| 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.
- ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
- ^ "Handbook of Mineralogy" (PDF). Archived from the original (PDF) on 2016-03-03. Retrieved 2011-12-10.
- ^ Mindat.org
- ^ Webmineral data
- ^ Frondel, C. (1958). Systematic Mineralogy of Uranium and Thorium. United States Government Printing Office.
- ^ Coomaraswamy, A.K. (1904). "Uraninite". Spolia Zeylanica. Pt. 6 (2): 57.
- ^ Dunstan, Wyndham R. (1904-03-31). "The occurrence of Thorium in Ceylon". Nature. 69 (1796): 510–511. Bibcode:1904Natur..69..510D. doi:10.1038/069510d0. S2CID 4032255.
- ^ One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Thorianite". Encyclopædia Britannica. Vol. 26 (11th ed.). Cambridge University Press. p. 878.
