Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium aluminium silicate
Chemical Formula: CaAl2Si2O8
Method(s) of Verification: polarizing microscope.

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
Introduction: anorthite is the calcium-rich end member of the plagioclase feldspar supergroup of minerals (see for details). At the opposite end of the series is albite (sodium aluminium silicate), and most examples of plagioclase feldspar sit somewhere in between, containing both calcium and sodium. The composition of plagioclase is always expressed in terms of the amount of anorthite and albite components present, reflecting the calcium and sodium contents respectively. This is expressed in the form AnxAby, where X =100 would equate to pure anorthite and Y=100 would be pure albite. Pure anorthite is, however, relatively rare: it is restricted in its occurrence to basic and ultrabasic igneous rocks such as gabbro and picrite.
Occurrence in Wales: many basic rocks and some ultrabasic rocks in Wales would have contained anorthite as a primary rock-forming mineral, however most of these rocks subsequently have been altered, resulting in the formation of new plagioclase with a lower calcium content. Greenly (1919) suggested that the plagioclase in the post-Carboniferous (Palaeogene) dykes of Anglesey approached anorthite in composition, although no particular occurrence is detailed. Analyses from the largest of these dykes at Plas Newydd, Anglesey, however, indicates plagioclase of An75Ab25 composition (NMW unpublished data). Cattermole (1976) recorded the presence of An90Ab10 plagioclase, from plagioclase cumulate rocks (heteradcumulates) of the Ordovician Rhiw intrusion, Llŷn, although the composition was based on optical determinations and no analyses are given.

Key Localities:

There are no key localities for this specimen.


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