Hematite

Hematite
Hematite.jpg
Hematite (blood ore) from the US state of Michigan (unknown scale)
General
Category Oxide mineral
Chemical formula iron(III) oxide, Fe2O3, α-Fe2O3
Identification
Color Metallic Grey to earthy red tones
Crystal habit Tabular to thick crystals
Crystal system Hexagonal (rhombohedral)
Cleavage None
Fracture Uneven to sub-conchoidal
Mohs Scale hardness 5.5 - 6.5
Luster Metallic to splendent
Refractive index Opaque
Pleochroism None
Streak Bright red to dark red
Specific gravity 4.9 - 5.3
References [1][2]

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.

Hematite in SEM, magnification 100x
Close-up of hematitic Banded Iron Formation specimen from Upper Michigan. Scale bar is 5.0 mm.

Good specimens of hematite come from England, Mexico, Brazil, Australia, United States and Canada.

Contents

Ochre

Main article: Ochre

The name hematite is derived from the Greek word for blood (haima) because hematite can be red, as in rouge, a powdered form of hematite. The color of hematite lends it well in use as a pigment.

Ochre is a clay that is colored by varying amounts of hematite, varying between 20% and 70% [3]. 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[3].

Magnetism

Hematite is an antiferromagnetic material below the Morin transition at 260 K, and a canted antiferromagnet or weakly ferromagnetic [1] above the Morin transition and below its Néel temperature at 948K, 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 (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.[4] Two other end-members are referred to as protohematite and hydrohematite.

Hematite on Mars

Image mosaic from the Mars Exploration Rover Microscopic Imager shows Hematite spherules partly embedded in rock at the Opportunity landing site. (Scale: image is approximately 5 cm (2 in) across)

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.[5] The mineral was seen in abundance at two sites.[6] on the planet, the Terra Meridiani site, near the Martian equator at 0° longitude, and the second site Aram Chaos near the Valles Marineris.[7] Several other sites also showed hematite, e.g., Aureum Chaos.[8] 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.

Jewelry

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.

When polished the stone sometimes gives a look of silver. It is still popularly used in art works for making hollow portraits which go by the name of intaglios. Care should be taken in handling hematite items due to the material's susceptibility to damage.

See also

Hematite carving, 5 cm (2 in) long.

References

  1. ↑ http://webmineral.com/data/Hematite.shtml Webmineral data
  2. ↑ http://www.mindat.org/min-1856.html Mindat mineral data
  3. ↑ 3.0 3.1 "Ochre". Industrial Minerals. Minerals Zone. Retrieved on 2008-04-29.
  4. ↑ M.-Z. Dang, D.G. Rancourt, J.E. Dutrizac, G. Lamarche, and R. Provencher. Interplay of Surface Conditions, Particle Size, Stoichiometry, Cell Parameters, and Magnetism in Synthetic Hematite-like Materials. Hyperfine Interactions 117 (1998) 271-319.
  5. ↑ NASA MGS TES Press Release, May 27 1998 "Mars Global Surveyor TES Instrument Identification of Hematite on Mars", available here
  6. ↑ Bandfield, J.L., Global mineral distributions on Mars, J. Geophys Res., 107, 2002. See: Mars Global Data Sets: Hematite Abundance
  7. ↑ Glotch, T. D., and P. R. Christensen (2005), "Geologic and mineralogic mapping of Aram Chaos: Evidence for a water-rich history," J. Geophys. Res., 110, E09006, doi:10.1029/2004JE002389 abstract here
  8. ↑ T. D. Glotch, D. Rogers, and P. R. Christensen, A Newly Discovered Hematite-Rich Unit in Aureum Chaos: Comparison of Hematite and Associated Units With Those in Aram Chaos, Lunar and Planetary Science Conference XXXVI, 2005

External links