Introduction: gibbsite is a secondary mineral which develops typically by the break-down of aluminous or aluminosilicate minerals in tropical or sub-tropical conditions. It is characteristic of laterites, clay deposits and highly-weathered soils; being the most commonly occurring crystalline aluminium hydroxide mineral in the latter. In general gibbsite is very stable under surface conditions, however under exceptional conditions it alters to clay minerals such as kaolin.

Occurrence in Wales: the two occurrence of gibbsite from Wales both have a supergene origin, although one occurs on a natural substrate, the other on mine waste. Although gibbsite is indicative of weathering under tropical or sub-tropical conditions, it’s occurrence at Glasdir Mine, first worked in the mid 1800s, clearly shows that it can form under range of conditions that is typically reported in the literature.

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