Descloizite

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Descloizite
Descloizite-162484.jpg
Descloizite specimen from Berg Aukas (Berg Aukus), Namibia, 9.5 x 8.9 x 4.9 cm
General
Category Vanadate mineral
Formula
(repeating unit)
(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 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.[4]

Discovery and occurrence[edit]

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 (1817–1897).[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 ("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").[5] 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.

References[edit]

  1. ^ a b Mindat.org
  2. ^ Webmineral data
  3. ^ a b Handbook of Mineralogy
  4. ^ Arsendescloizite on Mindat.org
  5. ^ Boni et al., 2007, Genesis of vanadium ores in the Otavi Mountainland, Namibia. Economic Geology v.102 p.441-469.