Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Sodium calcium aluminium silicate
Chemical Formula: (Na,Ca)Al(Al,Si)Si2O8
Method(s) of Verification: all occurrences by polarizing microscope.

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
Introduction: oligoclase, once a valid species name, is now an informal term used for plagioclase feldspar with anorthite/albite ratios in the range 10:90 to 30:70. It occurs as a typical primary rock-forming mineral in intermediate composition igneous rocks (SiO2 contents over 66% by weight) and also as an alteration product of more anorthite-rich plagioclase in more basic rocks (SiO2 less than 52% by weight).
Occurrence in Wales: oligoclase is widespread in Wales, occurring as a primary rock-forming mineral in acid to intermediate igneous rocks, such as granodiorite and granite intrusions and rhyolite.

Key Localities:

There are no key localities for this specimen.


  1. 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.