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identified from Benallt Mine, Llŷn; occurring as small greenish-brown to reddish brown prismatic crystals (N. Hubbard, unpublished data).

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