Alloclasite

Alloclasite ((Co,Fe)AsS) is a sulfosalt mineral. 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 lustre 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]

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