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.
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Available BOURNONITE specimens:
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