Copper lead antimony sulfide
A rare and well formed Bournonite Crystal. This is a great metal specimen, displaying the characteristic ‘cogwheel’ crystal formation of this material. A truly special gift for any mineral collector. Like yourself! This specimen is mounted on a perspex pad for enhanced display.
2.1cm x 2.6cm x 2cm, 28g.
Yaogangxian Mine, Chenzhou Prefecture, Hunan Province, China.
Named after Count de Bournon, the famous French crystallographer and mineralogist. Count de Bournon was one of the founders of the Geological Society in 1807. He also authored several important mineralogy catalogs. Called “cog wheel ore” because of the cog wheel shape that the twinned crystals form. Twinning is common in Bournonite and if a crystal repetitively 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.
Colour 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), colour and density.