MISPICKEL  , a

mineral consisting of iron sulpharsenide, FeAsS; it contains 46% of arsenic, and is of importance as anore of this element . It is known also as arsenopyrite or arsenical pyrites (Ger . Arsenikkies) : mispickel is an old name of German origin, and in the form Mistpuckel was used by G . Agricola in 1546 . The crystals are orthorhombic, with angles similar to those of marcasite; they are often prismatic in habit, and the prism M is usually terminated by the deeply striated faces of an obtuse dome r . Twinning is not uncommon, the twin- planes . M (no) and g (1o1) being the same as in marcasite . The colour of the mineral is silver-white or steel-grey, with a metallic lustre, but it is often tarnished yellow; the streak is greyish-black . The hardness is 51-6, and the specific gravity 5.9–6.2 . Mispickel occurs in metalliferous veins with ores of tin, copper, silver, &c . It is occasionally found as embedded crystals, for example, in serpentine at Reichenstein, Silesia . In Cornwall and Devon it is associated with cassiterite in the tin-lodes, but is also found in the copper-lodes: well crystallized specimens have been obtained from the neighbourhood of Tavistock, Redruth and St Agnes .

Mispickel is the

principal source of arsenious oxide or the " white arsenic " of commerce (see ARSENIC) . The chief supplies are from Cornwall and Devon, and Freiberg in Saxony, and from Canada and the United States . Danaite is a cobaltiferous variety of mispickel, containing up to 9% of cobalt replacing iron; it was first noticed by J . F . Dana in 1824 at Franconia in New Hampshire . This variety forms a passage to the species glaucodote, (Co,Fe)AsS, which is found as well-developed orthorhombic crystals in copper ore at Hakansboda in Ramberg parish, Vestmanland, Sweden . Other species belonging to this isomorphous group of orthorhombic minerals are marcasite (FeS2), lollingite (FeAs2), safflorite (CoAs2) and rammelsbergite (NiAs2) . (L . J .