Thorianite | |
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General | |
Category | Oxide mineral |
Formula (repeating unit) | 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] |
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.
Based on color, specific gravity and composition three types of thorianite are distinguished:[9]
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 U possibly making up to 46.50% and Th ranging up to 87.88%.[11] Rare earths, chiefly Ce, substitute for Th in amounts up to 8% by weight.[10][12] 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.
Usually found in alluvial deposits, beach sands, heavy mineral placers, and pegmatites.