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What is a Mineral?

‘In general terms, a mineral is an element or chemical compound that is normally crystalline and which has been formed as a result of geological processes’ - E.H. Nickel (1995)

However, this definition means that some minerals, such as those found in cavities in weathered smelter-slags or encrusting building-rubble, are no longer regarded as minerals. The distinction is based on the notion that Man has had too much involvement in their formation because the matrix or host-material is itself man-made. At present such minerals continue to be included within The Mineralogy of Wales database as do a few organic compounds.

Copper slag from the Neath Valley, containing vesicles filled with bright-coloured supergene copper minerals including botallackite. D. Evans Collection. Photo T.F. Cotterell, © National Museum of Wales.