Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Mercury sulphide
Chemical Formula: HgS
Method(s) of Verification: Machen Quarry - qualitative energy dispersive analysis (T.F. Bridges, unpublished data).

Chemical Group:

  • Sulphides

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Introduction: cinnabar occurs as a primary mineral, typically forming in near-surface, low-temperature ore deposits. Primary mercury deposits of this type are, however, not known from Wales and are in general very scarce in the UK. Nevertheless, small amounts of mercury may occur as a trace element in polymetallic vein deposits and then minute quantities of cinnabar may be found in supergene mineral assemblages derived by their weathering. Cinnabar, when occurring as a supergene mineral, can be difficult to identify positively by visual means alone, especially so because there are a number of similar-looking rare minerals such as the lead oxide, litharge.
Occurrence in Wales: only two occurrences of cinnabar are known from Wales - one from the Central Wales Orefield and the other from South Wales. They both fall into the supergene category in their mode of occurrence. Their association with supergene lead mineralization may reflect minor occurrences of mercury-bearing inclusion-forming phases in galena, such as tetrahedrite.

Key Localities:

  • Bwlch-glas Mine, Tal-y-bont, Ceredigion: Braithwaite (1982) noted the presence of very rare cinnabar in the underground workings, as brick-red powdery spots in gossany, cerussite-bearing material.
  • Machen Quarry, Caerphilly, South Wales: cinnabar has been visually identified within the extensively-developed supergene Pb-Zn-Cu assemblage at this locality. This record requires confirmation.


  1. Braithwaite, R.S.W., 1982b. Pyromorphite, wulfenite and other minerals from the Bwlch-Glas mine, Central Wales. Mineralogical Record, 13, 151-153.