Alloclasite
Alloclasite, or (Co,Fe)AsS, is a sulfosalt mineral (IMA symbol: Acl).[1] 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 5.91โ5.95.[2]
It was first described in 1866 for an occurrence in Romania.[3] 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.[4][2]
The mineral is monoclinic in the P21 space group.[5]