Clarkeite

{{Short description|Uranium oxide mineral}}

{{Infobox mineral

| name = Clarkeite

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| 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{{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}}

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| strunz = 4.GC.05

| dana = 5.4.1.1

| system = Trigonal

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| symmetry = R{{overline|3}}m (No. 166)

| unit cell = 239.11 ų (Calculated from unit cell)

| colour = Dark red-brown, dark brown

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| cleavage = None observed

| fracture = Conchoidal, sub-conchoidal

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| mohs = 4–4.5

| luster = Resinous, waxy, greasy

| streak = Yellow-brown

| diaphaneity = Translucent

| gravity = 6.39

| density = 6.39 g/cm³ (Measured)

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| birefringence = 0.111

| pleochroism = Weak

| 2V = Measured 30° to 50°, Calculated 32°

| dispersion = Relatively weak

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| other = 25px Radioactive

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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.