Hematite

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Hematite
Image:HematiteUSGOV.jpg Image:Hematite.bear.660pix.jpg
General
Category	Mineral
Chemical formula	iron (III) oxide, Fe₂O₃, α-Fe₂O₃
Identification
Colour	Metallic grey to earthy red
Crystal habit	Tabular to thick crystals
Crystal system	Hexagonal (rhombohedral)
Cleavage	None
Fracture	Uneven to sub-conchoidal
Mohs Scale hardness	5.5 - 6.5
Luster	Metallic to splendent
Refractive index	Opaque
Pleochroism	None
Streak	Bright red to dark red
Specific gravity	4.9 - 5.3
Fusibility	?
Solubility	?
Major varieties
Specularite

Hematite (AE) or haematite (BE) is the mineral form of Iron (III) oxide, (Fe₂O₃), one of several iron oxides. The ore sometimes contains slight amounts of titanium. When shaped into ornaments, it is often called black diamond.

Image:Hematite.jpg

Hematite is a very common mineral, coloured black to steel or silver-gray, brown to reddish brown, or red. It is mined as the main ore of iron. Varieties include Bloodstone, Iron Rose, Kidney Ore, Martite, Paint Ore, Specularite (Specular Hematite), Rainbow Hematite and Titano-hematite. While the forms of hematite vary, they all have a rust-red streak. Hematite is harder than pure iron, but much more brittle.

Deposits of grey hematite are typically found in places where there has been standing water or mineral hot springs, such as those in Yellowstone. The mineral can precipitate out of water and collect in layers at the bottom of a lake, spring, or other standing water. But hematite can also occur without water, 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.

The name hematite is derived from the Greek word for blood, since sometimes hematite can be red, as in Rouge, a powderized form of hematite. It shares this root with the word hemoglobin. Hemoglobin is the oxygen-transporting molecule in red blood cells, the iron of which causes blood to be red. The color of hematite lends it well in use as a pigment.

Image:Rainbow hematite MG.jpg

Especially nice specimens of hematite come from England, Mexico, Brazil, Australia and the Lake Superior region of the United States and Canada.

Hematite is an antiferromagnetic material below the Morin transition at 260K, and a canted antiferromagnet or weakly ferromagnetic above the Morin transition. [1].

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Hematite on Mars

Hematite can be formed by aqueous processes, weathering processes, or without water. In 2004, NASA's Mars rover Opportunity discovered small spheres ("blueberries") believed to be made partly or mostly of hematite. The spheres are only a few millimeters in diameter and are believed to have formed as rock deposits under watery conditions billions of years ago when Mars was a wetter world. The rover used its instruments (a Mössbauer spectrometer) to test how the hematite found on Mars at Meridiani Planum was formed.

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