Hematite is an iron-oxide mineral of the Oxides and Hydroxides group, with structural formula [alpha-Fe2O3]. The structure is similar to that of corundum, and consists essentially of a dense arrangement of Fe3+ ions in octahedral coordination with oxygens in hexagonal closest-packing. The structure can also be described as the stacking of sheets of octahedraly (six-fold) coordinated Fe3+ ions between two closed-packed layers of oxygens. Since Fe is in a trivalent state (ferric Fe), each of the oxygens is bonded to only two Fe ions, and therefore, only two out of three available oxygen octahedrons are occupied. This arrangement makes the structure neutral with no charge excess or deficit. The Fe-O sheets are held together by strong covalent bonds and this results in a very hard and dense structure.
Most hematite is relatively pure, with only very minor inclusions of Fe2+, Ti, Al, and/or Mn. Very limited solid solutions have been reported with magnetite [Fe3O4], ilmenite [FeTiO3], and bixbyite [(Fe,Mn)2O3].
The crystal system of hematite is hexagonal, but crystals appear in a wide variety of forms. Well crystallized forms, also called specularite, tend to develop as flat trigonal crystals. Reniform (kidney ore) or botryoidal forms are common as dehydrited geothite and break down to fibers or splinters. Oolitic and fossiliferous hematite is often found as replacement of carbonate fossils. Crystals may also be massive, or soft and earthy. Hematite is usually opaque, steel-gray, to bright red or brown. The luster is bright metallic to submetallic.
The name "hematite" is from the Creek "haimatites", meaning blood-like in reference to the bright-red color of the powder hematite. The crystal structure displayed to the left is based on an x-ray diffraction structural refinement of the original structural data of Linus Pauling from 1925.
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Hematite is the alteration product of many Fe-bearing minerals, especially, magnetite, siderite, and pyrite, and is precipitated in seas and lakes by chemical or organic processes. Its occurence may be attributed, but not limited, to intense weathering under normal and oxidizing environments. Sedimentary deposits of hematite may be extensive and constitute major ores of iron. Hematite deposits may occur as intermixed layers with chert and quartz, especially in cambrian sediments that have been moderately metamorphosed. The banded iron formations common in the Mesabi Iron Range are a good example.
Hematite is exceedingly common in surface environments and soils as a result of the weathering of Fe-containing minerals, and is responsible for the red and red-brown staining of soils and surface rocks. Partally hydrated hematite may induce yellow-brown staining. Several iron ore localities occur in the USA, mainly along Lake Superior, New York and Arizona. Other major mines are found in tropical and subtropical zones in Africa, Asia, and South America.
Igneous rocks such as granite, rhyolite, and similar rocks may contain primary accessory grains of hematite, suggesting oxygen rich magmas. The specularite form of hematite is commonly deposited as euhedral or platy crystals with quartz in veins and cavities by volcanic gases, and in quartz veins by high-temperature hydrothermal fluids. Intergrowths of ilmenite are common with hematite.
Hematite may occur in many soils as a product of weathering, or as an inherited mineral from parent rocks, or both. It is more common in tropical and warm temperate regions than in colder areas, where goethite usually predominates. Also, more hematite is found under well drained soils than in soils with limited drainage. Hematite is often a constituent of concretions and nodules and of plinthite. Hematite is responsible for the red, ochre-red and red-brown colors of many soils, especially those evolving under strongly-contrasting alternating wet and dry seasons, such as in mediterranean type environments. Hematites may exist as a dehydrated form of goethite and lepidocrocite. The latter minerals may reverse to hematite under dry conditions. Fine textured hematite plays an important role as a cementing agent for soil aggregates.
Blake, R.L., R.E. Hessevick, T. Zoltai, and L.W. Finger. 1966. Refinement of the hematite structure. American Mineralogist. 51:123-129.