Prehnite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Calcium aluminium silicate hydroxide
Chemical Formula: Ca2Al(Si,Al)4O10(OH)2
Method(s) of Verification: Llanelwedd – EMPA (Metcalfe, 1990); Pencaer Peninsula – EMPA (Metcalfe, 1990); Penmaenmawr – XRD (National Museum of Wales, NMW X-59); St. David’s Head – polarizing microscopy (Roach, 1969).

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
Coarsely crystalline prehnite forming a bed around a gemmy prismatic quartz crystal from St. David's Head, Pembrokeshire. National Museum of Wales Collection (NMW 98.78G.M.1). Photo D.I. Green, © National Museum of Wales.
Introduction: prehnite is a mineral found in low-temperature alteration and is commonly associated with zeolite minerals. In metamorphic rocks, the prehnite-pumpellyite facies represents very low grade conditions. Under these conditions chlorite + prehnite breaks down to pumpellyite + quartz at 250° at 7 kilobars (equivalent to approximately 23 kilometers depth).The only major variation that is seen in prehnite composition is substitution of ferric iron (Fe3+) for aluminium (Al).
Occurrence in Wales: prehnite is widely developed in Wales, typically occurring as an alteration mineral in basic lavas and intrusions, particularly those of Ordovician age. It occurs in a variety of settings, generally replacing groundmass minerals, but also infilling vesicles, replacing phenocrysts in cross-cutting veins or forming spherulitic ‘spots’. Prehnite was first described by Harrison (1897) from Gimlet Rock near Pwllheli, Gwynedd and subsequently by other authors from many localities around the Principality. Greenly (1919) noted prehnite from the margins of hornblende-picrites in Anglesey, but did not provide specific locality details. Bevins & Rowbotham (1983) suggested that the widespread occurrence of prehnite in altered basic igneous rocks in Wales is due to low-grade burial metamorphism. They also demonstrated by electron

Key Localities:

  • Gimlet Quarry, Pwllheli, Gwynedd: Harrison (1897) described prehnite associated with quartz, calcite, natrolite and brookite from joint planes in the Carreg yr Imbill dolerite intrusion.
  • Llanelwedd Quarry, Builth Wells, Powys: in Mid Wales prehnite has been recorded from altered lavas of the Builth Volcanic Group (Nicholls, 1958; Metcalfe, 1990), as inclusions in plagioclase phenocrysts. It also occurs in the tuffs at Llanelwedd Quarry, forming ovoid spots up to 20 mm cross, and also pseudomorphing original lithic fragments.
  • Pencaer Peninsula, Pembrokeshire: Metcalfe (1990) described alteration in dolerite and gabbro exposed on the hills of Garn Gilfach and Garn Folch. Prehnite occurs, within discrete pods of alteration, in association with pumpellyite, chlorite, epidote, titanite and quartz.
  • Penmaenmawr, Conwy: Schaub (1905) and Sargent (1915, 1925) described prehnite-bearing vein assemblages and acid segregations from the Penmaenmawr Intrusion. More recently Durham (2004) records fine-grained prehnite with or without pumpellyite replacing the cores of primary igneous plagioclase and prehnite and pumpellyite in quartz-rich veins and segregations, which presumably equate to the acid segregations described previously.
  • Porthgain Quarry, near St David's, Pembrokeshire: prehnite occurs as thin veins, some with small well-formed crystals in cavities (Bevins, 1994).
  • St. David's Head, Pembrokeshire: Roach (1969) described prehnite from the St. David’s Head Intrusion, forming alteration patches in plagioclase feldspar and attributed its presence to deuteric alteration (alteration by fluids dervived from the magma). Prehnite is also found in veins and pods associated with quartz.
  • Strumble Head, Pembrokeshire: prehnite spots are present in acidic tuffs exposed east of Strumble Head (Bevins, 1994).

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Bevins, R.E. & Rowbotham, G., 1983. Low-grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal, 18, 141-167
  3. Durham, J., 2004. The Petrogenesis of the Penmaenmawr Intrusion, North Wales. Unpublished M.Phil thesis, The Open University, 307pp.
  4. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  5. Harrison, W.J., 1897. An occurrence of prehnite in Wales. Mineralogical Magazine, 11, 198
  6. Metcalfe, R., 1990. Fluid-rock interaction and metadomain formation during low-grade metamorphism in the Welsh marginal basin. Unpublished Ph.D. thesis, University of Bristol.
  7. Nicholls, G.D., 1957. Autometasomatism in the Lower Spilites of the Builth Volcanic Series. Proceedings of the Geological Society of London, 1543, 8-10.
  8. 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.
  9. Sargent, H.C., 1915. The Penmaenmawr Intrusion. Geological Magazine, New Series, Decade, 6,2,15-27.
  10. Sargent, H.C., 1925. Notes on the petrology of the Penmaenmawr Intrusion (Part II). Proceedings of the Liverpool Geological Society, 14, 123-142.
  11. Schaub, L., 1905. Ueber den Quartznorit von Penmaenmawr in Wales und seine Schlierenbildungen. Neues Jahrbuch für Mineralogie, 1, 93-121.