*** Welcome to piglix ***

Haematite

Hematite
WLA hmns Hematite.jpg
Brazilian trigonal hematite crystal
General
Category Oxide minerals
Formula
(repeating unit)
iron(III) oxide, Fe2O3, α-Fe2O3
Strunz classification 4.CB.05
Crystal system Trigonal
Crystal class Hexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space group R3c
Unit cell a = 5.038(2) Å;
c = 13.772(12) Å; Z = 6
Identification
Color Metallic gray, dull to bright red
Crystal habit Tabular to thick crystals; micaceous or platy, commonly in rosettes; radiating fibrous, reniform, botryoidal or stalactitic masses, columnar; earthy, granular, oolitic
Twinning Penetration and lamellar
Cleavage None, may show partings on {0001} and {1011}
Fracture Uneven to sub-conchoidal
Tenacity Brittle
Mohs scale hardness 5.5–6.5
Luster Metallic to splendent
Streak Bright red to dark red
Diaphaneity Opaque
Specific gravity 5.26
Optical properties Uniaxial (-)
Refractive index nω = 3.150–3.220, nε = 2.870–2.940
Birefringence δ = 0.280
Pleochroism O = brownish red; E = yellowish red
References

Hematite, also spelled as haematite, is the mineral form of iron(III) oxide (Fe2O3), one of several iron oxides. Hematite crystallizes in the rhombohedral lattice 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 (1,740 °F).

Hematite is 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. Gray hematite is typically found in places that can have still standing water or mineral hot springs, such as those in Yellowstone National Park in North America. 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.


...
Wikipedia

...