Hematite
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. 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 | |
|---|---|
Brazilian trigonal hematite crystal | |
| General | |
| Category | Oxide minerals |
| Formula (repeating unit) | iron(III) oxide, Fe2O3, α-Fe2O3 |
| IMA symbol | Hem |
| 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 | |
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. 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.