Descloizite

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.[4] Appreciable gallium and germanium may also be incorporated into the crystal structure.

Descloizite
Descloizite specimen from Berg Aukas (Berg Aukus), Namibia, 9.5 x 8.9 x 4.9 cm
General
CategoryVanadate mineral
Formula
(repeating unit)		(Pb,Zn)2VO4OH
Strunz classification8.BH.40
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Unit cella = 7.593, b = 6.057
c = 9.416 [Å]; Z = 4
Identification
ColorBrownish red, red-orange, reddish to blackish brown, nearly black
Crystal habitZoned tabular crystals common, encrustations and plumose aggregates
CleavageNone
FractureIrregular, sub-conchoidal
TenacityBrittle
Mohs scale hardness3 - 3.5
LusterGreasy
StreakOrange to brownish red
DiaphaneityTransparent to opaque
Specific gravity6.1 - 6.2
Optical propertiesBiaxial (-)
Refractive indexnα = 2.185 nβ = 2.265 nγ = 2.350
Birefringenceδ = 0.165
PleochroismVisible
2V angle85° to 90°
DispersionStrong r > v rarely r < v
References[1][2][3]

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.[4] There is also an arsenate analogue called arsendescloizite.[5]

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 (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.[4]

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").[6] 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.

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References
  1. Mindat.org
  2. Webmineral data
  3. Handbook of Mineralogy
  4.  One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Descloizite". Encyclopædia Britannica. 8 (11th ed.). Cambridge University Press. p. 91.
  5. Arsendescloizite on Mindat.org
  6. Boni et al., 2007, Genesis of vanadium ores in the Otavi Mountainland, Namibia. Economic Geology v.102 p.441-469.
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