Introduction: pyrite is the commonest of the sulphide minerals and is far more widespread than its orthorhombic dimorph, marcasite. It occurs in many geological settings, including sedimentary, igneous and metamorphic rocks of all ages and in a great range of hydrothermal mineral deposits. It is also abundant in many placer deposits. Because of its widespread occurrence, associated minerals are highly diverse. Pyrite is readily identified by its cubic crystal shape when well-formed. In polished section it often shows high relief against other sulphides due to its greater hardness: it polishes well when fresh and has a relatively bright appearance. The instability of pyrite (pyrite-decay) is well-known to curators of both mineral and fossil collections. Pyrite can react with air and moisture, the reaction creating an acidic solution of iron which then goes on to attack the remaining pyrite plus other more stable sulphides such as galena and sphalerite. However, not all pyrite specimens are prone to this effect. Relatively high-temperature pyrite, formed in mesothermal mineral veins or as porphyroblasts in metamorphic rocks, is often relatively stable. Conversely, it is the lower-temperature varieties, including the botryoidal form known as melnikovite-pyrite, that are most prone to decay. These occur in sedimentary and low-grade metamorphic rocks and in low-temperature hydrothermal assemblages. Often such varieties are intergrown with the even more unstable dimorph, marcasite, and in such cases decay may set in within days of sample collection. Once initiated, the process is extremely difficult to arrest: prevention of its initiation is therefore the key. Any sample suspected to be at risk from pyrite decay must be kept in as cold and dry an atmosphere as possible: minimising moisture minimises the risk of decay initiation and the lower the temperature, the slower any decay reaction will be because, as with many chemical reactions, heat speeds up the rate of chemical activity.

Occurrence in Wales: pyrite occurs throughout most of Wales as a component of igneous, sedimentary and metamorphic rocks. It commonly replaces fossils, especially in black shale sequences, such as those found in the Lower Palaeozoic strata of Central Wales where pyritized graptolites are locally common. It is a familiar sight as thin golden sheens on household coal and in the past these have been mistaken by the unwary for gold - 'fool's gold' is a common nickname for pyrite. Gold is soft and malleable - ie. it bends under pressure. Pyrite is hard and brittle - it breaks and crumbles. There are few hydrothermal ore deposits in Wales that do not contain pyrite but in some it is very common and was actually mined in its own right (for sulphuric acid manufacture) in some places such as the Cae Coch Mine near Trefriw in the Conwy valley. Only those localities where pyrite is of paragenetic or specimen significance are noted here.

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