Descloizite
| Descloizite | |
|---|---|
Descloizite specimen from Berg Aukas (Berg Aukus), Namibia, 9.5 x 8.9 x 4.9 cm
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| General | |
| Category | Vanadate mineral |
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Formula (repeating unit) |
(Pb,Zn)2VO4OH |
| Strunz classification | 8.BH.40 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Unit cell | a = 7.593, b = 6.057 c = 9.416 [Å]; Z = 4 |
| Identification | |
| Color | Brownish red, red-orange, reddish to blackish brown, nearly black |
| Crystal habit | Zoned tabular crystals common, encrustations and plumose aggregates |
| Cleavage | None |
| Fracture | Irregular, sub-conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 3 - 3.5 |
| Luster | Greasy |
| Streak | Orange to brownish red |
| Diaphaneity | Transparent to opaque |
| Specific gravity | 6.1 - 6.2 |
| Optical properties | Biaxial (-) |
| Refractive index | nα = 2.185 nβ = 2.265 nγ = 2.350 |
| Birefringence | δ = 0.165 |
| Pleochroism | Visible |
| 2V angle | 85° to 90° |
| Dispersion | Strong r > v rarely r < v |
| References | |
Descloizite is a rare mineral species consisting of basic lead and zinc vanadate, (Pb,Zn)2(OH)VO4, crystallizing in the orthorhombic crystal system and isomorphous with olivenite. Appreciable gallium and germanium may also be incorporated into the crystal structure.
The color is deep cherry-red to brown or black, and the crystals are transparent or translucent with a greasy lustre; the streak is orange-yellow to brown; specific gravity 5.9 to 6.2; hardness 31/2. A variety known as cuprodescloizite is dull green in color; it contains a considerable amount of copper replacing zinc and some arsenic replacing vanadium. There is also an arsenate analogue called arsendescloizite.
It was discovered in the Sierra de Córdoba deposit in Córdoba, Argentina in 1854 and named in honor of the French mineralogist Alfred Des Cloizeaux (1817–1897). It occurs as small prismatic or pyramidal crystals, usually forming drusy crusts and stalactitic aggregates; also as fibrous encrusting masses with a mammillary surface.
Descloizite occurs in oxidised portions of veins of lead ores in association with pyromorphite, vanadinite, wulfenite, mottramite, mimetite and cerussite.
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