Cumengéite

Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Lead copper chloride hydroxide
Chemical Formula: Cu20Pb91Cl47(OH)40
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.

There are no key localities for this specimen.

References:

  1. Dossett, I. & Green, D.I., 1998. Atacamite, botallackite and cumengeite from Abersoch, Gwynedd, Wales. Journal of the Russell Society 7(1), 38-39.
  2. 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.

There are no references for this specimen.