Selenite, satin spar, desert rose, gypsum flower are crystal habit varieties of the mineral gypsum.
All varieties of gypsum, including selenite and alabaster, are composed of calcium sulfate dihydrate (meaning that it has two molecules of water), with the chemical formula CaSO4·2H2O. Selenite contains no significant selenium – The similar names both derive from Greek ( 'Moon').
Some of the largest crystals ever found are of selenite, the largest specimen found in the Naica Mine's Cave of the Crystals being 12 metres long and weighing 55 tons.
History and etymology
"Selenite" is mostly synonymous with gypsum, but from the 15th century, it has named the transparent variety that occurs in crystals or crystalline masses. The name derives through Middle English from Latin , ultimately from Greek (, ). It got this name because people historically believed the mineral waxed and waned with the cycles of the Moon.
Distinguishing characteristics
The main distinguishing characteristics of crystalline gypsum are its softness (hardness 2 on Mohs scale, soft enough to scratch with a fingernail) and its three unequal cleavages. Other distinguishing characteristics include its crystal habits, pearly lustre, easy fusibility with loss of water, and solubility in hot dilute hydrochloric acid.
Varieties
Though sometimes grouped together as "selenite", the four crystalline varieties have differences. General identifying descriptions of the related crystalline varieties are:
Selenite
Most often transparent and colorless
If selenite crystals show opacity or color, it is caused by the presence of other minerals, sometimes in druse
Satin spar
Most often silky and fibrous; chatoyant; can exhibit some coloration
The satin spar name has also been applied to fibrous calcite (a related calcium mineral), which can be distinguished from gypsum by its greater hardness (Mohs 3), rhombohedral cleavage, and reaction with dilute hydrochloric acid.
Desert rose
Rosette-shaped gypsum with outer druse of sand or with sand throughout – most often sand colored (in all the colors that sand can exhibit) The desert rose name can also be applied to barite desert roses (another related sulfate mineral) – barite is a harder mineral with higher density
Gypsum flower
Gypsum flowers are curved rosettes of fibrous gypsum crystals found in solution caves.
Use and history
Satin spar is sometimes cut into cabochons to best display its chatoyance.
Crystal habit and properties
Crystal habit refers to the shapes that crystals exhibit. Selenite crystals show a variety of habits, but the most common are tabular, prismatic, or acicular (columnar) crystals, often with no imperfections or inclusions. Twinned crystals are common, and often take the form of "swallow tail" twins.
Selenite crystals sometimes form in thin tabular or mica-like sheets and have been used as window panes as at Santa Sabina in Rome.
Selenite crystals sometimes will also exhibit bladed rosette habit (usually transparent and like desert roses) often with accompanying transparent, columnar crystals. Selenite crystals can be found both attached to a matrix or base rock, but can commonly be found as entire free-floating crystals, often in clay beds (and as can desert roses).
Satin spar is almost always prismatic and fibrous in a parallel crystal habit. Satin spar often occurs in seams, some of them quite long, and is often attached to a matrix or base rock.
Desert roses are most often bladed, exhibiting the familiar shape of a rose, and almost always have an exterior druse. Desert roses form in wet sand, unattached to a matrix or base rock.
Gypsum flowers are most often acicular, scaly, stellate, and lenticular. Gypsum flowers most often exhibit simple twinning (known as contact twins); where parallel, long, needle-like crystals, sometimes having severe curves and bends, will frequently form “ram’s horns”, "fishtail", "arrow/spear-head", and "swallowtail" twins. Selenite crystals can also exhibit “arrow/spear-head” as well as “duck-bill” twins. Both selenite crystals and gypsum flowers sometimes form quite densely in acicular mats or nets; and can be quite brittle and fragile. Gypsum flowers are usually attached to a matrix (can be gypsum) or base rock.
Color
Gypsum crystals are colorless (most often selenite), white (or pearly – most often satin spar), or gray, but may be tinted brown, yellow, red, or blue by the presence of impurities, such as iron oxides or clay minerals.