Selenite (mineral)
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Selenite, satin spar, desert rose, gypsum flower are crystal habit varieties of the mineral
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate 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, blackboard or sidewalk chalk, and drywall. ...
. All varieties of gypsum, including selenite and
alabaster Alabaster is a mineral or rock that is soft, often used for carving, and is processed for plaster powder. Archaeologists and the stone processing industry use the word differently from geologists. The former use it in a wider sense that includes ...
, are composed of
calcium sulfate Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Pari ...
dihydrate (meaning that it has two molecules of water), with the chemical formula CaSO4·2H2O. Selenite contains no significant
selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
 – The similar names both derive from
Greek Greek may refer to: Greece 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 ...
( '
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
') Some of the largest crystals ever found are of selenite, the largest specimen found in the
Naica Mine The Naica Mine of the Mexican state of Chihuahua, is a lead, zinc and silver mine. Located in Naica in the municipality of Saucillo, the Naica Mine is owned by Industrias Peñoles, the world's largest silver producer. Caverns discovered dur ...
's
Cave of the Crystals Cave of the Crystals or Giant Crystal Cave () is a cave connected to the Naica Mine at a depth of , in Naica, Chihuahua, Mexico. It takes the form of a chamber within the limestone host rock of the mine, and is about long with a volume of . ...
being 12 metres long and weighing 55 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 various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by th ...
, 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

* Most often
transparent Transparency, transparence or transparent most often refer to: * Transparency (optics), the physical property of allowing the transmission of light through a material They may also refer to: Literal uses * Transparency (photography), a still, ...
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 Fiber or fibre (from la, fibra, links=no) is a #Natural fibers, natural or Fiber#Artificial fibers, artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The stronge ...
;
chatoyant In gemology, chatoyancy ( ), or chatoyance or cat's eye effect, is an optical reflectance effect seen in certain gemstones, woods, and carbon fibre. Coined from the French "œil de chat", meaning "cat's eye", chatoyancy arises either from the ...
; 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 A cabochon (; ) is a gemstone that has been shaped and polished, as opposed to faceted. The resulting form is usually a convex (rounded) obverse with a flat reverse. Cabochon was the default method of preparing gemstones before gemstone cutti ...
to best display its
chatoyance In gemology, chatoyancy ( ), or chatoyance or cat's eye effect, is an optical reflectance effect seen in certain gemstones, woods, and carbon fibre. Coined from the French "œil de chat", meaning "cat's eye", chatoyancy arises either from the ...
.


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 Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
-like sheets and have been used as window panes as at
Santa Sabina The Basilica of Saint Sabina ( la, Basilica Sanctae Sabinae, it, Basilica di Santa Sabina all'Aventino) 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 Pre ...
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 most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...
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 optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are ''not'' prisms. The most familiar type of optical ...
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 Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...
s 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 In gemology, chatoyancy ( ), or chatoyance or cat's eye effect, is an optical reflectance effect seen in certain gemstones, woods, and carbon fibre. Coined from the French "œil de chat", meaning "cat's eye", chatoyancy arises either from the ...
(cat's eye effect). When cut across the fibers and polished on the ends, satin spar exhibits an
optical illusion Within 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 v ...
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 Ulexite (NaCaB5O6(OH)6·5H2O, hydrated sodium calcium borate hydroxide), sometimes known as TV rock or Television stone, is a mineral occurring in silky white rounded crystalline masses or in parallel fibers. The natural fibers of ulexite conduc ...
). Some selenite and satin spar specimens exhibit
fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
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 fluo ...
.


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 Sectility is the ability of a mineral to be cut into thin pieces with a knife. Minerals that are not sectile will be broken into rougher pieces when cut. Metals and paper are sectile. Sectility can be used to distinguish minerals of similar appea ...
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 various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by th ...
). 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 The Naica Mine of the Mexican state of Chihuahua, is a lead, zinc and silver mine. Located in Naica in the municipality of Saucillo, the Naica Mine is owned by Industrias Peñoles, the world's largest silver producer. Caverns discovered dur ...
of Chihuahua,
Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...
. 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 The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
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 Great Salt Plains Lake is a reservoir located within the Salt Plains National Wildlife Refuge in Alfalfa County, Oklahoma in the United States named because of the salt flats in the area and for the Salt Fork Arkansas River, which is dammed to fo ...
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 Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the ...
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, 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 Great Salt Plains Lake is a reservoir located within the Salt Plains National Wildlife Refuge in Alfalfa County, Oklahoma in the United States named because of the salt flats in the area and for the Salt Fork Arkansas River, which is dammed to fo ...
and
Great Salt Plains National Wildlife Refuge The Salt Plains National Wildlife Refuge is part of the United States system of national wildlife refuges. It is located in Alfalfa County in northern Oklahoma, north of Jet (pop. 230), along Great Salt Plains Lake, which is formed by a dam on ...
, Oklahoma, USA *
Lake Lucero Lake Lucero is a playa located within that section of the Tularosa Basin that is contained within White Sands National Park in the U.S. state of New Mexico. The playa is noted for the unusually high quantity of water-deposited and wind-deposi ...
, White Sands National Park, New Mexico, USA *
Lechuguilla Cave At , Lechuguilla Cave is the 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 formations, and pristine condition. The cave is named for the can ...
,
Carlsbad Caverns National Park Carlsbad Caverns National Park is an American national park in the Guadalupe Mountains of southeastern New Mexico. The primary attraction of the park is the show cave Carlsbad Cavern. Visitors to the cave can hike in on their own via the natural ...
, New Mexico, USA * Peñoles Mine,
Naica Naica is a town in the Mexican state of Chihuahua. It is located in the municipality of Saucillo. As of 2010, the town of Naica had a population of 4,938, up from 4,775 in the 2005 INEGI Census. It is a mining town and the location of the r ...
, 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

{{DEFAULTSORT:Selenite (Mineral) Calcium minerals Sulfate minerals Cave minerals