Posted by mining | Posted in iron ore | Posted on 24-01-2012
Hematite, also spelled as haematite, is the mineral form of iron(III) oxide (Fe2O3), one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum. Hematite and ilmenite form a complete solid solution at temperatures above 950 °C.
Hematite is a mineral, colored black to steel or silver-gray, brown to reddish brown, or red. It is mined as the main ore of iron. Varieties include kidney ore, martite (pseudomorphs after magnetite), iron rose and specularite (specular hematite). While the forms of hematite vary, they all have a rust-red streak. Hematite is harder than pure iron, but much more brittle. Maghemite is a hematite- and magnetite-related oxide mineral.
Huge deposits of hematite are found in banded iron formations. Grey hematite is typically found in places where there has been standing water or mineral hot springs, such as those in Yellowstone National Park in the United States. The mineral can precipitate out of water and collect in layers at the bottom of a lake, spring, or other standing water. Hematite can also occur without water, however, usually as the result of volcanic activity.
Clay-sized hematite crystals can also occur as a secondary mineral formed by weathering processes in soil, and along with other iron oxides or oxyhydroxides such as goethite, is responsible for the red color of many tropical, ancient, or otherwise highly weathered soils.
Hematite ?–Fe2O3
Crystal Data of Hematite
Hexagonal. Point Group: 32/m. Crystals complex rhombohedral, pseudocubic, prismatic, rarely scalenohedral, to 13 cm, with triangular striations on {0001} and {1011}; thin tabular, to micaceous or platy, commonly in rosettes; radiating fibrous, reniform, botryoidal or
stalactitic masses, columnar; earthy, granular, oolitic, concretionary.
Twinning: Penetration twins on {0001} or with {1010} as composition plane; also lamellar on {1011}.
Physical Properties: Cleavage: Parting on {0001} and {1011} due to twinning.
Fracture: Uneven to subconchoidal. Tenacity: Brittle; elastic in thin laminae. Hardness = 5–6
VHN = 1000–1100 (100 g load). D(meas.) = 5.26 D(calc.) = 5.255
Optical Properties of Hematite
Opaque, transparent on thin edges.
Color: Steel-gray, may have iridescent tarnish, dull to bright red; white to gray-white, with a bluish tint, in reflected light, with deep blood-red internal reflections.
Streak: Cherry-red or reddish brown.
Luster: Metallic or submetallic to dull.
Optical Class: Uniaxial (–).
Pleochroism: O = brownish red; E = yellowish red.
Mineral Group: Hematite group
Distribution: Widespread. Exceptional crystals from Switzerland, as at Fibbia, St. Gotthard, Uri; Binntal, Valais; Cavradi, Tavetsch, Gra¨ubunden; and many other places. At Ocna de Fier (Mor´avicza; Vask˝o), Romania. From Rio Marina, Elba, Italy. At Cleator Moor, Cumbria, England. From Kragerøand Hiassen, Norway. In Brazil, large crystals from Mesa Redonda and Congonhas do Campo, Minas Gerais; at Itabira and in the Brumado mine, Bahia; at Miguel Burnier, Ouro Prˆeto. From the Kuruman district, Cape Province, South Africa. At Nador, Algeria. In the USA, in the Thomas Range, Juab Co., Utah, and near Quartzsite, La Paz Co., Arizona.
Occurrences of Hematite
An accessory mineral in felsic igneous rocks, a late-stage sublimate in volcanic rocks, and in high-temperature hydrothermal veins. A product of contact metamorphism and in metamorphosed banded iron formations. A common cement in sedimentary rocks and a major constituent in oolitic iron formations. Abundant on weathered iron-bearing minerals. Association: Ilmenite, rutile, magnetite (metamorphic and igneous); goethite, siderite, lepidocrocite (sedimentary).