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The chemical formula of mineral Strontianite is indicated by SrCO3. Strontianite is a Carbonate mineral. It is known to crystallize in the orthorhombic system. In optical mineralogy, the orthorhombic crystal system comprises crystals having three mutually perpendicular axes, of which all are of different lengths. Strontianite is known to have biaxial negative figure that can be seen more clearly visible with the aid of petrographic polarizing microscope. The indices of refraction of the mineral vary according to composition. The birefringence of mineral Strontianite is commonly found ranging from 0.148 to 0.165. Mineral Strontianite has the same structure as that of Aragonite and it is consists of layers of triangular carbonate groups parallel to the basal plane and alternating with layers of Sr cations. There are significant amounts of calcium that substitute for Sr in most Strontianite minerals. There is also possible substitution of barium for Sr.

 

            Strontianite is known to have low negative to high positive relief, which can be seen clearly exhibited by the mineral under petrographic polarizing light microscope. This is however, depending on the orientation and it may show marked change with the rotation of the microscope stage. The hardness measure of the mineral Strontianite when it is evaluated with the aid of the Mohs scale method is 3.5. The specific gravity measure of the mineral is usually 3.75 grams per cubic centimeters. Strontianite usually appears colorless, gray, or white in hand sample. It may also appear yellowish, brownish, or greenish that can be very interesting when viewed under polarized light microscope. When the sample is rubbed on a white porcelain streak plate, it is usually found leaving a white streak. When viewed in reflected light of petrographic polarizing microscope, Strontianite is commonly found exhibiting a vitreous luster. Strontianite effervesces readily in cold dilute HCl. Strontianite is commonly found colorless in thin section or in grain mount.

 

            Strontianite is most commonly found as granular masses or aggregates of columnar, acicular, or fibrous crystals that are splendidly exhibited under polarizing microscopes used in optical mineralogy. The columnar crystals are usually pseudohexagonal like aragonite. Acicular crystals and the fibrous ones are usually elongate parallel to the c axis. With the aid of the petrographic polarizing microscope, it can be found that Strontianite has good prismatic cleavages in two directions that intersect at about 64 degrees.

 

            Strontianite commonly twins with composition planes. The twins may be found simple, polysynthetic, or cyclic. The longitudinal sections and elongate cleavage fragments are length fast with parallel extinction to cleavage and long dimension. Extinction to cleavage in basal sections is symmetrical. The indices of refraction of mineral Strontianite increase with substitution of calcium and barium for strontium. The indices are usually up 1.525, 1.686, and 1.690. With the aid of petrographic polarizing microscope, the birefringence of mineral Strontianite is usually found to be 0.165. The interference colors that are usually found for most orientations of the polarizing microscope in both grain mount and thin section are higher-order white. A near maximum birefringence is usually found displayed by fragments of the mineral Strontianite lying on the cleavage surfaces.

 

            Basal sections of mineral Strontianite usually yield centered acute bisectrix figures with numerous isochromes and a small optic angle. The optic angle of pure Strontianite is usually 7 degree and this value increases to about 10 degree for calcic samples. Strontianite fragments lying on cleavage surfaces usually yield off-center obtuse bisectrix figures that look like a flash figure due to small optic angle. The optic axis dispersion is commonly found very weak when the sample of the mineral is evaluated with the aid of the petrographic polarizing light microscope.

 

            Mineral Strontianite is commonly found as an alteration to celestite. This mineral Strontianite usually resembles the other orthorhombic carbonate minerals with extreme birefringence and parallel extinction that can be seen more clearly visible with the aid of the polarized microscopes used in optical mineralogy. Mineral Aragonite has higher indices when compared to Strontianite. The only one prominent cleavage is also a distinguishing feature of Aragonite. When compared to Witherite, Strontianite has lower indices and much smaller optic angle. Strontianite can be also best distinguished by the use of the flame test for it commonly displays a brilliant red flame for strontium and its indicative specific gravity that is very diagnostic for strontium. Strontianite commonly occurs in veins, cavities, and in irregular masses in limestone or calcareous clays where it was deposited by hydrothermal solutions. Strontianite is also sometimes found occurring as a gangue mineral in hydrothermal sulfide deposits.



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Wednesday, May 28th, 2008 at 5:02 am
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The Carbonates and Borates Mineral Class
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