Selenite

This article is about the mineral: for other meanings, see Selenite (disambiguation).

          Selenite, satin spar, desert rose, and gypsum flower are the four crystalline varieties of gypsum.

Gypsum is a very soft mineral (hardness: 2 on Mohs Scale) composed of calcium sulfate dihydrate (meaning has 2 molecules of water), with the chemical formula CaSO₄·2H₂O. Gypsum in all its varieties can be scratched with a fingernail. Alabaster is a massive form of gypsum, meaning that its crystal habit is not obvious to the naked eye.

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Varieties

The four crystalline varieties of gypsum are commonly grouped together as selenite; however, selenite, satin spar, desert rose, and gypsum flower have some differences.

The most important identifying characteristic is how soft gypsum is. If unable to easily scratch with a fingernail, then it is likely you have a different mineral. Also because gypsum has natural insulating properties, it feels warm to the touch.

General identifying descriptions of the four crystalline varieties are:

Selenite

• most often transparent and colorless
• if selenite crystals show translucency, opacity, and/or color, it is caused by the presence of other minerals (including druse)
• druse meaning crust of tiny, minute, or micro crystals that form or fuse either within or upon the surface of a rock cavity, geode, or another crystal

Satin Spar

• most often silky, fibrous, and translucent (pearly, milky) - can exhibit some coloration
• the satin spar name can also be applied to fibrous calcite (a related calcium mineral) - calcite is a harder mineral - and feels greasier, waxier, or oilier to the touch

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, too, is a harder mineral - and is heavier and not as warm to the touch

Gypsum Flower

• rosette shaped gypsum with spreading fibers - can include outer druse
• the difference between desert roses and gypsum flowers is that desert roses look like roses, whereas gypsum flowers form a myriad of shapes

Use and history

Because of the long history of the commercial value and use of gypsum and alabaster, the four crystalline varieties have been more or less ignored except as a curiosity or as rock collectibles.

Since the late 20th century, with the growing interest in crystal therapy and crystal healing in the New Age, Neo Pagan, and alternative healing countercultures, the four crystalline varieties of gypsum have increased in popularity and commercial value. This increased interest has translated itself into both the retail mineral and jewellery trades. In the retail mineral trade, all four crystalline varieties are offered as rough, carved, or tumbled specimens. In the retail jewellery trade, selenite crystals with interior druse are offered as a form of drusy jewellery.

Crystal habit and properties

Crystal habit refers to the shapes that crystals exhibit. (Also see^[1])

Selenite crystals commonly occur as tabular, reticular, and columnar crystals, often with no imperfections or inclusions; and thereby, appear water or glass-like. Nevertheless, there are many collectible selenite crystals that have interesting inclusions such as, accompanying related minerals, interior druse, dendrites, and fossils. In some rare instances, water was encased as a fluid inclusion when the crystal formed. Selenite sometimes forms in thin tabular or mica-like sheets; and has been used as glass panes. Selenite crystals sometimes will also exhibit bladed rosette habit (usually transparent) often with accompanying transparent, columnar crystals. Selenite 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.

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 are almost always unattached to a matrix or base rock; most often found lying around.

Gypsum flowers are most often acicular, scaly, stellate, and lenticular. Gypsum flowers most often exhibit simple twinning (known as contact twins); where that parallel, long, needle-like crystals, sometimes having severe curves and bends, will frequently form “ram’s horns”, "fishtail", "spear-head", and "swallowtail" twins. Gypsum flowers sometimes form quite densely in acicular mats; and can be quite fragile. Gypsum flowers often are attached to a matrix or base rock.

Color

Gypsum crystals are colorless (most often selenite), white (or pearly - most often satin spar), gray, brown, beige, orange, pink, yellow, light red, and green. Colors are caused by the presence of other mineral inclusions such as, copper ores, sulfur and sulfides, silver, iron ores, coal, calcite, dolomite, limestone, and opal.

Transparency

Gypsum crystals can be transparent (most often selenite), translucent (most often satin spar - also selenite), and opaque (most often the rosettes). Opacity can be caused by impurities, inclusions, druse, and crust - and can occur in all four crystalline varieties.

Luster

Both selenite and satin spar are often glassy or vitreous, pearly, and silky - especially on cleavage surfaces. 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.

It is not recommended that you leave any form or variety of gypsum in water - as extended submersion will either dissolve or degrade the mineral. Detergents and soaps should be avoided, as they affect luster, particularly with selenite and satin spar - even lukewarm water can affect luster of selenite and satin spar.

Play of Color

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; and is often called and sold as the “television stone” (as is ulexite). Print and pictures appear to be on the surface of the sample.^[2]

Some selenite and satin spar specimens exhibit fluorescence or phosphorescence.

Tenacity

All of the four crystalline varieties are slightly flexible, though will break if bent significantly. They are not elastic, meaning they can be bent, but will not bend back on their own. Both rosettes tend to be more fragile and brittle than selenite and satin spar.

All four crystalline varieties are sectile - soft enough to be easily be scratched with a fingernail. The rosettes are not as quite soft due to their exterior druse; nevertheless, they too can be scratched. Gypsum flowers can sometimes be brittle - particularly acicular gypsum flowers.

Care in handling is recommended for all four crystalline varieties.

Occurrence

Gypsum occurs on every continent and is the most common of all the sulfate minerals.

Gypsum is formed as an evaporative mineral, frequently found in alkaline lake muds, clay beds, evaporated seas, salt flats, and caves. Gypsum, also, is frequently found in conjunction with other minerals such as, copper ores, sulfur and sulfides, silver, iron ores, coal, calcite, dolomite, limestone, and opal. Gypsum has been dated to almost every geologic age.^[3]

In dry, desert conditions and arid areas, sand may become trapped both on the inside and the outside of gypsum crystals as they form. Interior inclusion of sand can take on shapes such as, an interior hourglass shape common to selenite crystals of the ancient Great Salt Plains Lake bed, Oklahoma, USA.^[4] Exterior inclusion occurs as embedded sand grains on the surface such as, commonly seen in the familiar desert rose.

When gypsum dehydrates severely, anhydrite is formed. If water is reintroduced, gypsum can and will reform - including as the four crystalline varieties. An example of gypsum crystals reforming in modern times is found at Philips Copper Mine (closed and abandoned), Putnam County, New York, USA where selenite micro crystal coatings are commonly found on numerous surfaces (rock and otherwise) in the cave and in the dump.^[5]

Whereas geology, mineralogy, and rockhounding groups, clubs, and societies as well as museums usually date, photograph, and note location of minerals, much of the retail mineral and jewellery trade can be somewhat casual about locations and descriptive claims.

See also

• Great Salt Plains Lake and Great Salt Plains National Wildlife Refuge, Oklahoma, USA - see also #4 in references below
• Lechuguilla Cave, Carlsbad Caverns National Park, New Mexico, USA - see also #18 in external links below
• Naica Mine, Mexico - see also #19, 20, 21 in external links below

References