1911 Encyclopædia Britannica/Pharmacosiderite
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., Würfelerz), the name pharmacosiderite, given by J. F. L. Hausmann in 1813, alludes to the arsenic and iron present (φάρμακον, and σίδηρος, 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 212; and the specific gravity 2·8. Recent analyses prove the presence of a small but variable amount of potassium (K2O, 2·68 – 4·13%) in the Cornish crystals, though in those from Hungary there is only 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 Königsberg near Schemnitz in Hungary, and in the Tintic district in Utah. (L. J. S.)