Alloclasite

Alloclasite ((Co,Fe)As S) 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 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.[1]

It was first described in 1866 for an occurrence in Romania.[2] 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.[3][1]

The mineral is monoclinic in the P2₁ space group.[4]

References

http://rruff.geo.arizona.edu/doclib/hom/alloclasite.pdf Handbook of Mineralogy
http://www.mindat.org/min-134.html Mindat data
http://www.webmineral.com/data/Alloclasite.shtml Webmineral data
Scott, J.D.; Nowacki, W. (1976). "The crystal structure of alloclasite, CoAsS, and the alloclasite-cobaltite transformation". The Canadian Mineralogist. 14: 561–566.

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[Handbook-1] ttp://rruff.geo.arizona.edu/doclib/hom/alloclasite.pdf Handbook of Mineralogy

[2] ttp://www.mindat.org/min-134.html Mindat data

[3] ttp://www.webmineral.com/data/Alloclasite.shtml Webmineral data

[4] Scott, J.D.; Nowacki, W. (1976). "The crystal structure of alloclasite, CoAsS, and the alloclasite-cobaltite transformation". The Canadian Mineralogist. 14: 561–566.