Hematite, also spelt hæmatite, is the mineral form of Iron(III) oxide (Fe2O3), one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and as corundum. Hematite and ilmenite form a complete solid solution at temperatures above 950°C.
Hematite is a mineral, 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. Grey hematite is typically found in places where there has been standing water or mineral hot springs, such as those in Yellowstone National Park in the United States. 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.
Ochre is a clay that is colored by varying amounts of hematite, varying between 20% and 70% . Red ochre contains unhydrated hematite, whereas yellow ochre contains hydrated hematite (Fe2O3 • H2O). The principal use of ochre is for tinting with a permanent color.
The magnetic structure of a-hematite was the subject of considerable discussion and debate in the 1950s because it appeared to be ferromagnetic with a Curie temperature of around 1000 K, but with an extremely tiny moment (0.002mB). Adding to the surprise was a transition with a decrease in temperature at around 260 K to a phase with no net magnetic moment.
Dzialoshinksi and later Moriya showed that the system is essentially antiferromagnetic but that the low symmetry of the cation sites allows spin–orbit coupling to cause canting of the moments when they are in the plane perpendicular to the c axis. The disappearance of the moment with a decrease in temperature at 260 K is caused by a change in the anisotropy which causes the moments to align along the c axis. In this configuration, spin canting does not reduce the energy.
Hematite is part of a complex solid solution oxyhydroxide system having various degrees of water, hydroxyl group, and vacancy substitutions that affect the mineral's magnetic and crystal chemical properties. Two other end-members are referred to as protohematite and hydrohematite.
The spectral signature of hematite was seen on the planet Mars by the infrared spectrometer on the NASA Mars Global Surveyor ("MGS") and 2001 Mars Odyssey spacecraft in orbit around Mars. The mineral was seen in abundance at two sites. on the planet, the Terra Meridiani site, near the Martian equator at 0° longitude, and the second site Aram Chaos near the Valles Marineris. Several other sites also showed hematite, e.g., Aureum Chaos. Because terrestrial hematite is typically a mineral formed in aqueous environments, or by aqueous alteration, this detection was scientifically interesting enough that the second of the two Mars Exploration Rovers was targeted to a site in the Terra Meridiani region designated Meridiani Planum. In-situ investigations by the Opportunity rover showed a significant amount of hematite, much of it in the form of small spherules that were informally tagged by the science team "blueberries" (a term which is somewhat confusing, since in spectrally-correct color images they are, in fact, silver-grey in color). Analysis indicates that these spherules are apparently concretions formed from a water solution.
Hematite's popularity in jewelry was at its highest in Europe during the Victorian era, while in the last 50 years it has been popular in North America, especially in the western United States where it is found in jewelry and art created by Native Americans. Care should be taken in handling hematite items due to the material's susceptibility to damage.