Wulfenite

Wulfenite


Composition: PbMoO4
Crystal System : Tetragonal
Colours: White, grey, beige, grey-yellow, orange, brown, blue
Lustre : Adamantine, resinous
Hardness (H) : 2.5-3.0
Specific Gravity (S.G) : 6.3-7.0
References: Gebhard (1999)

Distribution

Common. Occurs in all three oxidation zones.

Occurrence

  • Supergene
  • 1st Oxidation Zone
  • 2nd Oxidation Zone
  • 3rd Oxidation Zone

Notable Finds

In the first oxidation zone, wulfenite was found only to a depth of 160 m (Gebhard, 1999). Wulfenite discoveries peaked in the 1960s and 70s (presumably all second oxidation zone) and included the 1968 discovery of intergrown caramel-coloured crystals to 60 cm on edge (Gebhard, op. cit.). According to Gebhard, the third oxidation zone "did not produce any significant [wulfenite] crystals", however, the combination pieces of dioptase with chromate-yellow wulfenite from the third oxidation zone are surely noteworthy.

Paragenetic and General Notes

While it cannot really be claimed that Tsumeb has produced the world's best wulfenite specimens, the variety of colour and habit for wulfenite at Tsumeb is unmatched by any other single locality.

Tsumeb wulfenite crystals are generally tabular, modified only by pyramid faces to give bevelled edges. Crystals to 70 mm have been found, and one extraordinary occurrence contained large, thin, intergrown crystals allegedly to 60 cm (Pinch and Wilson, 1977).

Certain dark-blue crystals of wulfenite from Tsumeb have a steep pyramidal, or even a prismatic / pyramidal habit, and are informally known by the varietal name of chillagite, although Embrey et al. (1977) point out that they are different in habit and very much lower in WO3 content (< 1.5%) than the "chillagite" from Australia; they suggest that the blue colour may be due to partial reduction of Mo6+ to Mo4+ rather than to the presence of tungsten. However, tabular blue wulfenites have also been found with tungsten content up to 12%.

The following minerals are reported to form pseudomorphs after wulfenite:  bayldonite (rare); conichalcite (rare); dolomite (rare); malachite (rare); mottramite (rare); smithsonite (rare); stolzite (rare).