thorianite

{{Infobox mineral |boxbgcolor=#6d625d| name = Thorianite | boxtextcolor = #fff | category = Oxide mineral | image = Thorianite-54888.jpg | caption = Group of interpenetrating twinned thorianite crystals from Ambatofotsy, Madagascar (size: 1.6 x 1.4 x 1.3 cm) | formula = Thorium oxide, ThO2 | IMAsymbol = Tho | molweight = | strunz = 4.DL.05 | system = Isometric | class = Hexoctahedral (mm) H-M symbol: (4/m 2/m) | symmetry = Fm3m | unit cell = a = 5.595 Å; Z = 4 | color = Dark gray, brown-black | 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 = 6.5 – 7 | luster = Resinous, sub-metallic | refractive = n = 2.20 – 2.35 | opticalprop = Isotropic | birefringence = | pleochroism = | streak = Grey, grey green to black | gravity = 9.7 | density = | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Opaque, translucent on thin edges | other = 25px Radioactive | references = }}

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 , have been reported from Madagascar.