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Chemical formulaThorium Oxide, ThO2
ColorDark gray, brown-black
Crystal habitRounded grains
Crystal systemIsometric
TwinningPenetration twins on {111} very common
Mohs Scale hardness6½ - 7
StreakGrey, Grey green to black
Density9.7 g/cm3
DiaphaneityTranslucent on thin edges

Thorianite is a rare mineral,[1] originally discovered by Ananda Coomaraswamy in 1904 as uraninite,[2] but recognized as a new species by W. R. Dunston.[3] It was so named on account of its high percentage of thorium (about 70% ThO2); it also contains the oxides of uranium, lanthanum, cerium and didymium. Helium is present, and the mineral is slightly less radioactive than pitchblende but harder to shield due its high energy gamma rays. It is relatively more common in the alluvial gem-gravels of Sri Lanka, where it occurs mostly as water worn, small, heavy, black, cubic crystals. The largest crystals (sizes usually up to around 1.5 cm; very rare are sizes greater than 2.5 cm; largest is 6 cm and 2.2 kilos) came from Madagascar.



Based on color, specific gravity and composition three types of thorianite are distinguished:[4]

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

Thorianite and uraninite form a complete solid solution series in synthetic and natural material.[5] The division between the two species is at Th:U = 1:1 with U up to 46.50% and Th 45.3% to 87.9%.[6] Rare earths, chiefly Ce, substitute for Th in amounts up to 8% by weight. [7][8] 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.


  • Aldanite - a variety of thorianite containing 14.9% to 29.0% UO2 and 11.2% to 12.5% PbO.[9]
  • Uranothorianite[10]
  • Thorianite Cerian[11][12]
  • Thorianite La bearing[11]


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

  • Sri Lanka - In stream gravels, Galle district, Southern Province; Balangonda district; near Kodrugala, S'abaragamuwa Province; and from a pegmatite in Bambarabotuwa area.
  • India - Reported from beach sands of Travancore (Kerala).[13]
  • Madagascar - Found in alluvial deposits of Betroka and Andolobe.[14] Also as very large crystals from Tôlanaro (Fort Dauphin); at Andranondambo and other localities.
  • Russia - In black sands of a gold placer on Boshogoch River, Transbaikalia, Siberia; in the Kovdor massif, 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;[15] black sands in Nixon Fork and Wiseman districts, Alaska.[16][17]
  • Canada - Reported with uraninite in a pegmatite on Charlebois Lake, east of Lake Athabasca;[18] Uranon variety reported from pegmatite and metesomatized zones in crystalline limestones from many locations in Quebec and Ontario.[19]
  • South Africa - Occurs with baddeleyite as an accessory in carbonatite at Phalaborwa, Eastern Transvaal.[20]
  • Belgian Congo[21]

See also


  1. ^ Frondel, C. (1958). Systematic Mineralogy of Uranium and Thorium. United States Government Printing Office. 
  2. ^ Coomaraswamy, A.K. (1904). "Uraninite". Spolia Zeylanica Pt. 6 (2): 57.
  3. ^ Dunston, W.R. (1904-03-31). "The occurrence of Thorium in Ceylon". Nature 69: 510-511.
  4. ^ Kobayashi, M. (1912). "On the composition of thorianite". Tohuku Imp. Univ. Sci. Repts. 1 (Ist Ser): 201-206.
  5. ^ Palache, C.; H. Berman, C. Frondel (1944). "Dana's System of Mineralogy". John Wiley and Sons, New York 1: 478–480.
  6. ^ Heinrich, E. W. (1958). Mineralogy and Geology of Radioactive Raw Materials. McGraw-Hill. 
  7. ^ Palache, C.; H. Berman, C. Frondel (1944). "Dana's System of Mineralogy". John Wiley and Sons, New York 1: 478–480.
  8. ^ Graham, A. R. (1955). "CERIANITE CeO2: A NEW RARE-EARTH OXIDE MINERAL". Am. Mineralogist 40.
  9. ^ Bespalov, M.M. (1941). "On discovery of a new mineral of the thorianite group [in Russian]". Sovietskaya Geologiya II (6): 105-107.
  10. ^ Uranothorianite mineral information and data. Retrieved on 2007-06-28.
  11. ^ a b Minerals with crystal structure determined. Retrieved on 2007-06-27.
  12. ^ ICSD for WWW : Details (Thorianite Cerian). Retrieved on 2007-06-27.
  13. ^ Viswanathan, P. (1953). "Thorianite in Travancore". Mineralog. Mag. 88: 282.
  14. ^ Lacroix, A. (1923). Minéralogie de Madagascar. Augustin Challamel, éditeur, Librairie maritime et coloniale. 
  15. ^ George, D'Arcy (1949). "Mineralogy of Uranium and Thorium Bearing Minerals". USAEC Technical Information Service, Oak Ridge, Tennessee: 198.
  16. ^ White, M. G. (1952). Radioactivity of Selected Rocks and Placer Concentrates from Northeastern Alaska. GS-C-195, Geological Survey. 
  18. ^ Lang, A. H.; J. W. Griffith, H. R. Steacy (1962). Canadian Deposits of Uranium and Thorium. Geological Survey of Canada. 
  19. ^ Robinson, S. C.; A. P. Sabina (1955). "URANINITE AND THORIANITE FROM ONTARIO AND QUEBEC". Am. Mineralogist 40.
  20. ^ Hiemstra, S. A. (1955). "Baddeleyite from Phalaborwa, Eastern Transvaal". American Mineralogist 40: 275-282.
  21. ^ Ledoux, A.. "Les roches cristallines du Kasai". Soc. Geol. Belgique Annales 40: C177.
  • This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Thorianite". A list of authors is available in Wikipedia.
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