Arsenopyrite

Formula:  FeAsS

Origin of name:  From arsenic and pyrite

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning:  Common on {100} or {001} on {101}, contact or penetration, on {012}, cruciform, star-shaped trillings

Unit cell:   a = 5.74 Å, b = 5.68 Å, c = 5.79 Å, β = 112.3°

Colour:  Silver-white to steel-gray

Diaphaneity: Opaque

Luster:  Metallic

Habit:  Crystals short prismatic, may be elongated parallel to c-axis or b-axis; massive, compact, granular

Hardness:  5.5 to 6

Specific gravity:  6.07

Cleavage:  2; {101} distinct, {010} in traces

Tenacity:  Uneven; brittle

Streak:  Black

Synonyms/varieties:  Dalarnite,  mispickel, arsenical pyrite, danaite (Co-rich var.), Bismuthian arsenopyrite (Bi-containing var.), Cobaltian arsenopyrite (Co-containing var.)

Comments: Of hydrothermal origin, typically one of the earliest minerals to form. Found in pegmatites, high-temperature gold-quartz and tin veins and in contact-metamorphic sulphide deposits. Also in gneisses, schists and other metamorphic rocks.