Phosgenite

Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Lead carbonate chloride
Chemical Formula: Pb2CO3Cl2
Method(s) of Verification: Cwmavon Valley Copper Smelting Slags - XRD (Natural History Museum, film no. 12022); Eaglebrook Mine - visual, based on the distinctive morphology of phosgenite crystals; Penrhyn Du - XRD (Dr. P.A. Williams).

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
  • Supergene: minerals on an artificial substrate
Scanning electron micrograph of tabular phosgenite crystals lining a cavity in weathered galena from Penrhyn Du, Ll?n. National Museum of Wales specimen (NMW 98.16G.M.118). © National Museum of Wales.
Introduction: phosgenite is a secondary lead mineral typically developed in the oxidized zone of lead-bearing ore bodies and through post-mining oxidation of lead-bearing veinstone and metalliferrous slags, particularly where there is prolonged contact with sea water.
Occurrence in Wales: in Wales phosgenite is restricted to a small number of dominantly post-mining occurrences where, crystals are small and inconspicuous except to the trained eye. Hubbard (1991) provided the first account of phosgenite in Wales describing, prismatic and tabular microcrystals on loose blocks of galena-rich veinstone, below high water in a gulley beneath the Penrhyn Du Mine on the Llŷn. Smaller tabular crystals are reported in-situ where the vein outcrops with the sea. In actual fact, phosgenite was known from Wales since 1964 when, R.S.W. Braithwaite collected a specimen from dumps near Halkyn in Flintshire (Bevins, 1994). Phosgenite has since been discovered to a limited extent at Eaglebrook Mine in the Central Wales Orefield (Rust et al., 1995), and a recent investigation by Plant (2003) into the mineralogy of copper smelter slag in the Cwmavon Valley, South Wales has identified minute (0.15 mm) phosgenite crystals, perched on the slag surface.

Key Localities:

  • Cwmavon Valley Copper Smelting Slags, Aberavon, South Wales: the only lead mineral to have been identified in the copper smelting slags. Phosgenite occurs on the slag surface as single isolated crystals and in small groups, typically forming transparent to translucent multi-faceted barrel-shaped crystals, displaying growth zones, and long prismatic crystals with polyhedral terminations (Plant, 2003).
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: Rust et al. (1995) record phosgenite on five micro-specimens derived from material collected from the eastern shaft dump. Phosgenite occurs as blocky, colourless crystals to 0.75 mm. The crystals show a large pinacoid, and striated prism faces parallel to the c-axis, consistent in form with phosgenite. The prism edges have a rounded appearance. On some specimens, tiny drum-like phogenite crystals are scattered on cerussite. On others, phosgenite occurs in small cavities in relatively fresh galena, commonly with anglesite and cerussite. On two of the specimens covellite is also associated.
  • Pen-y-bryn shaft dumps, Halkyn, Clwyd: a specimen in the R.S.W. Braithwaite Collection (no. 64-403) collected on 15 November 1964 contains phosgenite (Bevins, 1994).
  • Penrhyn Du Mine, Llŷn, Gwynedd: phosgenite was discovered by Hubbard (1991) in loose blocks of veinstone below high water mark in a gulley below Penrhyn Du Mine. Crystals take the form of pink-white striated prisms and colourless tabular plates, up to 1.5 mm in size. Phosgenite is also reported by Hubbard (1991) in-situ, within cavities in a similar galena vein, exposed approximately 100 m to the south. Crystals are smaller (to 0.25 mm), colourless and tabular in form.

There are no key localities for this specimen.

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Hubbard, N., 1991. Phosgenite, the first Welsh occurrence. Journal of the Russell Society, 4 (1), 35.
  3. Plant, S., 2003. Secondary minerals from the Cwmavon Valley Copper Smelting Slags, Glamorgan, South Wales. Journal of the Russell Society, 8(1), 9-15.
  4. Rust, S., Burchmore, S. & Foy, E., 1995. Some interesting new finds from the Nant-y-cagle mine. British Micromount Society Newsletter, 40, 8-10.

There are no references for this specimen.