The composition of tsumebite from Roughton Gill Mine, Caldbeck Fells, Cumbria

There is extensive substitution of arsenate for phosphate in tsumebite from Roughton Gill Mine in the Caldbeck Fells, Cumbria. Calculated chemical formulae are in excellent agreement with the general brackebuschite group formula.The most arsenate-rich material has more than 50 atom substitution in the phosphate site and can therefore be described as arsentsumebite. Arsentsumebite occurs as rims a few tens of micrometres thick, which surround tsumebite cores. This indicates an increasing arsenate activity as the mineral crystallised. Tsumebite is commonly found with well crystallised brochantite and is often associated with slightly corroded cerussite. This association provides an indication of the chemical conditions in which it formed, probably at pH of about 6 with a high sulphate ion activity.