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The chemical formula of mineral Selenite is indicated by CaSO4 – 2(H2O) or Hydrated Calcium Sulfate. Selenite is actually a Sulfate mineral. Selenite is actually the colorless and transparent variety of mineral gypsum. Selenite is most commonly found showing a pearl-like luster when viewed in reflected light of petrographic polarizing microscope used in the field of optical mineralogy. Selenite has been described as having a moonlike glow that can be seen more splendidly wonderful when viewed with the aid of petrographic polarizing light microscopes for mineralogists. Selenite derived its name from the Greek word for moon and means moon rock. Selenite and other gypsum minerals are used as plaster, wall board, fertilizer, ornamental stone, some cements, etc.. Gypsum has several varieties that were widely used in the mineral trade.

             Selenite is most commonly found colorless and transparent in appearance. Selenite crystals can be found quite large and sheets of clear Selenite crystals can be easily peeled from a larger specimen. Selenite is most commonly found showing pearly luster when viewed in reflected light of petrographic polarizing light microscopes used in the field of optical mineralogy. Selenite is known to crystallize in the monoclinic system of crystal formation. In optical mineralogy, the monoclinic system of crystal formation comprises crystals having three axes of unequal lengths. Two of which are usually found in a position that is oblique or not perpendicular to one another. However, both of which are commonly found perpendicular to the third axis. The crystal habit of Selenite as other gypsum minerals usually includes tabular, bladed or blocky crystals with a slanted parallelogram outline. The pinacoid faces usually dominate with jutting prism faces on the edges of the tabular crystals. Long thin Selenite crystals show bends and some  specimens bend into spirals called ram’s horn Selenite. There can be two twinning type found wherein one produces a spear head twin or swallowtail twin while the other type produces a fishtail twin.  Selenite ad other gypsum minerals can be also found massive, crusty, fibrous and earthy. 

            Selenite is usually found showing good cleavage in only one direction and also distinct in two others when evaluated with the aid of polarized microscopes used in optical mineralogy. Selenite and other gypsum minerals are found showing uneven fracture but this is usually rarely seen except when specimen is closely evaluated with the aid of petrographic polarizing microscope for mineralogists. The hardness measure of Selenite when evaluated using the Mohs scale method is usually 2 and this can be scratched by a fingernail. The specific gravity measure of mineral Selenite is approximately 2.3+ grams per cubic centimeters, which is considered light. Selenite is commonly found leaving a white streak when specimen is rubbed on a white porcelain streak plate. Selenite is usually found associated with other interesting minerals including calcite, borax, halite, sulfur, pyrite and many others. The best field indicators of mineral Selenite include hardness, cleavage, crystal habit and flexible crystals. Selenite thin crystals are usually found flexible but not elastic. In optical mineralogy, this means they can be bent but will not bend back on their own. Some samples are found fluorescent. Gypsum minerals are known to have very low thermal conductivity and crystals will noticeably warmer than a like crystal of quartz. Selenite and other gypsum minerals are one of the more common minerals in sedimentary environments. They are major rock forming minerals that produce massive beds, usually from precipitation out of highly saline waters. They usually form easily from saline water, they can have many inclusions of other minerals and even trapped bubbles of air and water, all of which can be seen more clearly visible when specimens are evaluated under petrographic polarizing light microscopes used in the field of optical mineralogy. Selenite and other gypsum minerals notably occurs at several localities including Naica in Mexico, Sicily, Utah and Colorado in USA and many other localities throughout the world.

 



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Thursday, July 30th, 2009 at 9:06 am
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The Sulfates Mineral Class
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