Selenite is a mostly clear, transparent variety of the sulfate mineral gypsum. The name selenite is also commonly used for other varieties of gypsum, including satin spar gypsum, desert roses, and gypsum flowers. All varieties of gypsum, including selenite, satin spar, 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 selenium; the similar names both derive from Greek selḗnē (σελήνη '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 meters long and weighing 12 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 selenite from Latin selenites, ultimately from Greek selēnítēs líthos (σεληνίτης λίθος, lit. 'moon stone'). 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 Selenite is most often transparent and colorless. If selenite crystals show opacity or color, these are 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.
Transparency Gypsum crystals can be transparent (most often selenite), translucent (most often satin spar but also selenite and gypsum flowers), and opaque (most often the rosettes and flowers). Opacity can be caused by impurities, inclusions, druse, and crust, and can occur in all four crystalline varieties.
Luster Selenite typically shows vitreous luster, but may show pearly luster on cleavage surfaces. Satin spar shows characteristic silky luster. Luster is not often exhibited in the rosettes, due to their exterior druse; nevertheless, the rosettes often show glassy to pearly luster on edges. Gypsum flowers usually exhibit more luster than desert roses.
Other optical properties Fibrous satin spar exhibits chatoyancy (cat's eye effect). When cut across the fibers and polished on the ends, satin spar exhibits an optical illusion when placed on a printed or pictured surface: print and pictures appear to be on the surface of the sample. It is often called and sold as the “television stone” (as is ulexite). Some selenite and satin spar specimens exhibit fluorescence or phosphorescence.
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