Alloclasite

Formula:  (Co, Fe)AsS

Origin of name:  From Greek allos = another, and klas = to cleave or to break, because it was believed to be different from marcasite, which it resembles

Crystal system:  Monoclinic

Crystal class:  2

Twinning:

Unit cell: a = 4.661 Å, b = 5.602 Å, c = 3.411 Å, b = 90.2°

Colour:  Steel-gray to silver

Diaphaneity: Opaque

Luster:  Metallic

Habit:  Crystals short prismatic; massive; columnar to hemispherical aggregates.

Hardness:  5

Specific gravity: 5.95 to 6.17 166

Cleavage:  2; {101} perfect; {010} distinct.

Tenacity: Brittle

Streak:  Nearly black.

Synonyms/varieties: none

Comments:  Dimorphous with glaucodot. In calcite or quartz veins of apparently low temperature, late stage hydrothermal origin. Also in silicified, recrystallised metamorphic rocks