MISPICKEL, a mineral consisting of iron sulpharsenide,
FeAsS; it contains 46% of arsenic, and is of importance as an
ore 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 (110) and g (101) 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 512–6, 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), löllingite (FeAs2), safflorite (CoAs2) and rammelsbergite (NiAs2). (L. J. S.)