Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Magnesium silicate
Chemical Formula: Mg2Si206
Method(s) of Verification: optical in all occurrences; Bodelias and Carnfor intrusions (Croudace, 1982); Penmaenmawr - EMPA (Durham, 2004); St. David’s Head - EMPA (Bevins et al., 1994).
Introduction: enstatite is a member of the pyroxene supergroup and forms a series with ferrosilite; therefore up to 50% of the magnesium can be replaced by iron. Enstatite is also dimorphous with clinoenstatite, a mineral with the same chemical composition but a different crystal lattice structure. Enstatite is a rock-forming mineral occurring in ultrabasic, basic and intermediate composition igneous rocks.
Occurrence in Wales: enstatite occurs most typically in Wales in basic rocks of Ordovician age. In many instances hydrothermal alteration or low grade metamorphism has resulted in a breakdown of enstatite and replacement by amphibole or chlorite. The locatities listed below represent the confirmed occurrences of this mineral although it is likely that there are many other occurrences that have not been documented in detail or mineral analyses made.
- Garnfor, Llŷn, Gwynedd: Croudace (1982) reported enstatite from microtonalites from the Inner Garnfor Intrusion and the Penrhyn Bodeilias Intrusion and cited average compositions of Ca4Mg60Fe36 for phenocrysts and Ca4Mg48Fe48 for the groundmass crystals.
- Great House, Usk, Monmouthshire: the only occurrence of enstatite in SE Wales from peridotite nodules within the Great House diatreme near Usk (D.T. Moffat, unpublished data).
- Penmaenmawr, Conwy: the diorite to quartz-diorite/granodiorite rocks of the Penmaenmawr Intrusion are enstatite bearing, although in many instances the enstatite is replaced or partially replaced by secondary minerals. Teall (1888) illustrated ‘enstatite-diorite’ from Penmaenmawr in his classic book ‘British Petrography’, and the enstatite was re-examined by Harker (1889) and Sargent (1915) who both considered it to be ‘bronzite’, a variety of enstatite, although in a more detailed petrological work by Sargent he once again referred to this as enstatite (Sargent, 1924). A more recent study of the intrusion provides microprobe data (Durham, 2004).
- St. David's Head, Pembrokeshire: Roach (1969) described chlorite pseudomorphs after enstatite from the St. David’s Head Intrusion and more recently enstatite has been identified from cognate xenoliths within the laminated olivine gabbro unit (Bevins et al., 1994).
- Bevins, R.E., Lees, G.J. Roach, R.A. Rowbotham, G. & Floyd, P.A., 1994. Petrogenesis of the St David’s Head Layered Intrusion, Wales: a complex history of multiple magma injection and in situ crystallisation. Transactions of the Royal Society of Edinburgh: Earth Sciences, 85, 91-121
- Croudace, I.W., 1982. The geochemistry and petrogenesis of the Lower Palaeozoic granitoids of the Lleyn Peninsula, North Wales. Geochemica et Cosmochemica Acta, 46, 609-622.
- Durham, J., 2004. The Petrogenesis of the Penmaenmawr Intrusion, North Wales. Unpublished M.Phil thesis, The Open University, 307pp.
- Harker, A., 1889. The Bala Volcanic Series of Caernarvonshire. The Cambridge University Press, 130pp
- Roach, R.A., 1969. The composite nature of the St. David's Head and Carn Llidi intrusions of North Pembrokeshire. In: Wood, A. (ed.) The Pre-Cambrian and Lower Palaeozoic rocks of Wales, University of Wales Press, Cardiff, 409-433.
- Sargent, H.C., 1924. Notes on the petrology of Penmaenmawr Mountain (Part 1). Proceedings of the Liverpool Geological Society, 14, 82-89.
- Teall, J.J.H., 1888. British Petrography. Dulau, London, 469pp.