| 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 (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.
