Selenite (mineral)
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Selenite, satin spar, desert rose, and gypsum flower are
crystal habit In mineralogy, crystal habit is the characteristic external shape of an individual crystal or aggregate of crystals. The habit of a crystal is dependent on its crystallographic form and growth conditions, which generally creates irregularities d ...
varieties of the mineral
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...
. All varieties of gypsum, including selenite and
alabaster Alabaster is a mineral and a soft Rock (geology), rock used for carvings and as a source of plaster powder. Archaeologists, geologists, and the stone industry have different definitions for the word ''alabaster''. In archaeology, the term ''alab ...
, are composed of
calcium sulfate Calcium sulfate (or calcium sulphate) is an inorganic salt with the chemical formula . It occurs in several hydrated forms; the anhydrous state (known as anhydrite) is a white crystalline solid often found in evaporite deposits. Its dihydrate ...
dihydrate (meaning that it has two molecules of water), with the chemical formula CaSO4·2H2O. Selenite contains no
selenium Selenium is a chemical element; it has symbol (chemistry), symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elem ...
; the similar names both derive from
Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...
( '
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
'). 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,Selenite on Mindat.org
/ref> 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.Etymology of selenite from the New Collegiate Dictionary


Distinguishing characteristics

The main distinguishing characteristics of crystalline gypsum are its softness (hardness 2 on
Mohs scale The Mohs scale ( ) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by the Ger ...
, 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 Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...
.


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, 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 Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
(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)Desert rose on Mindat.org
/ref> * The desert rose name can also be applied to
barite Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate (Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...
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 In gemology, chatoyancy ( ), also called chatoyance or the cat's eye effect, is an optical reflectance effect seen in certain gemstones. (Historically, the term has applied specifically to gems; in woods and other materials the effect is more ...
.


Crystal habit and properties

Crystal habit In mineralogy, crystal habit is the characteristic external shape of an individual crystal or aggregate of crystals. The habit of a crystal is dependent on its crystallographic form and growth conditions, which generally creates irregularities d ...
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 Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...
-like sheets and have been used as window panes as at
Santa Sabina The Basilica of Saint Sabina (, ) is a historic church on the Aventine Hill in Rome, Italy. It is a titular minor basilica and mother church of the Roman Catholic Order of Preachers, better known as the Dominicans. Santa Sabina is the oldest ex ...
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 Matrix (: matrices or matrixes) or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the m ...
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 An optical prism is a transparent optics, optical element with flat, polished surfaces that are designed to refraction, refract light. At least one surface must be angled—elements with two parallel surfaces are ''not'' prisms. The most fami ...
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 oxide An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. Iron ...
s or
clay mineral Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay minera ...
s.


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 In gemology, chatoyancy ( ), also called chatoyance or the cat's eye effect, is an optical phenomenon, optical reflectance effect seen in certain gemstones. (Historically, the term has applied specifically to gems; in woods and other materials ...
(cat's eye effect). When cut across the fibers and polished on the ends, satin spar exhibits an
optical illusion In visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual perception, percept that arguably appears to differ from reality. Illusions come in a wide varie ...
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 Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with colore ...
or
phosphorescence Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluor ...
.


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 Mohs scale ( ) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by the Ger ...
). 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 Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...
. 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 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. It 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.Anthony’s Nose, New York: A Review of Three Mineral Localities
by John Betts, Fine Minerals – Philips Copper Mine and the re-formation of selenite crystals


Images

File:SeleniteGypsumUSGOV.jpg, Selenite, a gypsum crystal File:Selenite - Rice NW Museum in Oregon.JPG, Selenite from Rio Grande Do Sul, Brazil on display at the Rice Northwest Museum of Rocks and Minerals in Hillsboro, Oregon, USA. File:Gypsum-67904.jpg, Desert rose. Cluster of sharp, bladed selenite crystals File:Gypse, ankérite.jpeg, Gypsum flowers, Bou Azer East deposit, Bou Azer District, Tazenakht, Ouarzazate Province, Souss-Massa-Draâ Region, Morocco File:Gypsum-223940.jpg, Stereotypic cluster for "ram’s-horns" selenite


See also

* Great Salt Plains Lake and Great Salt Plains National Wildlife Refuge, Oklahoma, USA * Lake Lucero,
White Sands National Park White Sands National Park is a national park of the United States located in New Mexico and completely surrounded by the White Sands Missile Range. The park covers in the Tularosa Basin, including the southern 41% of a field of white sand ...
, New Mexico, USA *
Lechuguilla Cave At , Lechuguilla Cave is the longest cave, eighth-longest explored cave in the world and the second deepest () in the continental United States. It is most famous for its unusual geology, rare speleothem, formations, and pristine condition. The ...
, Carlsbad Caverns National Park, New Mexico, USA * Peñoles Mine, Naica, Chihuahua, Mexico


References


External links


Cavern of Crystal Giants
Cueva de los Cristales, Peñoles Mine, Naica, Chihuahua, Mexico
Mindat.org
- scientific description of gypsum plus list of localities
Gypsum flowers, National Speleological Society
telegraph.co.uk, 2008 {{DEFAULTSORT:Selenite (Mineral) Calcium minerals Sulfate minerals Dihydrate minerals Cave minerals