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Thorite crystal from the Kemp uranium mine in Ontario (size: 2.2 x 2.2 x 1.6 cm)
Category Silicate mineral
(repeating unit)
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
ColorYellow-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 hardness4.5 – 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 propertiesUniaxial (-)
Refractive index nω = 1.790 – 1.840 nε = 1.780 – 1.820
Birefringence δ = 0.010 – 0.020
Alters toCommonly metamict
Other characteristicsRadioactive
References [1] [2] [3]

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. It was named in 1829 to reflect its thorium content. Thorite was discovered in 1828 on the island of Løvøya, Norway, by the vicar and mineralogist, Hans Morten Thrane Esmark, who sent the first specimens of this black mineral to his father, Jens Esmark, who was a professor of mineralogy and geology. [4] [5] [6]

Silicate minerals Rock-forming minerals with predominantly silicate anions

Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of the Earth's crust.

Thorium Chemical element with atomic number 90

Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately hard, malleable, and has a high melting point. Thorium is an electropositive actinide whose chemistry is dominated by the +4 oxidation state; it is quite reactive and can ignite in air when finely divided.

Crystal solid material whose constituent atoms, molecules, or ions are arranged in an ordered pattern extending in all three spatial dimensions

A crystal or crystalline solid is a solid material whose constituents are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are usually identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations. The scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification.



Thorite in Prague national museum Thorit 1.jpg
Thorite in Prague national museum
Small crystals of green thorite under magnification Thorites - CAMR 10-crop.jpg
Small crystals of green thorite under magnification

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. [3]

Igneous rock Rock formed through the cooling and solidification of magma or lava

Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, platforms, orogens, basins, large igneous provinces, extended crust and oceanic crust.

Pegmatite Very coarse grained plutonic rock

A pegmatite is an igneous rock, formed underground, with interlocking crystals usually larger than 2.5 cm in size (1 in). Most pegmatites are found in sheets of rock near large masses of igneous rocks called batholiths.

Metamorphic rock Rock which was subjected to heat and pressure causing profound physical or chemical change

Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat and pressure, causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock.

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.

Uranium Chemical element with atomic number 92

Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weakly radioactive because all isotopes of uranium are unstable; the half-lives of its naturally occurring isotopes range between 159,200 years and 4.5 billion years. The most common isotopes in natural uranium are uranium-238 and uranium-235. Uranium has the highest atomic weight of the primordially occurring elements. Its density is about 70% higher than that of lead, and slightly lower than that of gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite.

Ontario Province of Canada

Ontario is one of the 13 provinces and territories of Canada. Located in Central Canada, it is Canada's most populous province accounting for 38.3 percent of the country's population, and is the second-largest province in total area. Ontario is fourth-largest jurisdiction in total area when the territories of the Northwest Territories and Nunavut are included. It is home to the nation's capital city, Ottawa, and the nation's most populous city, Toronto, which is also Ontario's provincial capital.

Calcium Chemical element with atomic number 20

Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to its heavier homologues strontium and barium. It is the fifth most abundant element in Earth's crust and the third most abundant metal, after iron and aluminium. The most common calcium compound on Earth is calcium carbonate, found in limestone and the fossilised remnants of early sea life; gypsum, anhydrite, fluorite, and apatite are also sources of calcium. The name derives from Latin calx "lime", which was obtained from heating limestone.


Thorite is commonly metamict and hydrated, making it optically isotropic and amorphous. 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 also brownish black, orange, yellowish-orange and dark green.

Specific gravity Relative density compared to water

Specific gravity, also called relative density, is the ratio of the density of a substance to the density of a reference substance; equivalently, it is the ratio of the mass of a substance to the mass of a reference substance for the same given volume. 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 for liquids is nearly always water at its densest ; for gases it is air at room temperature. Nonetheless, the temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm (101.325 kPa).

Because thorite is highly radioactive, specimens are often metamict . 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.

Related Research Articles

Uraninite oxide mineral

Uraninite, formerly pitchblende, is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2, but due to oxidation the mineral typically contains variable proportions of U3O8. Additionally, due to radioactive decay, the ore also contains oxides of lead and trace amounts of helium. It may also contain thorium and rare earth elements.

