Crystal System: Triclinic
Status of Occurrence: Unconfirmed Occurrence
Distribution: Uncommon
Chemical Composition: Sodium potassium aluminium silicate
Chemical Formula: (Na,K)AlSi3O8
Method(s) of Verification: none.

Chemical Group:

  • Silicates
Introduction: anorthoclase is member of the feldspar group, more specifically the orthoclase feldspars, and is a rock-forming mineral. It most commonly occurs in high temperature sodic volcanic and high level intrusive rocks
Occurrence in Wales: the only reported occurrence of anorthoclase from Wales is provided by Greenly (1919) who described anorthoclase with albite in a folded pegmatitic quartz augen within the Gwna Group schists exposed in the vicinity of Craig-y-don house, near Menai, Anglesey. These schists formed under low temperature (probably less than 300°C) metamorphic conditions which are not consistent with the conditions of formation of anorthoclase. This reported occurrence must therefore be treated as a suspected misidentification.

Key Localities:

There are no key localities for this specimen.


  1. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).