Thorianite
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Thorianite is a rare thorium oxide mineral, ThO2.[5] It was originally described by Ananda Coomaraswamy in 1904 as uraninite,[6] but recognized as a new species by Wyndham R. Dunstan.[7] 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,[8] 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.
| Thorianite | |
|---|---|
Group of interpenetrating twinned thorianite crystals from Ambatofotsy, Madagascar (size: 1.6 x 1.4 x 1.3 cm) | |
| General | |
| Category | Oxide mineral |
| Formula | Thorium oxide, ThO2 |
| IMA symbol | Tho[1] |
| Strunz classification | 4.DL.05 |
| Crystal system | Isometric |
| Crystal class | Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m) |
| Space group | Fm3m |
| Unit cell | a = 5.595 Å; Z = 4 |
| Identification | |
| Color | Dark gray, brown-black |
| Crystal habit | Cubic crystals, usually rounded to some degree in detrital deposits |
| Twinning | Penetration twins on {111} common |
| Cleavage | Poor/Indistinct |
| Fracture | Irregular to uneven, sub-conchoidal |
| Mohs scale hardness | 6.5 – 7 |
| Luster | Resinous, sub-metallic |
| Streak | Grey, grey green to black |
| Diaphaneity | Opaque, translucent on thin edges |
| Specific gravity | 9.7 |
| Optical properties | Isotropic |
| Refractive index | n = 2.20 – 2.35 |
| Other characteristics |  Radioactive |
| References | [2][3][4] |
Chemistry
Based on color, specific gravity and composition three types of thorianite are distinguished:[9]
- α-thorianite
- β-thorianite
- γ-thorianite
Thorianite and uraninite form a complete solid solution series in synthetic and natural material.[10] The division between the two species is at Th:U = 1:1 with uranium possibly making up to 46.50% and thorium ranging up to 87.88%.[11] Rare earths, chiefly cerium, substitute for thorium in amounts up to 8% by mass.[10][12] Cerium is probably present as Ce4+. Complete series is known in synthetic material between CeO2 - PrO2 - ThO2 - UO2. Small amounts of Fe3+ and zirconium also may be isomorphous with thorium. Lead present is probably radiogenic.
Varieties
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).[17]
- Madagascar – Found in alluvial deposits of Betroka and Andolobe.[18] Also as very large crystals from 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;[19] black sands in Nixon Fork and Wiseman districts, Alaska.[20][21]
- Canada – Reported with uraninite in a pegmatite on Charlebois Lake, east of Lake Athabasca;[22] Uranon variety reported from pegmatite and metesomatized zones in crystalline limestones from many locations in Quebec and Ontario.[23]
- South Africa – Occurs with baddeleyite as an accessory in carbonatite at Phalaborwa, Eastern Transvaal.[24]
- Democratic Republic of Congo - Kasaï region[25]
