Wurtzite skyscraper
Resembling some kind of habitat around a distant asteroid on a science fiction book cover, these prisms of zinc sulphide are made of a close cousin of the zinc sulphide sphalerite (seehttp://on.fb.me/1GaU0GY). Wurtzite shares the same chemical formula but crystallises in a different structure, quite a common feature in the mineral world with the varied forms being known as polymorphs (from the Greek for many shapes). It has another unusual feature to its structure, known as hemimorphy (from the Greek for half shape) , in which there is no mirror image symmetry between top and bottom of the crystal, meaning that the top and bottom ends display different faces.
The common form is a six sided prisms or plates often striated parallel to the base, and the usual colour a very dark reddish-orangey brown, often transparent in a strong fibre optic light. It is a soft mineral (3.5=4 on Mohs scale) and relatively dense. It was named in 1861 after a French chemist and is found in veins that have precipitated from metal rich hydrothermal fluids or as concretions in sedimentary rocks during the long arduous passage from sediment to stone.
The photo was taken at magnification, but I sadly don't have the field of view scale data to share with you.
Loz
Image credit: under the scope min
