Pyrite

Formula: FeS₂

Origin of name: from Greek pyr = fire

Crystal system: cubic

Crystal class: 2/m -3

Twinning: about [110] axis, penetration (iron cross)

Unit cell: a = 5.417 Å

Colour: brass-yellow

Diaphaneity: opaque

Luster: metallic

Habit: crystals cubic, pyritohedral, octahedral and combinations, abnormally developed, rarely acicular, massive, granular

Hardness: 6 to 6.5

Specific gravity: 5.018 to 5.028

Cleavage: 3; {100} and {311} indistinct, {110} occasional parting

Tenacity: brittle

Streak: greenish black

Synonyms/varieties: bravoite (Ni-rich variety), fool's gold

Comments: dimorphous with marcasite, series with cattierite, ferroselite