Introduction: marcasite occurs in a variety of settings but is most commonly seen as a primary mineral in low-temperature hydrothermal mineral veins, where it is associated with pyrite, sphalerite, wurtzite, galena and other sulphides. Supergene marcasite is also widespread and is commonly formed during the weathering of pyrrhotite. It also occurs in sedimentary rocks, where it is most commonly found forming, or occurring within, concretionary nodules. Marcasite is easy to identify when well-crystallized, forming prismatic crystals, often twinned into cogwheel-like forms. Massive marcasite is more of a yellow-white colour compared to pyrite but it rapidly tarnishes due to the onset of decay, after which its noxious, sulphurous smell is distinctive. The instability of marcasite when exposed to moist air is a well-known problem: the initial products include sulphuric acid which goes on to attack other sulphides and wallrock minerals, thus liberating other metals which can lead to a significant pollution problem in waters draining from mineworkings. The same process can affect inappropriately-stored marcasite specimens, the conservation of which is a well-known problem to museum curators.

Occurrence in Wales: marcasite is a widespread mineral in Wales, occurring in considerable amounts in some mining districts, notably in the Central Wales and Llanrwst Orefields, where its abundance and ready decomposition into acidic iron-rich solutions causes problems in some areas due to polluted minewater. Although some fine specimens have been obtained, the mineral's instability has made the preservation of many of these impossible. Locality data below focuses on the mineral's distribution as this is in most cases far more relevant than the specimens recovered.

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