Thorite
Encyclopedia
Thorite,SiO4, is a rare nesosilicate
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...

 of thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....

 that crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

lizes in the tetragonal system and is isomorphous with zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...

 and hafnon
Hafnon
Hafnon is a hafnium silicate mineral, chemical formula SiO4 or SiO4. In natural zircon ZrSiO4 part of the zirconium is replaced by the very similar hafnium and so natural zircon is never pure ZrSiO4...

. It is the most common mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

 of thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....

 and is nearly always strongly radioactive. It was named in 1829 to reflect its thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....

 content. Thorite was discovered in 1828 on the island of Løvø, Norway, by the vicar and mineralogist, Hans Morten Thrane Esmark
Morten Thrane Esmark
Hans Morten Thrane Esmark was a Norwegian priest and mineralogist.He was born in Kongsberg as a son of Jens Esmark. He became known for describing several new minerals, especially from the Langesundsfjorden district.-References:...

, who sent the first specimens of this black mineral to his father, Jens Esmark
Jens Esmark
Jens Esmark was a Danish-Norwegian professor of mineralogy who contributed to many of the initial discoveries and conceptual analyses of glaciers, specifically the concept that glaciers had covered larger areas in the past....

, who was a professor of mineralogy and geology.

Occurrence

Specimens of thorite generally come from igneous
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...

 pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....

s and volcanic extrusive rocks, hydrothermal veins and contact metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...

s. 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
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...

, monazite
Monazite
Monazite is a reddish-brown phosphate mineral containing rare earth metals. It occurs usually in small isolated crystals. There are actually at least four different kinds of monazite, depending on relative elemental composition of the mineral:...

, gadolinite
Gadolinite
Gadolinite, sometimes also known as Ytterbite, is a silicate mineral which consists principally of the silicates of cerium, lanthanum, neodymium, yttrium, beryllium, and iron with the formula 2FeBe2Si2O10...

, fergusonite
Fergusonite
Fergusonite is a mineral comprising a complex oxide of various rare earth elements. The chemical formula of fergusonite species is NbO4, where RE = rare-earth elements in solid solution with Y. Yttrium is usually dominant , but sometimes Ce or Nd may predominate in molar proportion...

, uraninite
Uraninite
Uraninite is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2, but also contains UO3 and oxides of lead, thorium, and rare earth elements...

, yttrialite
Yttrialite
Yttrialite or Yttrialite- is a rare yttrium thorium sorosilicate mineral with formula: 2Si2O7. It forms green to orange yellow masses with conchoidal fracture. It crystallizes in the monoclinic-prismatic crystal system. It has a Mohs hardness of 5 to 5.5 and a specific gravity of 4.58...

 and pyrochlore
Pyrochlore
Pyrochlore 2Nb2O6 is a solid solution between the niobium end member , and the tantalum end member .-Occurrence:...

.

Thorite is currently an important ore of uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

. A variety of thorite, often called "uranothorite", is particularly rich in uranium and has been a viable uranium ore at Bancroft in Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....

, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

. Other varieties of thorite include "orangite", an orange variety, and "calciothorite", an impure variety with trace amounts of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

.

Properties

Thorite is commonly metamict and hydrated, making it optically isotropic and amorphous. Owing to differences in composition, the specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...

 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 also brownish black, orange, yellowish-orange and dark green.

Because thorite is highly radioactive, specimens are often metamict
Metamictization
Metamictization is a natural process resulting in the gradual and ultimately complete destruction of a mineral's crystal structure, leaving the mineral amorphous...

. This is a condition found in radioactive minerals that results from the destructive effects of its own radiation on its crystal lattice. The effect can destroy a crystal lattice completely while leaving the outward appearance unchanged.
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