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The chemical formula of mineral Colemanite is indicated by Ca2B6O11 5H2O. This Colemanite is a Borate mineral. Mineral Colemanite is known to crystallize in the monoclinic system. 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. When viewed closely under petrographic polarizing microscope, Colemanite is found having biaxial positive figure. The indices of the mineral vary from 1.586 to1.614. The birefringence of the mineral is commonly found to be 0.028. The optic angle is about 56 degree. The composition of the mineral Colemanite does not vary significantly. This is despite the fact that some magnesium may replace calcium in the composition.

 

            Colemanite is commonly found exhibiting a moderate positive relief when viewed closely with the aid of the geological polarizing light microscope. When evaluated using the Mohs scale method, the hardness measure of the mineral is commonly found ranging from 4 to 4.5. The specific gravity measure of the mineral is around 2.42 grams per cubic centimeters. Colemanite is commonly found colorless, white, gray, or yellowish in hand sample. When rubbed on a white porcelain streak plate, it is commonly found leaving a white streak. When viewed in reflected light of the polarized microscope used in optical mineralogy, it is commonly found exhibiting a vitreous luster. Colemanite usually decrepitates on heating.

 

            Mineral Colemanite is commonly found colorless in thin section and in grain mount. Colemanite often forms as massive, granular, cleavable masses. The crystals formed by Colemanite are usually found as stubby prisms, equant, or rhomblike with many faces. Colemanite is commonly found having perfect cleavage on one direction and is at right angles to a good cleavage that can be seen clearly exhibited under petrographic polarizing light microscope. There is no reported twinning for mineral Colemanite.

 

            There are sections that are cut parallel to one direction that yield approximately centered acute bisectrix figures having an optic angle of about 56 degree. The cleavage fragments that lie on two directions in the optic axis yield obtuse bisectrix and off-center flash figures. The indices of refraction of this mineral Colemanite do not vary significantly. The interference colors that are usually found exhibited by the mineral in standard thin sections range up to middle second order.

 

            Colemanite may be altered to calcite. It may be also produces as an alteration product after other borate minerals. Colemanite has restricted occurrence and has higher indices of refraction than many other borate minerals known in the field of optical mineralogy with which Colemanite is commonly associated. Mineral Colemanite is commonly found in saline lake deposits. This is commonly associated with borax and other borate minerals.



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Wednesday, May 28th, 2008 at 4:58 am
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The Carbonates and Borates Mineral Class
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