Introduction: antigorite belongs to the kaolinite-serpentinite group of minerals and is one of three minerals (antigorite, lizardite and chrysotile) commonly referred to as ‘serpentine’. Antigorite and lizardite are soft green platy minerals, whereas chrysotile is fibrous. These minerals commonly result from the hydrothermal or retrograde metamorphism of mafic minerals such as olivine, pyroxene or amphibole, usually occurring in basic and ultrabasic igneous rocks. In general antigorite is less common than lizardite.

Occurrence in Wales: all recorded occurrences of antigorite from Wales are associated with exposures of serpentinite, or rocks linked to the serpentinization of ultrabasic and basic rocks, within the New Harbour Group of the Monian Supergroup in Anglesey. These rocks typically form scattered exposures, but they can be grouped into two main occurrences, Holy Island and NW Anglesey. The mineral associations are broadly similar in all occurrences although the abundance and nature of associated minerals may vary locally. It is these serpentinized rocks that were once referred to as ‘Mona Marble’ and worked as an ornamental stone (see Horák, 2005 for further details).

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