Hematite, also spelled as haematite, is the mineral form of iron(III) oxide, one of several
iron oxides. Hematite crystallizes in the rhombohedral lattice system, and it has the
same crystal structure as ilmenite and 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, iron rose and specularite. 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. Gray hematite is typically
found in places where there has been standing water or mineral hot springs, such as those
in Yellowstone National Park in North America. 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.
Etymology and history
The name hematite is derived from the Greek word for blood αἷμα haima because hematite
can be red, as in rouge, a powdered form of hematite. The color of hematite lends itself
to use as a pigment. The English name of the stone is derived from Middle French: Hématite
Pierre, which was imported from Latin: Lapis Hæmatites, which originated from Ancient
Greek: αἱματίτης λίθος. 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. The principal use of ochre is for tinting
with a permanent color. The red chalk writing of this mineral was
one of the earliest in the history of humans. The powdery mineral was first used 164,000
years ago by the Pinnacle-Point man possibly for social purposes. Hematite residues are
also found in old graveyards from 80,000 years ago. Near Rydno in Poland and Lovas in Hungary,
palaeolitic red chalk mines have been found that are from 5000 BC, belonging to the Linear
Pottery culture at the Upper Rhine. Rich deposits of hematite have been found
on the island of Elba that have been mined since the time of the Etruscans.
Jewelry
Hematite's popularity in jewelry was at its highest in Europe during the Victorian era,
and has since seen a strong resurgence in North America, especially in the western United
States. Certain types of hematite or iron oxide-rich clay, especially Armenian bole,
have been used in gilding. Hematite is also used in art such as in the creation of intaglio
engraved gems. Hematine is a synthetic material sold as magnetic hematite.
Magnetism
Hematite is an antiferromagnetic material below the Morin transition at 250 K, and a
canted antiferromagnet or weakly ferromagnetic above the Morin transition and below its Néel
temperature at 948 K, above which it is paramagnetic. 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. 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.
Iron from mine tailings 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. Discovery on Mars
The spectral signature of hematite was seen on the planet Mars by the infrared spectrometer
on the NASA Mars Global Surveyor 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,"
.. "One big question, of course, is whether life ever started on Mars. This mission probably
won't tell us that, but it may well lead to future missions that can answer that question."
See also
References
External links Martite – Mindat w/ location data
Iron rose – Mindat w/ locations Abandoned Mine Research.
Mars spheres image. Mars trench image showing a shiny texture
of half-buried spheres Florence Mine, Cumbria, UK.
MineralData.org