Cotunnite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Lead chloride
Chemical Formula: PbCl2
Method(s) of Verification: Penrhyn Du Mine - XRD at Manchester Museum (X-ray reference number MANCH:XRD727).

Chemical Group:

  • Halides

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
  • Supergene: in situ natural oxidation & weathering deposits
Introduction: cotunnite occurs as a volcanic sublimate and as an alteration product of galena under saline conditions.
Occurrence in Wales: cotunnite has been described (Hubbard & Green, 2005) within weathered galena-rich veinstone from the phosgenite locality at Penrhyn Du on the Llŷn. It was found as bladed microcrystals in material collected in situ from a vein exposed below the High Water mark. It is likely that a reaction between chlorine in the seawater and lead within galena in the veinstone has produced this rare mineral.

Key Localities:

  • Penrhyn Du, Abersoch, Llŷn, Gwynedd: cotunnite was described by Hubbard & Green (2005) forming whitish translucent tapering bladed crystals to 4 mm in a cavity in oxidized galena and was associated with a few tiny sulphur crystals. Hubbard & Green note that phosgenite is far more common than cotunnite at Penrhyn Du.

References:

  1. Hubbard, N. & Green, D.I., 2005. Cotunnite from Penrhyn Du Mine, Gwynedd, Wales. Journal of the Russell Society, 8(2), 106-107.