Thorite
Thorite crystal from the Kemp uranium mine in Ontario (size: 2.2 x 2.2 x 1.6 cm)
General
Category	Silicate mineral
Formula (repeating unit)	(Th,U)SiO4
IMA symbol	Thr[1]
Strunz classification	9.AD.30
Crystal system	Tetragonal
Crystal class	Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m)
Space group	I41/amd
Unit cell	a = 7.13, c = 6.32 [Å]; Z = 4
Identification
Color	Yellow-orange, brownish yellow, brownish black, black, green
Crystal habit	In square prisms, or pseudo-octahedral crystals; also massive
Cleavage	Distinct on {110}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	4.5 – 5
Luster	Vitreous to resinous
Streak	Light orange to light brown sometimes even an alien magenta
Diaphaneity	Nearly opaque, transparent in thin fragments
Specific gravity	6.63 – 7.20
Optical properties	Uniaxial (−)
Refractive index	nω = 1.790 – 1.840 nε = 1.780 – 1.820
Birefringence	δ = 0.010 – 0.020
Alters to	Commonly metamict
Other characteristics	Radioactive
References	[2][3][4]

Thorite, (Th,U)SiO₄, is a rare nesosilicate of thorium that crystallizes in the tetragonal system and is isomorphous with zircon and hafnon. It is the most common mineral of thorium and is nearly always strongly radioactive. Thorite was discovered in 1828 on the island of Løvøya, Norway, by the vicar and mineralogist, Hans Morten Thrane Esmark. First specimens of Thorite were sent to his father, Jens Esmark, who was a professor of mineralogy and geology.^[5][6][7] It was named in 1829 to reflect its thorium content.

Specimens of thorite generally come from igneous pegmatites and volcanic extrusive rocks, hydrothermal veins and contact metamorphic rocks. It is also known to occur as small grains in detrital sands. Crystals are rare, but when found can produce nicely shaped short prismatic crystals with pyramidal terminations. It is commonly associated with zircon, monazite, gadolinite, fergusonite, uraninite, yttrialite and pyrochlore.^[4]

Thorite is currently an important ore of uranium. A variety of thorite, often called "uranothorite", is particularly rich in uranium and has been a viable uranium ore at Bancroft in Ontario, Canada. Other varieties of thorite include "orangite", an orange variety, and "calciothorite", an impure variety with trace amounts of calcium.

Due to the radioactive elements contained, Thorite is commonly metamict. With the destructive effects of radioactivity on the crystal lattice, hydrated specimens are often amorphous and optically isotropic. Owing to differences in composition, the specific gravity varies from 4.4 to 6.6 g/cm³. Hardness is 4.5 and the luster is vitreous or resinous. The color is normally black, but can range from brownish black to orange, yellowish-orange and dark green.

Wikimedia Commons has media related to Thorite.

Wikisource has the text of the 1911 Encyclopædia Britannica article "Thorite".