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The chemical formula of mineral Coesite is indicated by SiO2 or Silicon Dioxide. Coesite is actually a Silicate mineral. The first Coesite mineral crystal was discovered at the Meteor Crater in Arizona. But before the discovery of this mineral species in the said meteor impact site, Coesite was first synthesized in the year 1953. It was Loring Coes Jr who first created Coesite crystals in 1953. And in the year 1960, Eugene Shoemaker found the naturally occurring Coesite mineral in the Barringer Crater or Meteor Crater in Arizona, USA. And this piece of material found was actually used as evidence that the crater must have been formed by an impact. Coesite 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.

 

            Coesite is most commonly used as an indicator of high pressure crystallization, possibly a meteorite impact. Coesite is also used as a mineral specimen and more often, it is found exhibiting nice and interesting microscope images when viewed under petrographic polarizing light microscope used in optical mineralogy. Coesite is a known polymorph of mineral Quartz. In optical mineralogy, this means that the two minerals, Coesite and Quartz, share the same chemistry but they certainly possess different structure. The two minerals belong to the same group where all its members are polymorphs of each other. The said group is called the Quartz Group where several other interesting minerals such as stishovite, cristobalite and tridymite also belong. When the structure of the mineral Coesite is usually composed of SiO4 tetrahedrons, which are usually linked together into four member rings. These rings are then linked together to form a structure that is chain like. This chain like structure is considered much more compact than the other members of the quartz group, except of course stishovite. This structure can be reflected in the index of refraction and high density of the mineral, which can be clearly seen under a petrographic polarizing light microscope.

 

            Well formed Coesite mineral crystals are considerably rare because Coesite minerals are commonly not getting enough time to pursue a crystal face formation. Coesite has been found forming quickly at high pressures of about 20 kilo bars. It has been thought that this high pressure environment can be possibly found at a meteor impact site. In this site, the pressures are considerably great but the temperature is not that high. And one of the most well studied meteor impact sites is the meteor crater in Arizona called the Meteor Crater. In this meteor impact site, the very first tiny Coesite crystal and of its cousin stishovite, were found. The presence of the mineral stishovite and Coesite is considered a diagnostic evidence of the meteor impact when craters of the unknown origin are examined. Mineral Coesite has been found also as identified in the location called kimberlites where diamond and other high pressure minerals are reportedly found. At normal surface temperatures, mineral Coesite can be found metastable. This means that mineral Coesite can actually slowly convert to quartz structure if it possibly could. But this process is said to be very complicated and very slow since it could take thousand of years to complete the transformation. The transformation process is very slow because it involves bond breaking and atom rearrangements.

 

            Coesite is also most commonly found colorless or white in appearance when viewed with the aid of petrographic polarizing microscopes used in the field of optical mineralogy. Coesite is also most commonly found showing a vitreous luster when viewed in reflected light of polarizing microscope used in optical mineralogy. Coesite has no cleavage found visible even when it is closely evaluated between crossed nicols of petrographic polarizing light microscope used in optical mineralogy. Coesite is commonly found showing conchoidal fracture when mineral specimen is evaluated under several adjustments on the aperture diaphragm of the polarizing light microscopes for mineralogists. The specific gravity measure of the mineral is commonly found 3.0+ grams per cubic centimeters, which is considered average for translucent minerals. The hardness measure of mineral Coesite when it is evaluated using the Mohs scale method is usually found 8. Coesite is commonly found leaving a clear streak when mineral specimen is rubbed on a white porcelain streak plate.

 

            Mostly Coesite mineral crystals are found transparent to translucent in appearance. The crystal habit of Coesite as described in optical mineralogy commonly includes the always small, usually microscopic, and very rare but well formed prismatic crystals that can be more clearly exhibited under polarizing microscopes for mineralogists. The refractive index of mineral Coesite is approximately 1.59. Mineral Coesite is commonly found showing biaxial positive figure between crossed nicols of petrographic polarizing light microscopes used in optical mineralogy. The maximum birefringence of mineral Coesite is usually found ranging from 0.004 to 0.005. Coesite is also most commonly found exhibiting moderate surface relief when the mineral is evaluated under polarized light microscope used in optical mineralogy. Coesite has weak dispersion display when viewed in transmitted light of petrographic polarizing light microscope. There is no specific data found on the toxicity and health dangers for mineral Coesite. However, the specimens of this mineral should be treated with great care and use of sensible precaution is advised upon handling them.

 

            Coesite is commonly associated with several other interesting minerals such as garnets, stishovite, pyroxenes, sanadine, iron meteorites and diamond. The best field indicators of mineral Coesite commonly include density, environment of formation and index of refraction. But generally, Coesite mineral crystals are too small to be identified by ordinary means. Mineral Coesite notably occurs at some famous mineral localities like the Canyon Diablo, Meteor Crater, Arizona in the United States and also in some other meteorite craters around the world like Kimberly in South Africa.



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Monday, December 29th, 2008 at 6:32 am
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The Silicates Mineral Class
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