Introduction: calcite occurs in a great diversity of parageneses and geological settings. It is an important rock-forming mineral (e.g. as the major constituent of limestone); it is a widespread product of low to medium-grade metamorphism and hydrothermal alteration in many rock-types and it is a common hydrothermal vein-mineral. The range of associated minerals is too extensive to quote. When well-crystallized the distinctive crystal habit and perfect rhombohedral cleavage of calcite make identification relatively straightforward.

Occurrence in Wales: calcite occurs throughout Wales and is known in fine specimens from many areas. In the older rocks of Wales it is chiefly found in hydrothermal vein-systems. It occurs throughout the Dolgellau Gold-belt, for example, both as a constituent of the mesothermal gold-bearing lodes and as a major product of the pervasive regional hydrothermal alteration that has affected the igneous intrusions of the area. It is a major vein mineral across Snowdonia in post-Caledonian Pb-Zn bearing veins, such as those mined in the Llanrwst area. Further south, in the Central Wales Orefield, calcite is a major constituent of certain assemblages. In the Upper Palaeozoic and Mesozoic rocks of Wales, calcite is an important rock forming mineral, given the abundance of limestones, marls and calcareous sandstones. It is also a major component of both the Fe-oxide and the Pb-Zn veins that are hosted by Carboniferous limestone-dominated strata in north-east and South Wales. Calcite is also an important supergene mineral in some weathered ore deposits, particularly at the Great Orme copper mines. There are in fact few localities in Wales that lack calcite in some form or other. Because it is so widespread, only the most notable specimen localities are listed here.

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