Powellite


Composition: CaMoO4
Crystal System : Tetragonal
Colours: Light-brown, yellowish-green, grey, pale-blue
Lustre : Vitreous, greasy
Hardness (H) : 3.5-4.0
Specific Gravity (S.G) : 4.27

Distribution

Extremely rare. Second oxidation zone. Third oxidation zone (?)

Occurrence

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

Paragenetic and General Notes

Powellite occurs as small, straw-yellow tetragonal crystals on scorodite with tennantite, with powellite crystals to 3 mm (Pinch and Wilson, 1977). Gerdtremmelite is also part of this paragenesis (Gebhard, 1999).

Powellite forms a series with scheelite (CaWO4), and crystals of intermediate composition (c. 60% scheelite : 40% powellite) have been analysed recently on a specimen in the collection of Mark Feinglos (personal communication to M.Southwood, September, 2015).  These golden-brown, pseudo-octahedral crystals, to a few mm in size, are associated with quartz and fraiponite, on a sulphide matrix.