Magnesite

Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Magnesium carbonate
Chemical Formula: MgCO3
Method(s) of Verification: Ochyrwyth Quarry - XRD (Natural History Museum, x14629).

Chemical Group:

  • Carbonates

Geological Context:

  • 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.

Key Localities:

  • 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.

References:

  1. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  2. Henry, W., 1830. On the magnesite discovered in Anglesey. Edinburgh Journal of Science, new series, 2, 155-156.