Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium manganese iron aluminium silicate hydroxide
Chemical Formula: (Ca,Pb,Ce)2(Mn3+,Fe3+)Al2(Si2O7)(SiO4)(O,OH)2
Method(s) of Verification: all occurrences identified by optical means.
- Metamorphic: low-grade
Clove-brown prismatic piemontite microcrystals from Benallt Mine. Field of view 2.5 mm wide. National Museum of Wales Collection (NMW 2002.60G.M.3). Photo T.F. Cotterell, © National Museum of Wales.
Introduction: piemontite (previously referred to as piedmontite) is a member of the epidote group of minerals. In thin section it is recognised by its distinct pink to violet colour. Piemontite typically occurs in low-grade metamorphic rocks, but is known from rocks formed under greenschist, blueschist and amphibolite facies conditions. It may also be produced by hydrothermal processes associated with manganese deposits and as a late-stage magmatic phase in some acid and intermediate lavas (Deer et al., 1986).
Occurrence in Wales: three occurrences of piemontite have been recorded from Wales, although, only substantiated by optical identification. Greenly (1919) identified the mineral in a thin section of ‘green-mica-schist’ from the New Harbour Group (part of the Monian Supergroup), collected in the vicinity of Holyhead on Holy Island, Anglesey. Subsequently Williams (1927) described the presence of granules of a rose-pink, strongly pleochroic epidote (presumed to be piemontite by Bevins, 1994), in altered rhyolitic intrusions in the Llanberis Pass district, Gwynedd. More recently, piemontite has been identified from Benallt Mine, Llŷn; occurring as small greenish-brown to reddish brown prismatic crystals (N. Hubbard, unpublished data).
There are no key localities for this specimen.
- Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
- Deer, W.A., Howie, R.A., & Zussman, J., 1986. Rock-Forming Minerals Volume 1B 2nd Edition, Dilsilicates and Ring Silicates Longman Scientific & Technical.
- Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
- Williams, H., 1927. The geology of Snowdon (North Wales). Quarterly Journal of the Geological Society of London, 83, 346-431.