Descloizite

Mineral
(repeating unit)(Pb,Zn)2VO4OHIMA symbolDcz[1]Strunz classification8.BH.40Crystal systemOrthorhombicCrystal 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 [Å]; Z = 4IdentificationColorBrownish red, red-orange, reddish to blackish brown, nearly blackCrystal habitZoned tabular crystals common, encrustations and plumose aggregatesCleavageNoneFractureIrregular, sub-conchoidalTenacityBrittleMohs scale hardness3 – 3.5LusterGreasyStreakOrange to brownish redDiaphaneityTransparent to opaqueSpecific gravity6.1 – 6.2Optical propertiesBiaxial (−)Refractive indexnα = 2.185 nβ = 2.265 nγ = 2.350Birefringenceδ = 0.165PleochroismVisible2V angle85° to 90°DispersionStrong r > v rarely r < vReferences[2][3][4]

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.[5] 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.[5] There is also an arsenate analogue called arsendescloizite.[6]

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).[2] It occurs as small prismatic or pyramidal crystals, usually forming drusy crusts and stalactitic aggregates; also as fibrous encrusting masses with a mammillary surface.[5]

Descloizite occurs in oxidised portions of veins of lead ores in association with pyromorphite, vanadinite, wulfenite, mottramite, mimetite and cerussite.[4]

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").[7] 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. ^ a b Descloizite on Mindat.org
  3. ^ Webmineral data
  4. ^ a b "Handbook of Mineralogy" (PDF). Archived from the original (PDF) on 2021-02-27. Retrieved 2010-11-15.
  5. ^ a b c  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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