| Clarkeite | |
|---|---|
| General | |
| Category | Mineral |
| Formula (repeating unit) | (Na,Ca,Pb) 2(UO 2) 2(O,OH) 3or(Na,Ca,Pb)(UO 2)O(OH)•0-1H 2O |
| IMA symbol | Cke[1] |
| Strunz classification | 4.GC.05 |
| Dana classification | 5.4.1.1 |
| Crystal system | Trigonal |
| Space group | R3m (No. 166) |
| Unit cell | 239.11 Å3 (Calculated from unit cell) |
| Identification | |
| Colour | Dark red-brown, dark brown |
| Cleavage | None observed |
| Fracture | Conchoidal, sub-conchoidal |
| Mohs scale hardness | 4–4.5 |
| Luster | Resinous, waxy, greasy |
| Streak | Yellow-brown |
| Diaphaneity | Translucent |
| Specific gravity | 6.39 |
| Density | 6.39 g/cm3 (Measured) |
| Birefringence | 0.111 |
| Pleochroism | Weak |
| 2V angle | Measured 30° to 50°, Calculated 32° |
| Dispersion | Relatively weak |
| Other characteristics |  Radioactive |
Clarkeite is a uranium oxide mineral with the chemical formula(Na,Ca,Pb)
2(UO
2)
2(O,OH)
3or(Na,Ca,Pb)(UO
2)O(OH)·0-1H
2O.
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 Na[(UO
2)O(OH)](H
2O)
0–1.
It was named for Frank Wigglesworth Clarke (1847–1931), American mineral chemist, and former chief chemist of the United States Geological Survey.
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