The chemical formula of mineral Hematite is indicated by Fe2O3. Hematite is an Oxide mineral. It is known to crystallize in the hexagonal or trigonal system of crystallization. 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. Hematite is often found opaque with uniaxial negative figure that can be seen more clearly exhibited under polarizing microscopes used in optical mineralogy. The indices are commonly found ranging from 2.87 to 3.22. The birefringence found exhibited by Hematite under petrographic polarizing light microscope is usually 0.28. When evaluated closely, Hematite exhibits an extreme positive relief under several minor adjustments on the aperture diaphragm of the polarized microscope.
Hematite crystals are usually well-formed most especially the specular hematite. Specular Hematite is commonly found as flat hexagonal plates or scales that may look micaceous in hand samples. But Hematite in general is more commonly found as fine-grained anhedral aggregated or grains. It also forms oolites in sedimentary rocks and reniform masses with a fibrous or radiating structure. Hematite has no cleavage but may have a pronounced basal parting that is clearly exhibited under polarizing microscope. Rhombohedral parting can be also possibly found. Both partings are actually related to twinning. Hematite crystals are commonly twinned however, lamellar twinning is not usually visible because Hematite is commonly found opaque.
The hardness measure of the mineral Hematite is commonly found ranging from 5 to 6 when evaluated using the Mohs scale method. In optical mineralogy tests, it has been found that earthy varieties of Hematite are softer. The specific gravity of Hematite is usually found to be about 5.26 grams per cubic centimeters. But there are some lower density Hematite samples that were reported as evaluated in optical mineralogy. The color of the mineral Hematite in hand sample is usually metallic steel gray or sometimes red to red brown. When Hematite samples are rubbed on a white porcelain streak plate, it is commonly found leaving a red brown streak. Hematite are commonly found weakly magnetic.
The most common color of Hematite is deep red brown in every small crystal or along the thin edges of some samples. Hematite crystals are usually found opaque in appearance. Thin crystals of Hematite are pleochroic exhibiting a brownish red color that is very splendid under polarizing microscopes of optical mineralogy. Sometimes yellowish red or brown is also fascinatingly exhibited. When viewed in reflected light of petrographic polarizing microscope, it appears as metallic black with red internal reflection.
Hematite grains that need to be sufficiently thin to be transparent have extinction parallel to the basal partings and bottom faces of tabular crystals. It is also important to note that tabular crystals of Hematite should be length fast. However, with the rotation of the stage of the petrographic polarizing microscope, the extreme dispersion of the mineral commonly prevents the grains from becoming extinct. The mineral color and the extreme birefringence of the mineral make determining the sign of elongation impossible.
The indices of refraction of the mineral Hematite are well beyond the range of routinely used index oils. The dispersion of the indices is commonly found extreme. The birefringence of Hematite is also extreme but it has been found in optical mineralogy evaluations that the color of the mineral often masks the interference color in grains that are thin enough to be transparent. Although it has been proven as evaluated in optical mineralogy that Hematite is uniaxial negative mineral, the useful interference figures are generally not obtainable because of the opacity of the mineral Hematite, its extreme birefringence, and extreme dispersion.
Hematite is relatively stable in the weathering environment and is often the product of weathering or other alteration of iron-bearing minerals. Hematite may be altered to iron hydroxide minerals or siderite. Hematite commonly occurs as alteration product of other mafic minerals. It is also common as finely disseminated grains in many clastic sedimentary rocks, to which it gives a red color. Hematite is also a primary accessory mineral in igneous rocks and a common mineral in some hydrothermal deposits.
