Hematite, also spelled as haematite, is a common iron oxide and is widespread 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 which it forms 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. It is electrically conductive. Varieties include kidney ore, martite (pseudomorphs after magnetite hematite). While these forms vary, they all have a rust-red streak. Hematite is harder than pure iron, but much more brittle 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 can have still, standing water or mineral hot springs , such as those in Yellowstone National Park in North America out of water and collect in layers at the bottom of a 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 , and along with other iron oxides or oxyhydroxides such as goethite, ancient, or otherwise highly weathered soils.