<P> 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 magnetic moment (0.002 μ). Adding to the surprise was a transition with a decrease in temperature at around 260 K to a phase with no net magnetic moment . It was shown 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 . The magnetic properties of bulk hematite differ from their nanoscale counterparts . For example, the Morin transition temperature of hematite decreases with a decrease in the particle size . The suppression of this transition has also been observed in some of the hematite nanoparticles, and the presence of impurities, water molecules and defects in the crystals were attributed to the absence of a Morin transition . Hematite is part of a complex solid solution oxyhydroxide system having various contents of water, hydroxyl groups and vacancy substitutions that affect the mineral's magnetic and crystal chemical properties . Two other end - members are referred to as protohematite and hydrohematite . </P> <P> Enhanced magnetic coercivities for hematite have been achieved by dry - heating a 2 - line ferrihydrite precursor prepared from solution . Hematite exhibited temperature - dependent magnetic coercivity values ranging from 289 to 5,027 Oe . The origin of these high coercivity values has been interpreted as a consequence of the subparticle structure induced by the different particle and crystallite size growth rates at increasing annealing temperature . These differences in the growth rates are translated into a progressive development of a subparticle structure at the nanoscale . At lower temperatures (350--600 ° C), single particles crystallize however; at higher temperatures (600--1000 ° C), the growth of crystalline aggregates with a subparticle structure is favored . </P> <P> Hematite is present in the waste tailings of iron mines . A recently developed process, magnetation, uses magnets to glean waste hematite from old mine tailings in Minnesota's vast Mesabi Range iron district . Falu red is a pigment used in traditional Swedish house paints . Originally, it was made from tailings of the Falu mine . </P> <P> 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 Aram Chaos site 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 sent 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 named "blueberries" by the science team . Analysis indicates that these spherules are apparently concretions formed from a water solution . "Knowing just how the hematite on Mars was formed will help us characterize the past environment and determine whether that environment was favorable for life". </P>

What is not a commonly mined metallic substance