Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Magnesium carbonate
Chemical Formula: MgCO3
Method(s) of Verification: Ochyrwyth Quarry - XRD (Natural History Museum, x14629).
- Hydrothermal: serpentinization
- Supergene: in situ natural oxidation & weathering deposits
Introduction: magnesite typically occurs in altered (serpentinized) ultramafic igneous rocks, and it may also form as a replacement of dolomite in altered limestones.
Occurrence in Wales: the possible presence of magnesite in Wales was first reported by Henry (1830), who described a greenish-white foliated mineral, soapy to the touch, in narrow veins cutting serpentine from a 'low hill' located 'SW and within a mile' of Parys Mountain on Anglesey. This report has never been substantiated: neither has the inference by Greenly (1919) that some of the carbonates in the serpentinites and calc-silicate rocks associated with the Monian Supergroup (previously referred to as the 'Mona Complex'), also on Anglesey, may be magnesite. However, one isolated occurrence in South Wales has been verified.
- Ochrwyth Quarry, Risca, Gwent: at Ochr-Chwith (Ochyrwyth Quarry) magnesite was identified in 1968 as a component of a greyish-green crust consisting mainly of mixed smithsonite, hydrozincite, calcite and baryte. This appears to be an unusual example of supergene magnesite: however no more material of this nature has since been recorded despite much research into the mineralogy of the area.
- Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
- Henry, W., 1830. On the magnesite discovered in Anglesey. Edinburgh Journal of Science, new series, 2, 155-156.