Pargasite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: sodium calcium magnesium iron aluminium silicate hydroxide
Chemical Formula: NaCa2(Mg,Fe2+)4Al(Si6Al2)O22(OH)2
Method(s) of Verification: all examples cited have been verified by microprobe analyses.

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
Introduction: this is a calcic amphibole and forms a series with ferropargasite. Pargasite is a rock-forming mineral found in basic or intermediate composition igneous rocks.
Occurrence in Wales: three occurrences of pargasite are recorded from Wales, all from basic igneous rocks of Ordovician age. The presence of pargasite reflects the water-rich nature of these magmas (containing 1.5-3 weight percent water) and indicates that the magma was formed by subduction processes.

Key Localities:

There are no key localities for this specimen.

References:

  1. 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
  2. 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.
  3. Koklaar, P.B., 1977. The igneous history of the Robell Fawr area, Merioneth, North Wales. Unpublished Ph.D. thesis, University of Wales.