Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Calcium aluminium iron silicate hydroxide
Chemical Formula: Ca2(Al,Fe3+)3(SiO4)3OH
Method(s) of Verification: verified by XRD and EMPA.
- Metamorphic: low-grade
- Hydrothermal: alpine type veins
- Metamorphic: skarn mineralization
Lustrous, prismatic epidote crystals from Marloes Bay, Pembrokeshire. National Museum of Wales Collection (NMW 87.43G.M.28). Photo D.I. Green, © National Museum of Wales.
Epidote. © National Museum of Wales.
Radiating epidote crystals with calcite from metadolerite dyke in Dinorwic Quarry. Specimen National Museum of Wales (NMW 27.111.GR.295), ex G.J. Williams Collection (original label on specimen). Photo D.I. Green, © National Museum of Wales.
Introduction: epidote is frequently observed in low to medium-grade metamorphic mineral assemblages, particularly in basic igneous rocks. Likewise, it occurs in metamorphism-related mineral veins cutting such rocks. Hydrothermal alteration may also include the development of epidote and finally it is frequently observed in skarn-type associations developed during contact metamorphism of calcareous rocks. Care must be taken when examining material carrying epidote-like minerals because epidote forms a solid solution series with clinozoisite. However, in many Welsh occurrences of these minerals, colour has proved to be a reasonable guide: epidote is a characteristic pistachio-green colour while clinozoisite is greenish through to brown.
Occurrence in Wales: the presence of epidote in Wales was first mentioned by Greenly (1919), while more recently, electron microprobe analyses were provided by Bevins & Rowbotham (1983) and by Bevins & Merriman (1988). Epidote is a widespread accessory mineral in Lower Palaeozoic low-grade metabasites outcropping in many parts of Wales, particularly in the north and west where the majority of them outcrop. A good example of hydrothermally-added epidote (so-called epidotization) is within the Rhobell Volcanics of early Ordovician age, outcropping to the NE of Dolgellau in Gwynedd: the rocks themselves have a striking bright greenish colour due mostly to the abundant presence of epidote. The localities listed below are where well-crystallized epidote has been recorded.
- Dinorwic Quarry, Llanberis, Gwynedd: typical pistachio-green epidote occurs as a component of alpine fissure-type mineralization developed in the neck-zones of boudinaged basalt dykes hosted by Cambrian slate at this locality. Specimens typically comprise intergrown aggregates of prismatic crystals associated with quartz and chlorite (National Museum of Wales specimens, collected as part of the MINESCAN project in 1997).
- Dolgellau Gold-belt, Gwynedd: epidote is widespread, as is its close relative clinozoisite, in alpine fissure-type quartz-chlorite-epidote-albite-calcite dominated veins and pods hosted by altered metabasites (so-called greenstones). Sheaves of prismatic or fibrous crystals may reach 4 cm in length but free-standing crystals in vugs are very rare. Small nests of acicular green epidote crystals have been recorded in vugs in such veins from the Afon Wen Intrusive Complex (J.S. Mason & M.J. Liezers, unpublished data). In addition, similar specimens, found near Ganllwyd, are present in the National Museum of Wales Collection (ex G.J. Williams Collection, acquired in 1927).
- Marloes Bay, Pembrokeshire: well-crystallized epidote occurs with quartz in metamorphic segregation veins hosted by basic igneous rocks of the Skomer Volcanic Group.
- Bevins, R.E. & Rowbotham, G., 1983. Low-grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal, 18, 141-167
- Bevins,R.E. & Merriman, R. J., 1988. Compositional controls on co-existing prehnite-actinolite and prehnite-pumpellyite assemblages in the Tal y Fan metabasite intrusion, North Wales: implications for Caledonian metamorphism field gradients. Journal of Metamorphic Geology, 6, 17-39.
- Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).