| Clarkeite | |
| Formula: | or |
| Imasymbol: | Cke[1] |
| Strunz: | 4.GC.05 |
| Dana: | 5.4.1.1 |
| System: | Trigonal |
| Symmetry: | Rm (No. 166) |
| Unit Cell: | 239.11 Å3 (Calculated from unit cell) |
| Colour: | Dark red-brown, dark brown |
| Cleavage: | None observed |
| Fracture: | Conchoidal, sub-conchoidal |
| Mohs: | 4–4.5 |
| Luster: | Resinous, waxy, greasy |
| Streak: | Yellow-brown |
| Diaphaneity: | Translucent |
| Gravity: | 6.39 |
| Density: | 6.39 g/cm3 (Measured) |
| Birefringence: | 0.111 |
| Pleochroism: | Weak |
| 2V: | Measured 30° to 50°, Calculated 32° |
| Dispersion: | Relatively weak |
| Other: | Radioactive |
Clarkeite is a uranium oxide mineral with the chemical formula or .
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 .
It was named for Frank Wigglesworth Clarke (1847–1931), American mineral chemist, and former chief chemist of the United States Geological Survey.