Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Magnesium silicate
Chemical Formula: Mg2SiO4
Method(s) of Verification: all localities - polarizing microscope identification; Rhiw - EMPA (Hawkins, 1970; Cattermole, 1976).
Introduction: forsterite is a member of the olivine group. It forms two series with fayalite (Fe2+2SiO4) and with tephroite (Mn2+2SiO4) and is trimorphous with ringwoodite and wadsleyite. Forsterite is an important rock-forming mineral which crystallizes from ultrabasic and basic magmas. It may also be found in impure dolomitic (magnesium-rich) carbonate rocks affected by thermal metamorphism. Forsterite, like all members of the olivine group, is very prone to alteration, most typically forsterite changes to serpentine minerals or a mixture of smectite, chlorite and iron oxide referred to as iddingsite.
Occurrence in Wales: the susceptibility of forsterite to hydration means that many occurrence of olivine are either partially of totally pseudomorphed by alteration products. Bevins & Merriman (1988) provide a classic illustration of an olivine crystal pseudomorphed by chlorite in the Tal y Fan intrusion. The only reported metamorphic occurrence of forsterite in Wales was noted by Greenly (1919) from scant exposures of impure marble near Brynfuches in NE Anglesey. However a re-examination of these rocks, including inspection of Greenly’s thin sections has failed to confirm its presence (Horák, 1993).
- Great House, Usk, Monmouthshire: magnesian peridotite nodules within the Great House diatreme (volcanic vent) near Usk, contain forsterite with a typical composition of Fo90 (D.T. Moffat, unpublished data, as cited in Bevins, 1994). Haslett (1992) also notes olivine crystals (less than 3 mm) from Glen Court, although the exact member of the olivine family is not stated, this is likely to be forsterite, which has been identified at Great House Quarry nearby.
- Rhiw, Llŷn, Gwynedd: forsterite is present within the Rhiw Intrusion, occurring throughout many of the layered units. Hawkins (1970) recorded compositions in the range Fo85 to Fo79, whereas Cattermole (1976) indicate compositions in the range Fo80 to Fo86.
- St. David's Head, Pembrokeshire: Bevins et al. (1994) describe olivine, orthopyroxene and clinopyroxene-rich mafic nodules within the laminated olivine gabbro of the St. David’s Head Intrusion. The olivine is contained within larger clinopyroxene crystals and has a rounded rounded form. These textures (and the low plagioclase content of the nodules) suggest that these mineral have been concentrated by settling out of the magma, producing cumulate rocks.
- Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
- 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.
- Cattermole, P.J., 1976. The crystallization and differentiation of a layered intrusion of hydrated alkali olivine-basalt parentage at Rhiw, North Wales. Geological Journal,11,45-70.
- Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
- Haslett, S.K., 1992. Petrology of a monchiquite from the Welsh Borderlands. Mercian Geologists,13,43-46.
- Hawkins, T.R.W., 1970. Hornblende gabbros and picrites at Rhiw, Caernarvonshire. Geological Journal, 7, 1-24.
- Horák, J.M., 1993. The Late Precambrian Coedana and Sarn Complexes, Northwest Wales - a Geochemical and Petrological study. Unpublished Ph.D. thesis, University of Wales, 415pp.