Oklahoma State Gemstone
Official state crystal
Selenite is a variety of gypsum with visible crystalline structure. It is composed of calcium sulfate dihydrate and may be colorless, transparent, or lightly colored. All varieties of gypsum are very soft, scoring two out of 10 on the Mohs scale, which means they can easily be scratched with a fingernail. All gypsum also has natural insulating properties, which make it warm to the touch. The most common of the sulfate minerals, selenite occurs on all continents.
Oklahoma is the country’s largest producer of gypsum. A variety of gypsum, hourglass selenite, is common to the ancient salt plains in north central Oklahoma, and in fact, this is the only place this variety occurs. Hourglass selenite was designated as the official state crystal on April 4, 2005.
The name selenite likely comes from the Greek word σελήυη, for "moon," referring to the white, moon-like reflections of the mineral. "Hourglass" describes the unique formation that occurs inside Oklahoma’s state crystal, hourglass selenite.
FORMATION AND OCCURRENCE
The 11,000-acre Salt Plains National Wildlife Refuge in Oklahoma is the only site in the world where selenite occurs in hourglass formation. The salt plains were formed millions of years ago by repeated seawater flooding. The seawater was cut off from the sea and evaporated, depositing thick layers of salt. The salt mixes with ground water to form a concentrated saline solution, which then combines with gypsum to promote selenite crystal growth in a portion of the salt flats.
Selenite sometimes forms in thin sheets that have been used as pane glass, and sometimes forms in rosettes known as desert roses.
While hourglass selenite is known to occur at only at the Great Salt Plains in Alfalfa County, Oklahoma, notable gypsum deposits around the word include the following:
Gypsum is used in making wallboard and plaster, as an ingredient in food and pharmaceuticals, and has other commercial applications. The crystal forms of gypsum, however, are mainly curiosities and collectibles. Selenite is quite popular among mineral collectors, especially specimens displaying inclusions, such as hourglass selenite. Giant selenite crystals measuring 36.9 feet (11 meters) long, as well as crystals up to 500,000 years old, have been found in caves in Chihuahua, Mexico. Hourglass selenite crystals measuring up to 7 inches (17.8 cm) long have been found in Oklahoma.
Chemical Formula: CaSO4·2H2O
Crystal Structure: Monoclinic (2/m) Space Group: A2/a
Hardness (Mohs): 2
Color: Brown green, brownish yellow, greenish, gray green, gray white
Birefringence: δ =0.0090-0.0100
Refractive Index: nα=1.519-1.521, nβ=1.522-1.523, nγ=1.529-1.53
Fracture: Fibrous; thin, elongated fractures produced by crystal forms or intersecting cleavages (e.g. asbestos).
Some data courtesy of the Mineralogical Society of America
Author: World Trade Press