arsenopyrite

{{Infobox mineral | name = Arsenopyrite | category = Sulfide mineral | boxwidth = | boxbgcolour = | image = Arsenopyrite-117874.jpg | imagesize = 260px | alt = | caption = | formula = FeAsS | strunz = 2.EB.20 | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = P21/c | unit cell = a = 5.744, b = 5.675 c = 5.785 [Å]; β = 112.3°; Z = 4 | colour = Steel grey to silver white | habit = Acicular, off-square prismatic, stubby; striated; also compact, granular, columnar | twinning = Common on {100} and {001}, contact/penetration twinning on {101} | cleavage = 110 (distinct) | fracture = Subconchoidal to rough | tenacity = Brittle | mohs = 5.5–6 | lustre = Metallic | refractive = | opticalprop = Anisotropism – strong red-violet | birefringence = | pleochroism = Weak, white or bluish tint, faint reddish yellow | streak = Black | gravity = 5.9–6.2 | melt = | fusibility = Yes | diagnostic = | solubility = Nitric acid | diaphaneity = Opaque | other = Garlic odour when struck, greenish tinge when weathered, green staining of wall rocks | references = }}

Arsenopyrite (IMA symbol: Apy) is an iron arsenic sulfide (FeAsS). It is a hard (Mohs 5.5–6) metallic, opaque, steel grey to silver white mineral with a relatively high relative density of 6.1.