Phosgenite

Phosgenite is a rare mineral consisting of lead chlorocarbonate, (PbCl)2CO3. The tetragonal, holosymmetric, crystals are prismatic or tabular in habit, and are bounded by smooth, bright faces: they are usually colorless and transparent, and have a brilliant adamantine lustre. Sometimes the crystals have a curious helical twist about the tetrad or principal axis. The hardness is 3 and the specific gravity 6.3. The mineral is rather sectile, and consequently was earlier known as corneous lead. The fanciful name phosgenite was given by A. Breithaupt in 1820, from phosgene, carbon oxychloride, because the mineral contains the elements carbon, oxygen and chlorine. At Cromford, near Matlock, it was long ago found in an old lead mine, being associated with afiglesite and matlockite (Pb2OC12) in cavities in decomposed galena: hence its common name cronfortite. Fine crystals are also found in galena at Monteponi near Iglesias in Sardinia, but the largest are those recently found near Dundas in Tasmania. Crystals of phosgenite, and also of the corresponding bromine compound PbBr2CO3, have been prepared artificially.