Introduction: andesine, once a valid species name, is now an informal term used for plagioclase feldspar with anorthite/albite ratios in the range 50:50 to 70:30. It occurs as a typical rock-forming mineral in intermediate composition igneous rocks (SiO₂ contents of 52-66% by weight) and also as an alteration product of more anorthite-rich plagioclase in more basic rocks (SiO₂ less than 52% by weight). Andesine gives its name to the intermediate composition volcanic rock andesite.

Occurrence in Wales: andesine is widespread in Wales, occurring as a primary rock-forming mineral in intermediate igneous rocks, such as diorite intrusions and andesite. It is additionally present as an alteration product of original, more calcic plagioclase feldspar, in basic igneous rocks such as dolerite and basalt. Bevins & Merriman (1988) record the presence of andesine as part of the low-grade metamorphic assemblage in the Tal y Fan intrusion, near Conwy in Gwynedd.

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