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Thorianite

Thorianite
Thorianite-54888.jpg
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.


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