Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: aluminium silicate hydroxide
Chemical Formula: Al2Si4O10(OH)2
Method(s) of Verification: all cited occurrences by XRD
Introduction: pyrophyllite is an uncommon mineral belongs to the talc group. It has two polytypes, that is, versions of the mineral with slightly different arrangements of the constituent atoms in the crystal lattice. This is why it can be classified as belonging to either the triclinic (pyrophyllite-1A) or the monoclinic (pyrophyllite -2M1) crystal systems. Pyrophyllite is most commonly found in aluminium-rich rocks up to greenschist facies grade or by the hydrothermal alteration of minerals (Deer et al. 1992). Experimental work suggests that it has a stability range from around 300 °C to 350-420°C.
Occurrence in Wales: Pyrophyllite has been described from low-grade metamorphic or hydrothermally altered rocks at several localities across Wales. Gill et al. (1977) recorded small amounts of pyrophyllite in some sandstones and shales of the Coal Measures (Upper Carboniferous) strata from the anthracite zone in the western part of the South Wales Coalfield. Further to the west, Nicholls (1979) and Robinson et al., (1980) identified pyrophyllite in sedimentary rocks of Silurian and Devonian age from the Marloes area, Pembrokeshire. The metamorphic grade of these rocks is insufficient for pyroplyllite to have formed by metamorphism, although a detrital origin is possible. Pyrophyllite, presumably of hydrothermal origin, is also found in altered rhyolites and tuffs in a borehole near Treffgarne (Brown and others, 1987). In North Wales Merriman & Roberts (1985) recorded pyprohyllite in Ordovician sedimentary rocks, forming the country rocks to Ordovician intrusions, such as those at Yr Eifel, Llanbedrog and Bwlch Mawr. Finally pyrophyllite also occurs in pod-like structures, associated with allanite, molybdenite and quartz, in the Ffestiniog Granite Quarry at Cwm Bychan, near Ffestiniog, Gwynedd (Roberts, 1979).
There are no key localities for this specimen.
- Brown, M.J. and others, 1987. Volcanogenic mineralisation in the Treffgarne area, south-west Dyfed, Wales. Mineral Reconnaissance Programme Report, British Geological Survey, No. 86.
- Deer, W.A., Howie, R.A. & Zussman, J., 1992. An Introduction to Rock-Forming Minerals. Longman Scientific & Technical, 696pp.
- Merriman, R.J. & Roberts, B., 1985. A survey of white mica crystallinity and polytypes in pelitic rocks of Snowdonia and Llŷn, North Wales. Mineralogical Magazine, 49, 305-319
- Roberts, B., 1979. The geology of Snowdonia and Llyn: an outline and field guide. Adam Hilger, Bristol.