Introduction: sanidine belongs to the alkali feldspar group of minerals. It is a high temperature feldspar, and if it undergoes slow cooling from magmatic temperatures it inverts to a lower temperature polymorph such as microcline. At high temperatures the sanidine forms a complete solid solution with albite, forming one component within the feldspar, but at lower temperature this does not occur and the two components unmix to form perthite (an intergrowth of sodic and potassic feldspar). Sanidine most commonly occurs in alkali and acid volcanic rocks and less commonly in high temperature metamorphic rocks characterised by the absence of hydrous minerals such as micas.

Occurrence in Wales: sanidine is an uncommon mineral in Wales, as there are no high grade metamorphic rocks formed under conditions conducive to sanidine crystallization, and almost all volcanic rocks have undergone low grade metamorphism resulting in the alteration or modification of the primary igneous feldspar. A single documented occurrence is reported by Davies & Bloxham (1990) from the Precambrian rhyolitic lavas exposed in the St David’s area, Pembrokeshire. Both sanidine and albite phenocrysts are recorded.

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