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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.
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.
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.