Zircon Zirconium silicate, a mineral belonging to the group of nesosilicates

Zircon ( or ) 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 (Zr1–y, REEy)(SiO4)1–x(OH)4x–y. Zircon forms in silicate melts with large proportions of high field strength incompatible elements. For example, hafnium is almost always present in quantities ranging from 1 to 4%. The crystal structure of zircon is tetragonal crystal system. The natural colour of zircon varies between colourless, yellow-golden, red, brown, blue and green. Colourless specimens that show gem quality are a popular substitute for diamond and are also known as "Matura diamond".

Pyrochlore mineral group, pyrochlore supergroup

Pyrochlore (Na,Ca)2Nb2O6(OH,F) is a mineral group of the niobium end member of the pyrochlore supergroup. The general formula, A2B2O7 (A and B are metals), represent a family of phases isostructural to the mineral pyrochlore. Pyrochlores are important class of materials from the point of view of diverse technological applications like in luminescence, ionic conductivity, nuclear waste immobilization, high temperature thermal barrier coatings, automobile exhaust gas control, catalysts, solid oxide fuel cell, ionic/electric conductors etc.

Monazite phosphate mineral series

Monazite is a reddish-brown phosphate mineral containing rare-earth metals. It occurs usually in small isolated crystals. It has a hardness of 5.0 to 5.5 on the Mohs scale of mineral hardness and is relatively dense, about 4.6 to 5.7 g/cm3. There are at least four different kinds of monazite, depending on relative elemental composition of the mineral:

Metamictisation is a natural process resulting in the gradual and ultimately complete destruction of a mineral's crystal structure, leaving the mineral amorphous. The affected material is therefore described as metamict.

Coffinite nesosilicate mineral

Coffinite is a uranium-bearing silicate mineral with formula: U(SiO4)1−x(OH)4x.

Samarskite-(Y) oxide mineral

Samarskite is a radioactive rare earth mineral series which includes samarskite-(Y) with formula: (YFe3+Fe2+U,Th,Ca)2(Nb,Ta)2O8 and samarskite-(Yb) with formula (YbFe3+)2(Nb,Ta)2O8. The formula for samarskite-(Y) is also given as: (Y,Fe3+,U)(Nb,Ta)O4.

Xenotime phosphate mineral

Xenotime is a rare-earth phosphate mineral, the major component of which is yttrium orthophosphate (YPO4). It forms a solid solution series with chernovite-(Y) (YAsO4) and therefore may contain trace impurities of arsenic, as well as silicon dioxide and calcium. The rare-earth elements dysprosium, erbium, terbium and ytterbium, as well as metal elements such as thorium and uranium (all replacing yttrium) are the expressive secondary components of xenotime. Due to uranium and thorium impurities, some xenotime specimens may be weakly to strongly radioactive. Lithiophyllite, monazite and purpurite are sometimes grouped with xenotime in the informal "anhydrous phosphates" group. Xenotime is used chiefly as a source of yttrium and heavy lanthanide metals (dysprosium, ytterbium, erbium and gadolinium). Occasionally, gemstones are also cut from the finer xenotime crystals.

Allanite epidote supergroup, sorosilicate series

Allanite (also called orthite) is a sorosilicate group of minerals within the broader epidote group that contain a significant amount of rare-earth elements. The mineral occurs mainly in metamorphosed clay-rich sediments and felsic igneous rocks. It has the general formula A2M3Si3O12[OH], where the A sites can contain large cations such as Ca2+, Sr2+, and rare-earth elements, and the M sites admit Al3+, Fe3+, Mn3+, Fe2+, or Mg2+ among others. However, a large amount of additional elements, including Th, U, Zr, P, Ba, Cr and others may be present in the mineral. The International Mineralogical Association lists four minerals in the allanite group, each recognized as a unique mineral: allanite-(Ce), allanite-(La), allanite-(Nd), and allanite-(Y), depending on the dominant rare earth present: cerium, lanthanum, neodymium or yttrium.

Thorianite oxide mineral

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.

Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised from about 1 million years to over 4.5 billion years ago with routine precisions in the 0.1–1 percent range.

Jens Esmark Danish-Norwegian professor of mineralogy

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.

