Mineral Database (Saesneg yn unig)

Mineral Database (Saesneg yn unig)
Mineralogy of Wales

Cumengéite

Crystal System: Tetragonal
Formula: Cu20Pb91Cl47(OH)40
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Lead copper chloride hydroxide
Method(s) of Verification: Abersoch - XRD (Manchester Museum, MANCH:XRD378).
Chemical Group:
  • Halides
Geological Context:
  • Supergene : post-mining oxidation & weathering deposits
Introduction:

cumengéite is a supergene mineral, the formation of which requires a particularly high concentration of chloride ions to be present. This rather specific requirement means that the environmental conditions for the formation of cumengéite are of a tightly-constrained nature (Humphreys et al., 1980). Its single Welsh occurrence, within an intertidal zone (where the chloride concentration is increased by evaporation of sea-water) and in association with other chloride-bearing phases, is consistent in such terms to the small number of other occurrences reported in Britain.

Occurrence in Wales:

only one very limited occurrence of cumengéite is known from Wales, on a beach on Llŷn adjacent to a number of disused metal mines (Dossett & Green, 1998).

Key Localities:
  • Abersoch, Gwynedd: cumengéite occurs but is rare on Abersoch beach, forming light blue crusts which are composed of minute (<0.1 mm) bipyramidal crystals (Dossett & Green, 1998). Associated minerals are atacamite and rare botallackite. The host matrix consists of pebbles of vein-breccia cemented by quartz and containing chalcopyrite and galena.
References:
  • Dossett, I. & Green, D.I., 1998 Atacamite, botallackite and cumengeite from Abersoch, Gwynedd, Wales.  Journal of the Russell Society 7(1), 38-39.
  • Humphreys, D.A., Thomas, J.H. & Williams, PA., 1980 The chemical stability of mendipite, diaboleite, chloroxiphite and cumengéite, and their relationships to other secondary lead(II) minerals.  Mineralogical Magazine, 43, 901-904.