Arsenic

Formula:  As

Origin of name:  From Greek arrenikos or arsenikos for arsenic (later from Latin arsenicum) meaning masculin, an allusion to its potent properties, originally applied to the mineral orpiment

Crystal system:  Hexagonal

Crystal class: -3 2/m

Twinning:  Rare on {10-14}; pressure twinning on {01-12}results in delicate lamellae

Unit cell: a = 3.76 Å, c = 10.55 Å

Colour:  Tin-white, tarnishes dark gray

Diaphaneity: Opaque

Luster:  Metallic on fresh fracture

Habit:  Small rhombohedral or acicular crystals; granular massive; stalactitic; reniform, reticulated

Hardness:  3.5

Specific gravity:  5.7 to 5.73

Cleavage:  2; {0001} perfect, {10-14} fair

Tenacity:  Uneven; brittle

Streak:  Tin-white

Synonyms/varieties:

Comments:  Dimorphous with arsenolamprite. Formed in hydrothermal veins and deposits that contain other arsenic minerals, sometimes in Co-Ag veins.