Thorianite is a rare thorium oxide mineral, ThO2.[4] It was originally described by Ananda Coomaraswamy in 1904 as uraninite,[5] but recognized as a new species by Wyndham R. Dunstan.[6] It was so named by Dunstan on account of its high percentage of thorium; it also contains the oxides of uranium, lanthanum, cerium, praseodymium and neodymium. Helium is present, and the mineral is slightly less radioactive than pitchblende,[7] but is harder to shield due to its high energy gamma rays. It is common in the alluvial gem-gravels of Sri Lanka, where it occurs mostly as water worn, small, heavy, black, cubic crystals. The largest crystals are usually near 1.5 cm. Larger crystals, up to 6 cm (2.4 in), have been reported from Madagascar.
Chemistry
Based on color, specific gravity and composition three types of thorianite are distinguished:[8]
- α-thorianite
- β-thorianite
- γ-thorianite
Thorianite and uraninite form a complete solid solution series in synthetic and natural material.[9] The division between the two species is at Th:U = 1:1 with U possibly making up to 46.50% and Th ranging up to 87.88%.[10] Rare earths, chiefly Ce, substitute for Th in amounts up to 8% by weight.[9][11] Ce is probably present as Ce4+. Complete series is known in synthetic material between CeO2 - PrO2 - ThO2 - UO2. Small amounts of Fe3+ and Zr also may be isomorphous with Th. Pb present is probably radiogenic.
Varieties
- Aldanite – a variety of thorianite containing 14.9% to 29.0% UO2 and 11.2% to 12.5% PbO.[12]
- Uranothorianite[13]
- Thorianite Cerian[14][15]
- Thorianite La bearing[14]
Occurrence
Usually found in alluvial deposits, beach sands, heavy mineral placers, and pegmatites.
- Sri Lanka – In stream gravels, Galle district, Southern Province; Balangoda district; near Kodrugala, Sabaragamuwa Province; and from a pegmatite in Bambarabotuwa area.
- India – Reported from beach sands of Travancore (Kerala).[16]
- Madagascar – Found in alluvial deposits of Betroka and Andolobe.[17] Also as very large crystals from Tôlanaro (Fort Dauphin); at Andranondambo and other localities.
- Russia – In black sands of a gold placer on Boshogoch River, Transbaikalia, Siberia; in the Kovdor Massif by Kovdor, Kola Peninsula; in the Yenisei Range, Siberia.
- United States – reported from Easton, Pennsylvania; black sands in Missouri River, near Helena, Montana; Scott River, Siskiyou County, California;[18] black sands in Nixon Fork and Wiseman districts, Alaska.[19][20]
- Canada – Reported with uraninite in a pegmatite on Charlebois Lake, east of Lake Athabasca;[21] Uranon variety reported from pegmatite and metesomatized zones in crystalline limestones from many locations in Quebec and Ontario.[22]
- South Africa – Occurs with baddeleyite as an accessory in carbonatite at Phalaborwa, Eastern Transvaal.[23]
- Democratic Republic of Congo - Kasaï region
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