Descloizite

Berg Aukas (Berg Aukus), Grootfontein District, Otjozondjupa Region, Namibia © Rob Lavinsky

Chemical Formula: Pb(Zn,Cu)(VO4)(OH)
Locality: Sierra de Córdoba, Punilla Department, Córdoba, Argentina
Name Origin: Named after the French mineralogist, Alfred Des Closzeaux (1817-1897).

Descloizite is a rare mineral species consisting of basic lead and zinc vanadate, Pb(Zn,Cu)(VO4)(OH), 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.

Discovery and occurrence

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.

The Otavi (“O-tarvi”) Mountainland of northern Namibia was once considered home to the greatest vanadium deposits in the world, including those at Berg Aukas (“OW-cuss”), Abenab (“UB-en-ub”), Baltika (“BUL-tika”) and Uitsab (“ATE-sub”). 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 Obir, Carinthia Austria.

Physical Properties of Descloizite

Cleavage: None
Color: Dark brownish black, Dark reddish brown, Orange red, Reddish brown, Black.
Density: 6.1 – 6.2, Average = 6.15
Diaphaneity: Transparent to Opaque
Fracture: Brittle – Generally displayed by glasses and most non-metallic minerals.
Hardness: 3.5 – Copper Penny
Luminescence: Non-fluorescent.
Luster: Greasy (Oily)
Streak: light brownish green

Photos :

Descloizite Berg Aukas, Grootfontein District, Otjozondjupa Region, Namibia Size: 8.5 x 6.5 x 5.5 cm © danweinrich
Descloizite Berg Aukas, Grootfontein District, Otjozondjupa Region, Namibia Size: 10.0 x 7.5 x 5.0 cm © danweinrich
Descloizite Mina da Preguiça, Sobral da Adiça, Moura, Beja  Portugal (2005-2006) Specimen size: 10.3 × 6.5 × 4.2 cm = 4.1” × 2.6” × 1.7” Main crystal size: 0.1 × 0.1 cm = 0.0” × 0.0” © Fabre Minerals
This sample of descloizite is displayed in the Smithsonian Museum of Natural History. The sample at left is about 10 cm across and is from Grootfontein, Namibia.
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