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Selenite

Selenite

Selenite, satin spar, desert rose, and gypsum flower are four varieties of gypsum; all four varieties show obvious crystalline structure. The four "crystalline" varieties of gypsum are sometimes grouped together and called selenite.

All varieties of gypsum, including selenite and alabaster, are a very soft mineral (hardness: 2 on Mohs Scale) composed of calcium sulfate dihydrate (meaning has two molecules of water), with the chemical formula CaSO4·2H2O.

The most important identifying characteristic is how soft gypsum is, as any variety of gypsum can be easily scratched with a fingernail. Also because gypsum has natural 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

* 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 is the 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 both gypsum and alabaster, the four crystalline varieties have been somewhat 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.[1]

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 forms in thin tabular or mica-like sheets; and has been used as glass panes.[2][3]

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; 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", "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, limestone, 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.

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.[4]

Some selenite and satin spar specimens exhibit fluorescence or phosphorescence.

Tenacity

Care in handling is recommended for all four crystalline varieties.

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), and like all gypsum varieties, can be scratched by a fingernail (hardness: 2 on Mohs Scale). The rosettes are not as quite 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 12 meters long such as, those found in the Penoles Mine.[5]

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.[6]

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.[7] 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, USA where selenite micro crystal coatings are commonly found on numerous surfaces (rock and otherwise) in the cave and in the dump.[8]

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.

See also

* Great Salt Plains Lake and Great Salt Plains National Wildlife Refuge, Oklahoma, USA: see also references below
* Lechuguilla Cave, Carlsbad Caverns National Park, New Mexico, USA: see also references and external links below
* Peñoles Mine, Naica, Chihuahua, Mexico: see also references and external links below

References

  1. ^ [1] The Mineralogical Society of America website offers a photographs and a further explanation of 'Mineral Identification Key Habit'
  2. ^ [2] Kristallgrotte - photograph of selenite crystals in the (below)
  3. ^ [3] Marienglashöhle Show Mine, Germany - selenite was commonly used in Germany during mediaeval times for glass panes in windows and, in particular, for coverings of pictures of the Madonna - in Germany, this form of selenite was usually referred to as Marienglas or Mary’s Glass.
  4. ^ [4] Will the real television stone please stand up? by Jeffrey Shallit, School of Computer Science and Peter Russell, Department of Earth Sciences, University of Waterloo, Canada - discussion whether ulexite or satin spar is the “real” television stone. When the optical illusion that some satin spar can exhibit was “discovered”, satin spar was “marketed” as ulexite - rather than as a gypsum variety. Ulexite is a different mineral.
  5. ^ [5] Electric Caverns - picture from Peñoles Mine - article also includes a link to a picture of a spectacular gypsum flower at Lechuguilla Cave
  6. ^ [6] Surface Mining - Industrial Minerals - Gypsum and Anhydrite, Richard H Olson, Edwin H Bentzen, III, and Gordon C Presley, Editors, SME - Society for Mining, Metallurgy, and Exploration, USA
  7. ^ [7] Salt Plains National Wildlife Refuge, Oklahoma, USA - website showing photographs of sand-colored hour-glass formations in clear selenite columnar crystals
  8. ^ [8] Anthony’s Nose, New York, USA: A Review of Three Mineral Localities, by John Betts, John Betts-Fine Minerals - page down to read about Philips Copper Mine and the re-formation of selenite crystals

External links

  • [9] Web Mineral - gypsum plus some information on the crystalline varieties
  • [10] MinMax Mineral Information System - gypsum plus some information on the crystalline varieties - type "gypsum" in search bar to reach page (in German)
  • [11] Answers.com - gypsum plus some information on the crystalline varieties
  • [12] Mineral Atlas - gypsum photograph gallery - page 1 - over 300 photographs of all four crystalline varieties showing numerous locations, habits, and coloration
  • [13] Mindat.org - scientific description of gypsum plus list of localities - extensive list of over 2000 gypsum occurrences, inclusive of crystalline varieties, worldwide on all continents including Antarctica - Mindat.org states that not all occurrences are confirmed - and listings do not necessarily have photographs
  • [14] Mindat.org - gypsum photograph gallery - page 1 - gallery of over 60 pages and over 600 photographs of all four crystalline varieties showing numerous locations, habits, and coloration
  • [15] Mindat.org - satin spar localities and some photographs
  • [16] From Google book search: A System of Mineralogy: Comprising the Most Recent Discoveries; James Dwight Dana; Wiley & Putnam, 1844; pp240-242 - notes on gypsum, alabaster, selenite, and anhydrite - including locations.

Giant gypsum crystals and gypsum speleothems sites (mainly selenite crystals):

  • [17] Kraushöhle, Gams bei Hieflau, Austria - gypsum speleothems (in German)
  • [18] Pilar de Jaravia, Spain - large columnar selenite crystals in massive geode at Quien Tal Pensara Mine - also see [19] The Giant Crystal Project
  • [20] Lechuguilla Cave, Carlsbad Caverns National Park, New Mexico, USA - speleothems and gypsum flowers
  • Peñoles Mine, Naica, Chihuahua, Mexico - large gypsum mine - massive selenite crystals in Cueva de los Cristales (Cave of Crystals) - selenite "blades" in Cueva de las Espadas (Caves of Swords or Xochitl Cave) - “shark (or dog) teeth” selenite formations throughout the caves - one cave contains selenite crystals described as "long, slender, slightly milky needles with tubular water-filled cavities and movable bubbles" - [21] 1927 article on the giant crystals of Niaca - [22] The Naica-Peñoles project - [23] The Giant Crystal Project Site-Naica - [24] National Geographic "Giant Crystal Cave" program information (includes airing times, short video, some pictures, and information)

* Wopenka, B. and J.D. Pasteris, A mineralogical perspective on the apatite in bone. Materials Science and Engineering: C. 25(2): 131, 2005

List of minerals

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