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The chemical formula of Siderite is indicated by FeCO3. Siderite 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. When viewed under petrographic polarizing light microscope, mineral Siderite is known to have uniaxial negative figure. The indices of refraction of the mineral ranges from 1.633 to 1.875. The birefringence of Siderite is usually 0.242. When viewed closely under polarizing microscopes used in optical mineralogy, mineral Siderite is commonly showing a moderate to high positive relief and this usually change with rotation.

 

            Mineral Siderite is known in optical mineralogy field to form a complete solid solution series with mineral Magnesite and Rhodochrosite. Most Siderite is actually found containing significant amount of magnesium or manganese substituting for iron. It has been also found that almost up to 10 mole percent of calcium may substitute for iron, as can minor amounts of cobalt can substitute for zinc. Mineral Siderite is usually found having a hardness measure ranging from 4 to 4.5 when evaluated with the use of the Mohs scale method. The specific gravity measure of the mineral Siderite when pure is about 3.96 grams per cubic centimeters and this becomes lower with substitution of magnesium, manganese, or calcium and with about 50 mole percent magnesium to about 3.48 grams per cubic centimeters. Siderite is generally found with some shade of yellowish, grayish, or reddish brown and may also appear for quite sometime as grayish green or gray in hand sample. When rubbed on a white porcelain streak plate, mineral Siderite is commonly found leaving a white streak. When viewed in reflected light of the polarized microscope, Siderite also exhibits a vitreous luster. It usually reacts with slow effervescence in cold dilute hydrochloric acid. Siderite also reacts vigorously if powdered or if the acid is hot.

 

            Prepared thin sections of mineral Siderite are usually found colorless, ash gray, pale yellow, or yellowish brown in color. This is the same with Siderite samples in grain mount. Some colored varieties of Siderite may be pleochroic when viewed under polarizing microscopes used in optical mineralogy. Siderite samples may appear darker in color when the long diagonal of the rhomb is parallel to the lower polar vibration direction.

 

            Siderite is usually found as coarse-grained anhedral aggragates, as nodular, as oolites, or as botryoidal forms that is composed of radiating coarse fibers, or as earthy aggregates. Siderite crystals are usually found as rhombohedrons, sometimes modified by a basal pinacoid. There is a perfect rhombohedral cleavage that can be found when the mineral is evaluated with the aid of the petrographic polarizing light microscope. This cleavage is very typical of rhombohedral carbonates. Fragments of the mineral Siderite typically lie on the cleavage face and this can be seen more clearly when viewed with the aid of the polarizing microscopes used in optical mineralogy. Siderite may show lamellar twins on one direction that is similar to that of calcite. Sometimes a simple twin may also be found when the mineral is closely evaluated under polarized microscope. The twin lamellae are usually found parallel to the edges or along the long diagonal of the rhombs.

 

            The extinction of the mineral Siderite is usually found inclined or symmetrical to cleavage traces when the section is closely examined under a petrographic polarizing microscope. The fast ray vibration direction is parallel to the short diagonal of rhombohedral faces. The indices of refraction of the mineral vary in an approximately linear manner with composition. The fragments of the mineral Siderite lying on one direction on the cleavage faces usually shows an index of about 1.748 for a pure Siderite. This value would decrease to about 1.675 for 50 mole percent magnesium and 1.725 for 50 mole percent manganese. The interference colors of the mineral that can be seen exhibited by the thin section or grain mount when evaluated under polarized microscopes are typically upper-order white and gray, even if the optic axis is nearly vertical.

 

            Basal sections of mineral Siderite usually yields a uniaxial negative figure with numerous isochromes, and isogyres that fan out significantly toward the edge of the field of view of the polarizing microscope. When closely evaluated under petrographic polarizing microscope, the cleavage fragments of Siderite usually yield strongly off-center figures. Siderite is often altered to goethite of less commonly to hematite or magnetite. Pseudomorph after Siderite by these minerals and others is relatively common.

 

            Siderite can be easily distinguished from other carbonate minerals. Siderite is the one with higher indices when compared to other rhombohedral carbonates. The yellowish or brownish color of Siderite is one of its distinctive properties and is a common alteration to iron oxides or hydroxides. Siderite may be sometimes mistaken for mineral Cassiterite. But then Cassiterite is optically positive and has higher indices and its birefringence is lower than Siderite. Another is titanite, but this mineral is biaxial and has a darker color compared to Siderite. Siderite commonly occurs as disseminated grains or fine-grained masses in sedimentary iron formations where it is commonly associated with clays and other various iron oxides, hydroxides, and silicates. Siderite can be also found in hydrothermal sulfide deposits, in sedimentary rock types, and rarely in mica schists. It can be also found in igneous rocks.



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Wednesday, May 28th, 2008 at 5:00 am
Category:
The Carbonates and Borates Mineral Class
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