{{Short description|Uranium oxide mineral}} {{Infobox mineral | name = Clarkeite | boxwidth = | boxbgcolor = | image = | imagesize = | alt = | caption = | struct image = | struct caption = | struct imagesize = | struct2 image = | struct2 caption = | struct2 imagesize= | SMILES = | Jmol = | category = | formula = {{chem|(Na|,Ca|,Pb)|2|(U|O|2|)|2|(O|,OH)|3}} or {{chem|(Na|,Ca|,Pb)|(U|O|2|)O|(OH)|•0-1H|2|O}} | IMAsymbol = Cke<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref> | molweight = | strunz = 4.GC.05 | dana = 5.4.1.1 | system = Trigonal | class = | symmetry = ''R{{overline|3}}m'' (No. 166) | unit cell = 239.11 Å<sup>3</sup> (Calculated from unit cell) | colour = Dark red-brown, dark brown | habit = | twinning = | cleavage = None observed | fracture = Conchoidal, sub-conchoidal | tenacity = | toughness = | mohs = 4–4.5 | luster = Resinous, waxy, greasy | streak = Yellow-brown | diaphaneity = Translucent | gravity = 6.39 | density = 6.39 g/cm<sup>3</sup> (Measured) | polish = | opticalprop = | refractive = | birefringence = 0.111 | pleochroism = Weak | 2V = Measured 30° to 50°, Calculated 32° | dispersion = Relatively weak | extinction = | other = 25px Radioactive | length fast/slow = | fluorescence = | absorption = | melt = | Curie temp = | fusibility = | diagnostic = | solubility = | impurities = | alteration = }} '''Clarkeite''' is a uranium oxide mineral with the chemical formula{{chem|(Na|,Ca|,Pb)|2|(U|O|2|)|2|(O|,OH)|3}} or {{chem|(Na|,Ca|,Pb)|(U|O|2|)O|(OH)|·0-1H|2|O}}.
Its color varies from dark brown to reddish orange. Clarkeite forms by oxidation and replacement of uraninite late during pegmatite crystallization. Although uraninite-bearing granite pegmatites are common, clarkeite is rare and occurs intimately intergrown with other uranium minerals.
It is known from only two localities; the Spruce Pine pegmatite district in western North Carolina, US, and Rajputana, in the Ajmer district, India. Clarkeite is the only known naturally occurring high-temperature uranate. The general formula for ideal clarkeite is {{chem|Na|[(U|O|2|)O|(OH)]|(H|2|O)|0–1}}.
It was named for Frank Wigglesworth Clarke (1847–1931), American mineral chemist, and former chief chemist of the United States Geological Survey.
==See also== * List of minerals * List of minerals named after people * Sodium uranate
==References== {{Reflist}} *[http://www.minsocam.org/MSA/AmMin/TOC/Articles_Free/1997/Finch_p607-619_97.pdf Clarkeite: New chemical and structural data] *[http://www.mindat.org/min-1059.html Clarkeite: Clarkeite mineral information and data] *[https://www.webmineral.com/data/Clarkeite.shtml WebMineral]
Category:Uranium minerals Category:Sodium minerals Category:Hydroxide minerals Category:Trigonal minerals Category:Minerals in space group 166
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