Augite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium iron magnesium aluminium silicate
Chemical Formula: (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)206
Method(s) of Verification: all occurrences optical. Electron microprobe analyses - Fishguard Volcanic Complex (Bevins, 1982); Penmaenmawr Intrusion (Durham, 2004); Rhobell Fawr Volcanic Complex (Koklaar, 1986); Tal y Fan Intrusion (Merriman et al., 1986).

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
Introduction: augite is a common rock-forming mineral in basic igneous rocks which belong to the pyroxene supergroup
Occurrence in Wales: augite is widespread in Wales as a primary mineral in basic igneous rocks where it is found in association with plagioclase feldspar. Its occurrence is widely reported and representative descriptions and analyses are presented here. In slowly cooled intrusions, such as the Tal y Fan (see below), augite can form large (‘ophitic’) plates enclosing plagioclase crystals. In more rapidly cooled volcanic rocks augite forms both groundmass and phenocryst crystals and in pillow lavas, formed by the rapid quenching of magma as the lava was erupted under water, spectacular dendritic to fan spherulitic growth forms occur.

Key Localities:

  • Fishguard, Pembrokeshire: Bevins (1982) described unaltered dendritic augite from pillow lavas of the Fishguard Volcanic Complex, and large Ti-rich skeletal augite interpreted as having formed during quenching. The clinopyroxene show considerable compositional variation with crystals that have cooled most quickly, as indicated by quench textures, being Ti and Al enriched relative to those that cooled more slowly.
  • Rhobell Fawr, Gwynedd: Koklaar (1986) describes pargasite-bearing basalts with zoned clinopyroxene changing from diopside cores to augite rims.
  • St. David's Head, Pembrokeshire: Roach (1969) described augite associated with plagioclase defining a pronounced magmatic layering in the gabbroic rocks of the St. David’s Head Intrusion. He also recorded gabbros containing pegmatitic patching with augite crystals up to 12 cm in length.
  • Tal y Fan, Conway: Merriman et al. (1986) described augite from a thick olivine dolerite sheet. In the central zone of the intrusion the augite forms anhedral crystals up to 20 mm in size which enclose altered olivine and partially altered plagioclase but towards the margins the crystals decrease to 5 mm and in the very marginal zones they are altered to actinolite, titanite, and chlorite.

There are no key localities for this specimen.

References:

  1. Bevins, R.E., 1982. Petrology and geochemistry of the Fishguard Volcanic Complex, Wales. Geological Journal, 17, 1-21
  2. Durham, J., 2004. The Petrogenesis of the Penmaenmawr Intrusion, North Wales. Unpublished M.Phil thesis, The Open University, 307pp.
  3. Koklaar, B.P., 1986. Petrology and geochemistry of the Rhobell Volcanic Comples: Amphibole-dominated fractionation at an early Ordovician arc volcano in North Wales. Journal of Petrology, 27, 887-914.
  4. Merriman, R. J., Bevins, R.E. & Ball, T.K., 1986. Petrology and geochemical variation within the Tal y Fan Intrusion: a study of element mobility during low-grade metamorphism with implications for petro-tectonic modelling. Journal of Petrology, 27, 1409-1436.
  5. 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.

There are no references for this specimen.