Precipitated in mineral veins from fluids with complex chemistry and dissolved elements, this ore of antimony also contains copper and lead in its formula. Dense and opaque it was first mentioned in 1797 and named in 1804 by the mineralogist who first described it, after an aristocratic French mineralogist who had done a lot of research in England during his exile there during the revolutionary period, including becoming a founder member of the Geological Society (1789-1815). It was first found in the antique mines of Cornwall (active since the Bronze age, when the tin ore cassiterite (see http://bit.ly/1xOVBAs ) was the main export, the first recorded name for the Isles being the Cassiterides), and was originally known as Endellionite (after the parish where it occurred), though the more colloquial miner's term for it has always been cart or cog wheel ore.
The crystals are often twinned, ie growing separately but sharing one or more planes of contact, leading to the typical shape, though single prism shaped examples are also known. The cogwheels are composed of four crystals in a trilling type twin (see http://bit.ly/1XRqRer for a superb gemmy example).
Dense and opaque while like many lead minerals possessed of bright often metallic reflectivity (a property known as lustre), the mineral is soft (2.5-3 on Mohs scale) and hence not often faceted for collectors. The usual colours are steel grey to black, though some iridescent specimens are known, a property induced by fine layering like petrol on a puddle. As well as the type region it has also turned up in the Harz Mountains of Germany, California, Mexico, Peru, Australia, Japan, China and Romania.
This 95 x 74 x 46 mm example comes from the Viboras mine in Bolivia.
Image credit: Carles Millan
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