This mineral commonly forms as octahedral or pseudocubic crystals. Crystal faces ar usually striated. Other habits include massive, granular, and compact. The color varies from grayish black to silvery white. When tested for streak, a grayish-black powder is produced.
Cobaltite is opaque; light is unable to pass through it, even when it is in thin pieces. It has a metallic luster on fresh crystal, or broken, surfaces.
FORMATION Forms in hydrothermal veins (fractures in the earth’s crust through which hot fluids circulate, depositing minerals as they cool) and also in metamorphic rocks with other arsenides and sulfides.
TESTS Fuses quite easily, forming a globule that is slightly magnetic. Cobaltite is soluble in nitric acid.
Group: SULFIDES
Composition: CoAsS
Hardness: 5½
SG: 6.33
Cleavage: Perfect
Fracture: Uneven
Silver