Oxide placer deposits
Magnetite, ilmenite and haematite. New Zealand
A river placer containing euhedral magnetite (brown) that has a slightly deeper colour than an irregular grain of ilmenite (brown, top left). The central magnetite crystals have oxidized to haematite. Blue-white haematite forms a rim around unaltered magnetite (centre right) but also forms martite (white, centre left) with relict magnetite (brown). The crystallographic control of the haematite oxidation along (111) planes of the original magnetite is clearly seen.
Grain mount, plane polarized light, x 180, oil
Columbite, cassiterite, ilmenite and zircon. Jos Plateau, Nigeria
A river placer that comprises a mixture of oxides and silicates. Cassiterite (light grey, centre) has small columbite (light brown, higher reflectance, centre) inclusions. Ilmenite (pink-brown, bottom left) has altered along its edges to porous haematite (grey), but the unaltered ilmenite core has higher reflectance and a stronger surface colour than columbite (brown-grey, top right, top left). Zircon (grey, bottom left) has fine-grained haematite inclusions (higher reflectance) which are responsible for the incipient red internal reflections.
Grain mount, plane polarized light. x 180, oil
Cassiterite, ilmenite and columbite. Jos Plateau, Nigeria
This is a concentrate from a river placer comprising a mixture of oxides. Cassiterite (grey) is twinned (bottom right) and shows bireflectance, faint reflection pleochroism and faint internal reflections (red, bottom left). It is intergrown with columbite (light brown-grey, bottom right), which has a slightly lower reflectance than ilmenite (pink-brown, left centre). Ilmenite has altered along its edge to a mixture of poorly crystalline haematite and TiO2 (blue-grey, left centre).
Grain mount, plane polarized light, x 180, oil
Zircon and haematite. Jos Plateau, Nigeria
This is a concentrate from a river placer. A euhedral zircon (centre) shows zoning and very thin haematite (grey-white) veinlets cross-cutting the growth zones. The other zircons have porous cores with a faint red surface colour due to submicroscopic haematite inclusions (left) within well polished inclusion-free outer zones. Pare higher reflectance spots (bottom centre) within zircon are discrete haematite crystals.
Grain mount, plane polarized light, x80, air
Rutile and zircon. Richards Bay, Republic of South Africa
This is a concentrate from a beach placer. Zircon (grey) shows complex zoning seen as slight reflectance differences (centre bottom), and faint internal reflections (centre top). Rutile grains (light grey) are euhedral (bottom left) to rounded (top right) and some show poor polishing due to the strong (110) cleavage of rutile (top left, bottom right).
Grain mount, plane polarized light, x80, air
Rutile. Richards Bay, Republic of South Africa
This is a concentrate from a beach placer. Rutile grains show strong internal reflections (red, centre top) in some grains but few reflections in others. Both simple twins (bottom left) and polysynthetic twinning (centre top) are clearly visible due to the strong anisotropy of rutile.
Grain mount, crossed polars, x 180, oil