THE MINERAL BOURNONITE

Chemistry: CuPbSbS3, Copper lead antimony sulfide
Class: Sulfides
Subclass: Sulfosalts
Uses: minor ore of lead and copper and as a mineral specimen
Specimens

Bournonite is also called "cog wheel ore" because of the cog wheel shape that the twinned crystals form. Twinning is common in bournonite and if a crystal repeatitively twins it can form a type of twin called a trilling. The trilling is composed of four "twins" or crystals connected in a plane and forms a wheel with a jagged edge that resemble the teeth of a cog wheel. Not all specimens show this type of twinning however.

PHYSICAL CHARACTERISTICS:

Color is silver gray or black.
Luster is metallic.
Transparency crystals are opaque.
Crystal System is orthorhombic; 2/m2/m2/m
Crystal Habits include tabular to prismatic crystals. Twinning is common and if repeated forms flat wheel shaped crystals called cog wheels. Also massive and granular.
Cleavage is poor in one direction.
Fracture is subconchoidal.
Hardness is 2.5 - 3
Specific Gravity is approximately 5.8 (above average for metallic minerals)
Streak is black.
Associated Minerals are siderite, fluorite, galena, sphalerite, calcite and pyrite.
Other Characteristics: although the luster can be bright, bournonite develops a dull tarnish. Crystals are usually striated on their sides which produces the "teeth" of the cog wheel.
Notable Occurrences include England; California, USA; Mexico; Peru and Australia.
Best Field Indicators are crystal habit (especially twinning), color and density.