Introduction: chalcanthite is a secondary mineral which develops in the oxide zone of copper-bearing ore bodies, particularly in arid regions. Natural crystals are very rare, although crystals are easily made artificially

Occurrence in Wales: the first written account of chalcanthite from Wales appears to be that by Aikin (1797), reporting the presence of ‘sulphate of copper, crystallized and in solution’ from Parys Mountain, on Anglesey. In 1858, Greg & Lettsom describe how at Pary’s mine, strips of iron are laid in the slime containing chalcanthite in solution and, when the iron is dissolved, copper is deposited in its place; for every ton of iron consumed there is thrown down a deposit of an equal weight of copper mud, containing about 12 per cent of copper. Clearly a considerable amount of copper was present within the mine waters, which, being highly acidic results in the deposition of chalcanthite on mine walls. A number of very rich, crystallized specimens reported as being from underground workings at Parys Mountain are held by the National Museum of Wales. Doubt remains over the authenticity of a number of these pieces as one particular specimen is perfect in terms of colour and form and looks to have been laboratory grown. Others however, show accurate provenancing and contain loose fragments of mine material within their growth structure.

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