According to Betterton (1989) "Birnessite has been identified by X-ray diffraction methods at the British Museum (Natural History),(X-ray number 6713F) on specimens submitted in 1987 from the Penberthy Croft Mine, St Hilary, Cornwall [SW 558 324]. The mineral occurs here in weathered blocks of partially-cemented slate-rich veinstone below the surface of an old dump. It forms black glossy coatings up to 2 mm thick on massive quartz, botryoidal woodwardite (6712F), earthy goethite, and dolomite with brochantite crystals (6711F), connellite (7407F), wroewolfeite (6710F), and chalcopyrite."
Penberthy Croft Mine, St Hilary, Cornwall, England, UK