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Descloizite

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Mineral
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
IMA symbolDcz
Strunz classification8.BH.40
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPnma (no. 62)
Unit cella = 7.593, b = 6.057
c = 9.416 ; 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

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.

Discovery and occurrence

Superb spear-point bladed crystals of descloizite, Berg Aukas, Namibia. Size 3.6 × 3.1 × .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). 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.

See also

References

  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ Descloizite on Mindat.org
  3. Webmineral data
  4. ^ "Handbook of Mineralogy" (PDF). Archived from the original (PDF) on 2021-02-27. Retrieved 2010-11-15.
  5. ^  One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Descloizite". Encyclopædia Britannica. Vol. 8 (11th ed.). Cambridge University Press. p. 91.
  6. Arsendescloizite on Mindat.org
  7. Boni et al., 2007, Genesis of vanadium ores in the Otavi Mountainland, Namibia. Economic Geology v.102 p.441-469.
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