- Descloizite
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Descloizite 
Descloizite specimen from Berg Aukas (Berg Aukus), Namibia, 9.5 x 8.9 x 4.9 cm]]General Category Vanadate mineral Chemical formula (Pb,Zn)2VO4OH Strunz classification 08.BH.40 Crystal symmetry Orthorhombic (2/m 2/m 2/m) - dipyramidal Unit cell a = 7.593 Å, b = 6.057 Å, 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 Crystal system Orthorhombic 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 [1][2][3] Descloizite is a rare mineral species consisting of basic lead and zinc vanadate, (Pb,Zn)2(OH)VO4, crystallizing in the orthorhombic system and isomorphous with olivenite.
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. Appreciable gallium and germanium may also be incorporated into the crystal structure.
Discovery and occurrence
Superb spear-point bladed crystals of Descloizite, Berg Aukas, Namibia. Size 3.6 x 3.1 x .9 cm.
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.[1] 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.[3]
The Otavi Mountainland of northern Namibia was once considered home to the greatest vanadium deposits in the world, including those at Berg Aukas, Abenab, Baltika and Uitsab.[4] Descloizite and mottramite were the main ore minerals in each of these deposits, which are now exhausted. Other localities are the Sierra de Cordoba in Argentina; Lake Valley in Sierra County, New Mexico; Arizona; Phoenixville in Pennsylvania and Kappel (Eisen-Kappel) near Klagenfurt in Carinthia.
References
- ^ a b Mindat.org
- ^ Webmineral data
- ^ a b Handbook of Mineralogy
- ^ Boni et al., 2007, Genesis of vanadium ores in the Otavi Mountainland, Namibia. Economic Geology v.102 p.441-469.
This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.
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