Thorianite
Thorianite is a rare thorium oxide mineral, ThO2. It was originally described by Ananda Coomaraswamy in 1904 as uraninite, but recognized as a new species by Wyndham R. Dunstan. 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, 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 (repeating unit) | Thorium oxide, ThO2 |
| IMA symbol | Tho |
| 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 | |
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