Gypsum, Desert rose var.

Gypsum, Desert rose var.

Gypsum, CaSO. 4· 2H2O is an evaporite mineral. It is precipitated from mineral rich bodies of water, either lakes or seas, as they dry up,and are no longer able to hold the minerals in suspension. In arid areas rich in silica sand, crystals are formed  which incorporate sand grains in their growth, and " Desert roses" are formed. There are a number of minerals which form Desert roses, the two most common being baryte or gypsum. The origin of this gypum rose is unknown,and a guess at it's age would be Permian.  

Quartz, Amethyst var.

Quartz, Amethyst var.

This specimen of amethyst var. quartz, SiO2 is very unremarkable in appearance, but is included, simply because it is listed in my mineral collection. The scalenahedral crystals are crisp, and unchipped. Amethyst quartz is coloured by irradiation, and trace amounts of iron. The origin of the specimen is unknown.

Collectors Paradise.

Collectors Paradise.

There are so many minerals coming from Broken Hill in Australia that there is a good chance that many mineral collections will have at least one specimen from there. The street names of the town reflect the importance of the trade to the town.

Prehnite, after laumonite.

Prehnite, after laumonite.

Prehnite is an inosilicate of calcium and aluminium with the formula Ca2AlSi3O12(OH) Although not classed as a zeolite, it is often found associated with minerals which are zeolites such as apophyllite, heulandite, datolite, etc. It's colour varies from light green to yellow, and may be colourless. Crystals of prehnite can be reniform, botryoidal or stalactitic. This specimen is a pseudomorph of prehnite after laumonite, in other words original laumonite has been replaced by prehnite. It's origin is only known as India, but it is probably from the Mumbai district.

Natrolite.

Natrolite.

Natrolite is a  tectosilicate  mineral, in the zeolite group. It is a hydrated sodium and aluminium silicate with the formula  Na2Al2Si3O10 . 2H2O[3] . The specimens locality is not known, but the type locality is basaltic rock.  From a base rock of basalt, prismatic dirty white to colourless crystals, with a hint of green at their bases, have obviously grown into a cavity,