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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

The chemical formula of Barite is indicated by BaSO4. Barite is a Sulfate mineral. Barite 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. When viewed with the aid of the petrographic polarizing light microscope, Barite is known to have a biaxial positive figure. The indices of refraction of mineral Barite varies from 1.634 to 1.649. The birefringence of the mineral when it is evaluated under petrographic polarizing microscope is usually 0.012. The optic angle of the mineral is usually found at about 36 to 40 degree.


            When viewed closely under polarized microscopes used in optical mineralogy, mineral Barite is commonly found exhibiting a moderately high positive relief. The hardness measure of the mineral when it is evaluated using the Mohs scale method is usually ranging from 3 to 3.5. The specific gravity measure of the mineral is usually found to be about 4.50 grams per cubic centimeters. Barite is commonly found white, yellow, gray, light blue, light green, red, or brown in hand sample. When rubbed on a white porcelain streak plate, Barite is commonly found leaving a white streak. When viewed in reflected light of the polarizing microscope, Barite is usually found exhibiting a vitreous luster.


            Barite is usually found colorless in thin section and grain mount. But there are also some colored varieties that may show pale colors and exhibit a weak pleochroism under petrographic polarizing microscope. Barite crystals are usually tabular in form or commonly prismatic parallel to the a or b crystal axes. Barite crystals are often integrown and are usually forming rosettes or platy aggregates that are commonly splendidly exhibited under polarizing microscopes. Barite may also form concretionary masses with fibrous texture. Sometimes granular or cleavable masses. Mineral Barite has four cleavages on one direction, two fair cleavages on the other direction, and a perfect basal cleavage on the other direction. All cleavage can be seen more clearly visible under polarizing microscopes used in optical mineralogy.


            Barite massive varieties may have deformation-induced twinning that can be found when the sample is evaluated with the aid of the polarized microscopes for optical mineralogy. Otherwise, this mineral is not twinned. For Barite mineral sections that are cut parallel to the c crystal axis, the extinction is parallel to the cleavage and the slow ray vibrates parallel to the long dimension of the section. In basal section of Barite mineral on the other hand, the extinction is usually found symmetrical to the prismatic cleavages. Fibrous or prismatic crystals are commonly found elongate along the a axis. Then it is usually length slow with parallel extinction. Those crystals that are elongate along b axis are relatively uncommon and usually show parallel extinction and may be either length fast or length slow.


            The indices of refraction of mineral Barite are usually found decreasing slightly with addition of strontium and increase of lead. The interference colors of thin section samples when evaluated under polarizing microscopes are usually ranging up to first-order yellow. There are sections that when cut would yield centered acute bisectrix figures with optic angle of 36 to 40 degree. This is with weak dispersion. There are also some sections, which show very low birefringence and parallel extinction. The cleavage fragments of Barite lying on the dominant cleavage usually produce an obtuse bisectrix figure.


            Barite may alter to mineral Witherite or may be replaced by a variety of minerals like dolomite, calcite, quartz, or pyrite. Mineral Barite is most commonly confused with gypsum, celestite, or anhydrite. But then gypsum is commonly found having an inclined extinction and negative relief. Anhydrite on the other hand has higher birefringence compared to Barite. Celestite has larger optic angle and lower indices when compared to Barite. Mineral Barite is commonly found as a gangue mineral in hydrothermal sulfide deposit and is also found as concretionary masses, veins, or irregular masses in limestone, dolomite, shale, or other sedimentary rocks. Mineral Barite can be also found in carbonatites.

Wednesday, May 28th, 2008 at 4:56 am
The Sulfates Mineral Class
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