Thorianite

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	Thorium oxide, ThO2
IMA symbol	Tho1
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	234

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.

Based on color, specific gravity and composition three types of thorianite are distinguished:⁹

• α-thorianite
• β-thorianite
• γ-thorianite

Thorianite and uraninite form a complete solid solution series in synthetic and natural material.¹⁰ The division between the two species is at Th:U = 1:1 with uranium possibly making up to 46.50% and thorium ranging up to 87.88%.¹¹ Rare earths, chiefly cerium, substitute for thorium in amounts up to 8% by mass.¹⁰¹² Cerium is probably present as Ce⁴⁺. Complete series is known in synthetic material between CeO₂ - PrO₂ - ThO₂ - UO₂. Small amounts of Fe³⁺ and zirconium also may be isomorphous with thorium. Lead present is probably radiogenic.

• Aldanite – a variety of thorianite containing 14.9% to 29.0% UO₂ and 11.2% to 12.5% PbO.¹³
• Uranothorianite¹⁴
• Thorianite Cerian¹⁵¹⁶
• Thorianite La bearing¹⁵

Usually found in alluvial deposits, beach sands, heavy mineral placers, and pegmatites.

• Sri Lanka – In stream gravels, Galle district, Southern Province; Balangoda district; near Kodrugala, Sabaragamuwa Province; and from a pegmatite in Bambarabotuwa area.
• India – Reported from beach sands of Travancore (Kerala).¹⁷
• Madagascar – Found in alluvial deposits of Betroka and Andolobe.¹⁸ Also as very large crystals from Fort Dauphin; at Andranondambo and other localities.
• Russia – In black sands of a gold placer on Boshogoch River, Transbaikalia, Siberia; in the Kovdor Massif by Kovdor, Kola Peninsula; in the Yenisei Range, Siberia.
• United States – reported from Easton, Pennsylvania; black sands in Missouri River, near Helena, Montana; Scott River, Siskiyou County, California;¹⁹ black sands in Nixon Fork and Wiseman districts, Alaska.²⁰²¹
• Canada – Reported with uraninite in a pegmatite on Charlebois Lake, east of Lake Athabasca;²² Uranon variety reported from pegmatite and metesomatized zones in crystalline limestones from many locations in Quebec and Ontario.²³
• South Africa – Occurs with baddeleyite as an accessory in carbonatite at Phalaborwa, Eastern Transvaal.²⁴
• Democratic Republic of Congo - Kasaï region²⁵