Betafite mineral group, pyrochlore supergroup

Betafite is a mineral group in the pyrochlore supergroup, with the chemical formula (Ca,U)2(Ti,Nb,Ta)2O6(OH). Betafite typically occurs as a primary mineral in granite pegmatites, rarely in carbonatites. Defined by the B-site atom Ti, Atencio et al.(2010) combined and considered the ideas portrayed in (Hatert and Burke)(2008) and a modernization of (Hogarth)(1977) system for nomenclature of pyrochlore and betafite in order to further rationalize the naming process of this grouping of minerals. Therefore, Atencio et al. (2010), states that only two of the mineral species that were formerly recognized under the previous nomenclature system of betafite in Hogarth (1977) are now recognized. They are oxyuranobetafite and oxycalciobetafite. Now the term betafite is a synonym or varietal group name under the pyrochlore super group (Christy and Atencio 2013).

Huttonite nesosilicate mineral

Huttonite is a thorium nesosilicate mineral with the chemical formula ThSiO4 and which crystallizes in the monoclinic system. It is dimorphous with tetragonal thorite, and isostructual with monazite. An uncommon mineral, huttonite forms transparent or translucent cream–colored crystals. It was first identified in samples of beach sands from the West Coast region of New Zealand by the mineralogist Colin Osborne Hutton (1910–1971). Owing to its rarity, huttonite is not an industrially useful mineral.

Hans Morten Thrane Esmark was a Norwegian priest and mineralogist. He is most noted for first locating the mineral thorite.

Løvøya, Telemark island in Norway

Løvøya is an island in Ormefjorden in Porsgrunn municipality, Norway.

Brockite is a rare earth phosphate mineral with formula: (Ca,Th,Ce)PO4·H2O. It crystallizes in the hexagonal system. It is typically granular to massive with only rare occurrence of stubby crystals. It is radioactive due to the thorium content.

Titanium in zircon geothermometry

Titanium in zircon geothermometry is a form of a geothermometry technique by which the crystallization temperature of a zircon crystal can be estimated by the amount of titanium atoms which can only be found in the crystal lattice. In zircon crystals, titanium is commonly incorporated, replacing similarly charged zirconium and silicon atoms. This process is relatively unaffected by pressure and highly temperature dependent, with the amount of titanium incorporated rising exponentially with temperature, making this an accurate geothermometry method. This measurement of titanium in zircons can be used to estimate the cooling temperatures of the crystal and infer conditions during which it crystallized. Compositional changes in the crystals growth rings can be used to estimate the thermodynamic history of the entire crystal. This method is useful as it can be combined with radiometric dating techniques that are commonly used with zircon crystals, to correlate quantitative temperature measurements with specific absolute ages. This technique can be used to estimate early Earth conditions, determine metamorphic facies, or to determine the source of detrital zircons, among other uses.

Grayite, ThPO4 • (H2O), is a thorium phosphate mineral of the Rabdophane group first discovered in 1957 by S.H.U. Bowie in Rhodesia. It is of moderate hardness occurring occasionally in aggregates of hexagonal crystals occasionally but more commonly in microgranular/cryptocrystalline masses. Due to its thorium content, grayite displays some radioactivity although it is only moderate and the mineral displays powder XRD peaks without any metamict-like effects. The color of grayite is most commonly observed as a light to dark reddish brown but has also been observed as lighter yellows with grayish tints. It has a low to moderate hardness with a Mohs hardness of 3-4 and has a specific gravity of 3.7-4.3. It has been found in both intrusive igneous and sedimentary environments.


  1. Webmineral data
  3. 1 2 Handbook of Mineralogy
  4. Berzelius, M. (1829). "Thorite, a new mineral, and thorina, a new earth" (PDF). Philosophical Magazine. Series 2. 6 (35): 392–393. doi:10.1080/14786442908675174.
  5. Berzelius, J. J. (1829). "Untersuchung eines neuen Minerals und einer darin enthaltenen zuvor unbekannten Erde". Annalen der Physik und Chemie. 92 (7): 385–415. Bibcode:1829AnP....92..385B. doi:10.1002/andp.18290920702.
  6. Marshall, J.L.; Marshall, V.R. (2001). "Rediscovery of the Elements- Thorium-Løvøya, Langesundsfjord, Norway" (PDF). The Hexagon. 93: 70–73. Archived from the original (PDF) on 2005-04-08.