Brochantite star
Many of us have grown similar crystals by hanging a piece of string into a copper sulphate solution and letting it evaporate. In nature, an arid climate or an oxidising (see http://bit.ly/1I4XWKt for an explanation) primary copper sulphide deposit is required for this mineral to form. Colour ranges from bright emerald green to blue green, though its softness (3.5 on Mohs scale) prevents faceting or jewellery use.
Since it forms in the same environments as malachite and azurite (copper carbonates), they are frequently found together, and sometime brochantite replaces the other minerals, taking on their crystal shape in a process called pseudomorphism (from pretend shape in Greek). The more usual distinguishing shape is as sprays of acicular (needle shaped) crystals, often radiating from a common centre. It go was named in 1824 after a French mineralogist, Brochant de Villiers and is an important ore mineral.
Main localities include the copper mines of the Atacama in Chile, the Zaire copper belt in the Democratic Republic of Congo, the Tsumeb mine (see http://bit.ly/1c2CjkD) and Arizona in the USA. A few stunning specimens (including the 2.5 x 2.5 x 1.8 cm one in the photo) have recently emerged from the Milpillas mine in Sonora, Mexico.
Loz
Image credit: Spirifer Minerals
http://www.mindat.org/min-779.html http://www.galleries.com/Brochantite http://bit.ly/1N0Tqox http://bit.ly/20pv7TD
