Selenite ( Desert rose var. )

Selenite ( Desert rose var. )

Selenite, Desert rose var. is one of the four crystal structures of gypsum, with the formula of CaSO4 . 2H2O, Calcium sulfate dihydrate.  Desert rose is a colloquial name given to this particular crystal structure, which is not exclusive to selenite. Desert rose structures form in drying up salt basins, and incorporate fine sand grains in their fibrous structure as the crystals grow. This specimem is from Chihuahua in Mexico.

Phrehnite

Phrehnite

Prehnite is an inosilicate with the formula Ca2Al(AlSi3O10)(OH)2  It is a hydrothermal mineral, filling cavities in basaltic volcanic rocks, and also lining fractures in various intrusive and metamorphic rocks. The specimen is from Loanhead quarry, Nr. Beith in Scotland, and is a botryoidal aggregate with no crystals observed. part of the surface is peppered with small calcite "rosettes" with a slightly orange colouration. Prehnite of suitable quality is much used in jewellery.

Gypsum, selenite var.

Gypsum, selenite var.

Gypsum, Calcium sulfate dihydrate,  CaSO4 . 2H2O  This specimen from Essone in France is referred to as selenite. However, selenite varieties of gypsum also includes desert rose, satin spar, and gypsum flower. The crystals of the specimen are clear and tabular with an angled termination. The crystals are relatively minute compared to the cave selenite crystals in the inset picture.

Pyrophyllite.

Pyrophyllite.

Pyrophyllite is a phyllosilicate mineral Al2Si4O10(OH)2 Aluminium silicate hydroxide. The crystalline folia habit is well represented in this specimen, which has the general location of California, USA. The specimen has a soapy feel, when handled.

Aragonite.

Aragonite.

Aragonite, Calcium carbonate, CaCO3  has the same chemical composition as calcite, and vaterite. These three minerals are polymorphs of calcium carbonate, each with a different crystal lattice, so each one with a different crystal habit. Aragonite crystals are often pseudohexagonal and prismatic, as are the twinned crystals in this specimen from Morocco. Aragonite is not a stable polymorph, and will eventually  alter to calcite.