Marcasite

Formula:  FeS₂

Origin of name:  From Arabic or Moorish for pyrite

Crystal system:  Orthorhombic

Crystal class:   2/m 2/m 2/m

Twinning:  Common on {101}, repeated; also on {011}

Unit cell: a = 4.443 Å, b = 5.424 Å, c = 3.387 Å

Colour:  Pale brass-yellow to tin-white, darkens on exposure

Diaphaneity: 

Luster:  Metallic

Habit:  Crystals tabular or pyramidal, prismatic or capillary; massive, fine granular; stalactitic; globular; reniform

Hardness:  6 to 6.5

Specific gravity:  4.92

Cleavage:  2; {101} distinct; {110} traces

Tenacity:  Uneven; brittle

Streak:  Greenish black

Synonyms/varieties:

Comments:  Dimorphous with pyrite