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The chemical formula of Magnesite is indicated by MgCO3. This Magnesite is a Carbonate mineral. It is known to crystallize in the hexagonal or trigonal crystal system. In optical mineralogy, the hexagonal system of crystallization comprises crystals having four axes. Three of which are positioned in a single plane with equal length and are symmetrically spaced. The fourth axis is found to be perpendicular to the other three axes. Magnesite is a uniaxial negative mineral. This mineral is commonly found exhibiting a low negative to high positive relief when viewed under several adjustments on the aperture diaphragm of the petrographic polarizing light microscope. Marked change with complete rotation of the stage of the polarized microscope. Magnesite is commonly found forming a complete solid solution series with siderite. There is also a limited substitution of Manganese and Calcium for Magnesium.

 

            When evaluated with the use of the Mohs scale method, Magnesite is usually found having a hardness measure ranging from 3.5 to 4.5. The specific gravity measure of the mineral Magnesite when pure is usually 3.01 grams per cubic centimeters and ranges up to 3.48 grams per cubic centimeters with 50 percent iron inclusion. Magnesite is usually found white or gray in hand sample, and may appear yellow or brown if iron-bearing. Magnesite is usually found leaving a white streak when rubbed on a white porcelain streak plate. When viewed in reflected light of polarized microscope, Magnesite is usually found vitreous to earthy luster. Fine granular varieties are commonly intergrown with microcrystalline quartz or opal that makes it difficult to determine hardness and specific gravity. Magnesite reacts vigorously with cold dilute hydrochloric acid only if powdered.

 

            Mineral Magnesite is most commonly found colorless in thin section and grain mount. It may be actually appear cloudy sometimes. Magnesite usually occurs in compact granular aggregates. This form of Magnesite usually appear chalky or like porcelain if evaluated in hand sample. There can be some times when Magnesite is found as lamellar or fibrous aggregates. Magnesite crystals are commonly found as rhombs or may be prismatic. When closely evaluated under petrographic polarizing microscope, Magnesite is usually found having perfect rhombohedral cleavage, like the other rhombohedral carbonates. Fragments are commonly found lying on cleavage surfaces and can be seen more clearly visible when viewed with the aid of the polarized microscope used in optical mineralogy. Magnesite crystals are typically not twinned although translation gliding on one direction may occur. Magnesite has an extinction that is inclined or symmetrical to cleavage traces and this can be seen more clearly with the aid of the petrographic polarizing microscope. The fast ray vibration direction is parallel to the short diagonal of the rhombohedral faces.

 

            The indices of refraction of Magnesite vary in an approximately linear manner. The same is true with its birefringence. The index for fragments that lies on cleavage surface may increase with the increasing iron content of the mineral. The interference colors seen in thin section or in grain mount are usually found typically creamy high-order colors. This is typically true even if the optic axis is close to vertical. The grains in thin section typically show a marked change of relief with rotation due to the extreme birefringence of the mineral.

 

            Basal sections of mineral Magnesite commonly yield uniaxial interference figures with numerous isochromes, and isogyres that broaden substantially toward the edge of the field of view of polarizing microscope. The cleavage fragments of Magnesite yield strongly off-center figures. Magnesite is generally not significantly altered. But iron-bearing varieties may show red staining or can be possibly altered to iron hydroxides and oxides. Mineral Magnesite is distinguished from the other rhombohedral carbonate minerals. This is because of its lack of twin lamellae, physical properties, and index of refraction.

 

            Magnesite most commonly occurs as fine to extremely fine-grained masses that are usually produced as an alteration product of magnesium-rich rocks such as peridotite, dunite, and many others. Magnesite may also occur as disseminated grains or stratified layers in talc, mica schists, or chlorite in metamorphic rocks. It can be also a result of an alteration of calcite by magnesium-bearing solutions. In sedimentary rocks, mineral Magnesite can be found occasionally in evaporate deposits either as the result of direct precipitation or of alteration of calcite or dolomite during diagenesis. Magnesite is also occasionally found as a gangue mineral in hydrothermal sulfide deposits. It is also sometimes reported as a primary mineral in igneous rocks where it is associated with magnesium-rich silicates.



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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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