PHARMACOSIDERITE  , a

mineral species consisting of hydrated basic ferric arsenate, 2FeAsO4 Fe(OH)3.5H2O . Crystals have the form of small, sharply defined cubes of an olive- or grass-green colour, and occur together in considerable numbers on the matrix of the specimens . On account of its cubic form the mineral was early known as " cube ore " (Ger., Wilrfelerz); the name pharmacosiderite, given by J . F . L . Hausmann in 1813, alludes to the arsenic and iron present (4 fipaaKov, poison, and rL pos, iron) . The faces of the cube are striated parallel to one diagonal, and alternate corners are sometimes replaced by faces of a tetrahedron . The crystals are feebly doubly refracting, and in polarized light exhibit a banded structure parallel to the cube faces . The hardness is 22 and the specific gravity 2.8 . Recent analyses prove the presence of a small but variable amount of potassium (K20, 2.68—4.13%) in the Cornish crystals, though in those from Hungary there isonly a trace; this constituent appears to take the place of basic hydrogen in the above formula . A curious property is to be observed when a crystal of pharmacosiderite is placed in a solution of ammonia—in a few minutes the green colour changes throughout the whole crystal to red; on placing the red crystal in dilute hydrochloric acid the green colour is restored . Natural crystals are sometimes honey-yellow to brown in colour, but this appears to be due to alteration .

Pharmacosiderite is a mineral of secondary origin, the crystals occurring attached to gozzany

quartz in the upper part of veins of copper ore . It was found in some abundance at the end of the 18th century in the copper mines of the St Day district in Cornwall, and has since been found at a few other localities, for example, at Konigsberg near Schemnitz in Hungary, and in the Tintic district in Utah . (L . J .