Hematite

Hematite
Brazilian trigonal hematite crystal
General
Category	Oxide minerals
Formula (repeating unit)	iron(III) oxide, Fe2O3, α-Fe2O3[1]
IMA symbol	Hem[2]
Strunz classification	4.CB.05
Dana classification	4.3.1.2
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H–M symbol: (3 2/m)
Space group	R3c (no. 167)
Unit cell	a = 5.038(2) Å; c = 13.772(12) Å; Z = 6
Identification
Color	Metallic grey, dull to bright "rust-red" in earthy, compact, fine-grained material, steel-grey to black in crystals and massively crystalline ores
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 subconchoidal
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
Density	5.3
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	[3][4][5]

Hematite (/ˈhiːməˌtaɪt, ˈhɛmə-/), also spelled as haematite, is a common iron oxide compound with the formula, Fe₂O₃ and is widely found in rocks and soils.^[6] Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of Fe
₂O
₃. It has the same crystal structure as corundum (Al
₂O
₃) and ilmenite (FeTiO
₃). With this it forms a complete solid solution at temperatures above 950 °C (1,740 °F).

Hematite naturally occurs in black to steel or silver-gray, brown to reddish-brown, or red colors. It is mined as an important ore mineral of iron. It is electrically conductive.^[7] Hematite varieties include kidney ore, martite (pseudomorphs after magnetite), iron rose and specularite (specular hematite). While these forms vary, they all have a rust-red streak. Hematite is not only harder than pure iron, but also much more brittle. Maghemite is a polymorph of hematite (γ-Fe
₂O
₃) with the same chemical formula, but with a spinel structure like magnetite.

Large deposits of hematite are found in banded iron formations. Gray hematite is typically found in places that have still, standing water or mineral hot springs, such as those in Yellowstone National Park in North America. The mineral can precipitate in the water and collect in layers at the bottom of the lake, spring, or other standing water. Hematite can also occur in the absence of water, 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, which is responsible for the red color of many tropical, ancient, or otherwise highly weathered soils.