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Piemontite

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.

Chemical Group:

  • Silicates

Geological Context:

  • 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identified from Benallt Mine, Llŷn; occurring as small greenish-brown to reddish brown prismatic crystals (N. Hubbard, unpublished data).

Key Localities:

There are no key localities for this specimen.

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Deer, W.A., Howie, R.A., & Zussman, J., 1986. Rock-Forming Minerals Volume 1B 2nd Edition, Dilsilicates and Ring Silicates Longman Scientific & Technical.
  3. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  4. Williams, H., 1927. The geology of Snowdon (North Wales). Quarterly Journal of the Geological Society of London, 83, 346-431.