Selenite (mineral)

History and etymology

The etymology of selenite is through Middle English selinete, from Latin selenites, from Greek selēnitēs (lithos), literally, moonstone or stone of the moon, from selēnē (Moon). The ancients had a belief that certain transparent crystals waxed and waned with the moon. From the 15th century, "selenite" has referred specifically to the variety of gypsum that occurs in transparent crystals or crystalline masses.

Identification of crystals as gypsum

All varieties of gypsum are very soft minerals (hardness: 2 on Mohs Scale). This is the most important identifying characteristic of gypsum, as any variety of gypsum can be easily scratched with a fingernail. Also, because gypsum has natural thermal insulating properties, all varieties feel warm to the touch.

Varieties

Though sometimes grouped together as "selenite", the four crystalline varieties have differences. General identifying descriptions of the related crystalline varieties are:

Selenite

Satin spar

Desert rose

Gypsum flower

Use and history

Varieties of gypsum known as "satin spar" and "alabaster" are used for a variety of ornamental purposes. But also because of the long history of the commercial value and use of both gypsum and alabaster, the four crystalline varieties have been somewhat ignored, except as a curiosity or as rock collectibles(not commonly used).

Crystal habit and properties

Crystal habit refers to the shapes that crystals exhibit.

Selenite crystals commonly occur as tabular, reticular, and columnar crystals, often with no imperfections or inclusions, and thereby can appear water or glass-like. Many collectible selenite crystals 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 (see Peñoles Mine reference in external links).

Selenite crystals sometimes form in thin tabular or mica-like sheets and have been used as glass 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 are almost always 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), 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, and opal.

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

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.

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.

Some selenite and satin spar specimens exhibit fluorescence or phosphorescence.

Tenacity

All 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.

All four crystalline varieties are sectile in that they can be easily cut, will peel (particularly selenite crystals that exhibit mica-like properties), and like all gypsum varieties, can be scratched by a fingernail (hardness: 2 on Mohs Scale). The rosettes are not quite as soft due to their exterior druse; nevertheless, they too can be scratched.

Selenite crystals that exhibit in either reticular or acicular habits, satin spar, in general (as fibrous crystals are thin and narrow), desert roses that are thinly bladed, and gypsum flowers, particularly acicular gypsum flowers, can be quite brittle and easily broken.

Size

All four crystalline varieties can range in size from minute to giant selenite crystals measuring 11 meters long such as those found in the caves of the Naica Mine of Chihuahua, Mexico. The crystals thrived in the cave's extremely rare and stable natural environment. Temperatures stayed at 58 °C, and the cave was filled with mineral-rich water that drove the crystals' growth. The largest of those crystals weighs 55 tons, is 33 feet long, and is over 500,000 years old.

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, salt springs, 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 since the Silurian Period 443.7 ± 1.5 Ma.

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, US. Exterior inclusion (druse) 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, US where selenite micro crystal coatings are commonly found on numerous surfaces (rock and otherwise) in the cave and in the dump.

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