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Click Here For Best Selection Of High Quality Polarizing Microscope

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The chemical formula of mineral Witherite is indicated by BaCO3. Witherite is a Carbonate mineral. It is known to crystallize in the orthorhombic structure. In optical mineralogy, the orthorhombic crystal system comprises crystals having three mutually perpendicular axes, of which all are of different lengths. Mineral Witherite is known to have biaxial negative figure that can be found more clearly exhibited when the sample is viewed under polarizing microscope used in optical mineralogy. The indices of refraction of Witherite range from 1.592 to 1.677. The birefringence of Witherite is commonly found to be 0.148. The optic angle of Witherite when it is evaluated under petrographic polarizing microscope is about 16 degrees. Mineral Witherite has the same structure as Aragonite. The composition of Witherite is usually close to the pure BaCO3 but there is possible substitution of small amounts of strontium or calcium to barium.

 

            When Witherite is examined closely with the aid of the petrographic polarizing light microscope, it can be found having low negative to high positive relief. This is, on the other hand, dependent on the orientation of the polarizing microscope. The mineral may actually show a marked change with stage rotation of the polarized microscope. The hardness measure of the mineral when it is evaluated with the use of the Mohs scale method is usually 3.5. The specific gravity measure of the mineral is usually at about 4.30 grams per cubic centimeters. When closely examined, mineral Witherite is usually colorless, white, or gray in hand sample. It may also appear to be pale yellow, brown, or green in color. When rubbed on a white porcelain streak plate, Witherite is commonly found leaving a white streak. When viewed in reflected light of petrographic polarizing light microscope, it commonly exhibits a vitreous luster. Witherite effervesces in cold dilute HCl.

 

            Mineral Witherite is commonly found colorless in grain mount or thin section. When found, Witherite is usually forming stubby pseudohexagonal dipyramids. Because of the cyclic twins, these hexagonal forms are produced. These cyclic twins are usually found producing six segments. Witherite also forms granular, columnar, or fibrous masses that may have rounded or botryoidal shapes, which are usually splendidly exhibited under petrographic polarizing microscopes used in optical mineralogy.

 

            Witherite commonly shows a single distinct cleavage on one direction and poor cleavages on the other two directions. These cleavages are usually seen more clearly visible when the sample is evaluated closely with the aid of the polarizing microscopes. Mineral Witherite is always twinned on one direction and forms cyclic twins with six segments. The extinction of the mineral when viewed under petrographic polarizing microscope is usually parallel to the cleavage traces in principal sections. The fast ray is usually found vibrating parallel to the cleavage traces in both basal and longitudinal sections.

 

            The indices of Witherite do not vary significantly because there is relatively little compositional variation. The interference colors of the mineral in thin section and grain mount are usually found to be upper-order white for most orientations. That is of course with the exception of those having an optical axis nearly vertical. The basal sections of mineral Witherite commonly yield centered acute bisectrix figures and when viewed between crossed nicols would display numerous isochromes and optic angle of about 16 degree. The fragments of the mineral lying on the dominant cleavage yield centered flash figures when viewed under polarizing microscope. The optic axis dispersion of Witherite is usually weak. Mineral Witherite may alter to barite or may be produced as an alteration product after barite.

 

            Just like other orthorhombic carbonate minerals, Witherite is distinguished from the rhombohedral carbonates by their biaxial character and lack of rhombohedral cleavage. The indices of Strontianite are significantly lower than Witherite. It also has two good cleavages that can be seen clearly with the aid of the polarized microscopes used in optical mineralogy. Mineral Aragonite has almost the same cleavage and indices of refraction but it has significantly lower specific gravity measure compared to Witherite. Witherite fragments noticeably sink on diodomethane while aragonite fragments float. With the use of the flame test method, the yellow green flame for barium is also very diagnostic. Mineral Witherite commonly occurs at low-temperature hydrothermal veins and masses that are deposited in limestone or other calcareous sediment. Witherite is commonly associated with mineral barite and galena.



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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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