Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Calcium molybdate
Chemical Formula: CaMoO4
Method(s) of Verification: Benallt Mine - XRD and EDS at Manchester Museum (Dossett et al., 2007).

Chemical Group:

  • Sulphites, Chromates, Molybdates & Tungstates

Geological Context:

  • Hydrothermal: amygdale infill & veins in volcanic rocks
  • Supergene: in situ natural oxidation & weathering deposits
  • Metamorphic: skarn mineralization
Creamy-white powellite microcrystals (to 0.4 mm on edge) from Benallt Mine. I. Dossett specimen. Photo D.I. Green, © D.I. Green.
Introduction: an uncommon secondary species formed by the alteration of molybdenite or other molybdenum-bearing minerals, but can also form in low-temperature hydrothermal environments, such as with zeolites in vugs in basalt and within skarns and contact metamorphic environments.
Occurrence in Wales: Dossett et al. (2007) described the first and only occurrence of powellite in Wales from a single specimen collected at Benallt Mine, near Rhiw, Pen Llyn in 2006. One small (5 mm across) cavity within a specimen of altered igneous rock was discovered to contain twenty microcrystals of powellite.

Key Localities:

  • Benallt Mine, Llŷn, Gwynedd: a single cavity containing twenty euhedral, translucent, white to very pale yellow blocky pyramidal crystals up to 0.4 mm on edge was described by Dossett et al. (2007). The cavity is within a medium-grained igneous rock.


  1. Dossett, I., Green, D.I. and Cotterell, T.F., 2007. Powellite from Benallt mine, Lleyn Peninsula, Gwynedd: a first Welsh occurrence. Journal of the Russell Society, 10, 54-55.