria

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Under oil immersion the twinning of cassiterite is distinct (top centre), as are faint brown-orange internal reflections (centre right). Although the light-coloured internal reflections of zircon are clear, its zoning is less apparent than in air (left centre). It is very difficult to see any surface features of the thorite but its presence is indicated by its strong orange-red internal reflections (centre), due to fine-grained haematite within it. The zircon above the upper thorite (centre top) encloses haematite (reflectance higher than that of cassiterite). The polishing pit (upper left) has trapped an air bubble (dark circle) in the immersion oil. Yellow high reflectance areas arc brass (bottom centre).

Polished thin section, plane polarized light, x 160, oil

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