Mineral Database
Powellite
Crystal System: Tetragonal
Formula: CaMoO4
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Calcium molybdate
Method(s) of Verification: Benallt Mine - XRD and EDS at Manchester Museum (Dossett et al., 2007).
Creamy-white powellite microcrystals (to 0.4 mm on edge) from Benallt Mine. I. Dossett specimen. Photo D.I. Green, © D.I. Green.
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
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, Gwyneddŷ: 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.
References:
- Powellite from Benallt mine, Lleyn Peninsula, Gwynedd: a first Welsh occurrence. Journal of the Russell Society, 10, 54-55.
