Thorianite

Thorianite
Group of interpenetrating twinned thorite crystals from Ambatofotsy, Madagascar (size: 1.6 x 1.4 x 1.3 cm)
General
Category	Oxide mineral
Formula (repeating unit)	Thorium oxide, ThO2
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
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 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.

Based on color, specific gravity and composition three types of thorianite are distinguished:

Thorianite and uraninite form a complete solid solution series in synthetic and natural material. 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 87.9%.Rare earths, chiefly Ce, substitute for Th in amounts up to 8% by weight. 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.