LIROCONITE  , a rare

mineral consisting of hydrous basic copper and aluminium arsenate, with the probable formula Cu9AI4(OH),5(AsO4)a30H2O . It crystallizes in the monoclinic system, forming flattened octahedra almost lenticular in shape (hence the German name Linsenkupfer) . Characteristic is the bright sky-blue colour, though sometimes, possibly owing to differences in chemical composition, it is verdigris-green . The colour of the streak or powder is rather paler; hence the name liroconite, from the Gr . Xeip6s, pale, and Kovia, powder . The hardness is 2, and the specific gravity 2.95 . The mineral was found at the beginning of the 19th century in the copper mines near Gwennap in Cornwall, where it was associated with other copper arsenates in the upper, oxidized portions of the lodes . (L . J .