Alloclasite, or , is a sulfosalt mineral (IMA symbol: Acl). It is a member of the arsenopyrite group. Alloclasite crystallizes in the monoclinic system and typically forms as columnar to radiating acicular prismatic clusters. It is an opaque steel-gray to silver-white, with a metallic luster and a black streak. It is brittle with perfect cleavage, a Mohs hardness of 5 and a specific gravity of 5.91–5.95.http://rruff.geo.arizona.edu/doclib/hom/alloclasite.pdf Handbook of Mineralogy
It was first described in 1866 for an occurrence in Romania.http://www.mindat.org/min-134.html Mindat data Its name is derived from Greek for "other" and "to break", in reference to its distinct cleavage which distinguished it from the similar appearing mineral marcasite.http://www.webmineral.com/data/Alloclasite.shtml Webmineral data
The mineral is monoclinic in the P21 space group.
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