Realgar

Formula: AsS

Origin of name: from Arabic Rahj al ghar = powder of the mine

Crystal system: monoclinic

Crystal class: 2/m

Twinning: on {100} contact

Unit cell: a = 9.32 Å, b = 13.53 Å, c = 6.59 Å, b = 106.4°

Colour: orange-red to dark red

Diaphaneity: 

Luster: resinous to greasy

Habit: crystals short prismatic, striated parallel to c-axis, massive, compact, coarse- to fine-grained, powdery coatings

Hardness: 1.5 to 2

Specific gravity: 3.56

Cleavage: 4; {010} distinct, {-101}, {100} and {120} indistinct

Tenacity: conchoidal, sectile

Streak: orange-yellow

Synonyms/varieties:

Comments: dimorphous with pararealgar, alters to yellow-orange powder on exposure to light