Hematite occurs naturally in black to steel or silver-gray, brown to reddish-brown, or red colors. It ismined as an importantore mineral of iron. It is electrically conductive.[7] Hematite varieties includekidney ore,martite (pseudomorphs aftermagnetite),iron rose andspecularite (specular hematite). While these forms vary, they all have a rust-redstreak. Hematite is not onlyharder than pure iron, but also much morebrittle. The termkidney ore may be broadly used to describebotryoidal, mammillary, or reniform hematite.[8]Maghemite is a polymorph of hematite (γ-Fe 2O 3) with the same chemical formula, but with aspinel structure like magnetite.
Large deposits of hematite are found inbanded iron formations. Gray hematite is typically found in places that have still, standing water, or mineralhot springs, such as those inYellowstone National Park inNorth America. The mineral mayprecipitate in the water and collect in layers at the bottom of the lake, spring, or other standing water. Hematite can also occur in the absence of water, usually as the result ofvolcanic activity.
Clay-sized hematite crystals also may occur as a secondary mineral formed byweathering processes insoil, and along with other iron oxides oroxyhydroxides such asgoethite, which is responsible for the red color of manytropical, ancient, or otherwise highly weathered soils.
The name hematite is derived from theGreek word for blood,αἷμα(haima), due to the red coloration found in some varieties of hematite.[6] The color of hematite is often used as apigment. The English name of the stone is derived fromMiddle Frenchhématite pierre, which was taken fromLatinlapis haematitesc. the 15th century, which originated fromAncient Greekαἱματίτης λίθος (haimatitēs lithos, "blood-red stone").
Ochre is a clay that is colored by varying amounts of hematite, varying between 20% and 70%.[9] Red ochre contains unhydrated hematite, whereas yellow ochre containshydrated hematite (Fe2O3 · H2O). The principal use of ochre is for tinting with a permanent color.[9]
Use of thered chalk of this iron-oxide mineral in writing, drawing, and decoration is among the earliest in human history. To date, the earliest known human use of the powdery mineral is 164,000 years ago by the inhabitants of thePinnacle Point caves in what now is South Africa, possibly for social purposes.[10] Hematite residues are also found in graves from 80,000 years ago. NearRydno inPoland andLovas inHungary red chalk mines have been found that are from 5000 BC, belonging to theLinear Pottery culture at theUpper Rhine.[11]
Rich deposits of hematite have been found on the island ofElba that have been mined since the time of theEtruscans.[12]
Underground hematite mining is classified as a carcinogenic hazard to humans.[13]
Hematite shows only a very feeble response to amagnetic field. Unlike magnetite, it is not noticeably attracted to an ordinary magnet. Hematite is anantiferromagnetic material below theMorin transition at 250 K (−23 °C), and acanted antiferromagnet or weaklyferromagnetic above the Morin transition and below itsNéel temperature at 948 K (675 °C), above which it isparamagnetic.
The magnetic structure of α-hematite was the subject of considerable discussion and debate during the 1950s, as it appeared to be ferromagnetic with a Curie temperature of approximately 1,000 K (730 °C), but with an extremely smallmagnetic moment (0.002 Bohr magnetons). Adding to the surprise was a transition with a decrease in temperature at around 260 K (−13 °C) to a phase with no net magnetic moment. It was shown that the system is essentially antiferromagnetic, but that the low symmetry of thecation sites allowsspin–orbit coupling to causecanting of the moments when they are in the plane perpendicular to thec axis. The disappearance of the moment with a decrease in temperature at 260 K (−13 °C) is caused by a change in theanisotropy which causes the moments to align along thec axis. In this configuration, spin canting does not reduce the energy.[14][15] 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 been observed in hematitenanoparticles and is attributed to the presence of impurities, water molecules and defects in the crystals lattice. Hematite is part of a complex solid solution oxyhydroxide system having various contents of H2O (water), hydroxyl groups and vacancy substitutions that affect the mineral's magnetic and crystal chemical properties.[16] Two other end-members are referred to as protohematite and hydrohematite.
Enhancedmagnetic coercivities for hematite have been achieved by dry-heating a two-line ferrihydrite precursor prepared from solution. Hematite exhibited temperature-dependent magnetic coercivity values ranging from 289 to 5,027oersteds (23–400 kA/m). The origin of these high coercivity values has been interpreted as a consequence of the subparticle structure induced by the different particle andcrystallite 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 (super small). At lower temperatures (350–600 °C), single particles crystallize. However, at higher temperatures (600–1000 °C), the growth of crystalline aggregates, and a subparticle structure is favored.[17]
Hematite is present in the wastetailings ofiron mines. A recently developed process,magnetation, uses magnets to glean waste hematite from old mine tailings inMinnesota's vastMesabi Range iron district.[18]Falu red is a pigment used in traditional Swedish house paints. It is made from tailings of theFalun Mine.[19]
Image mosaic from the Mars Exploration Rover Microscopic Imager shows hematitespherules partly embedded in rock at the Opportunity landing site. Image is around 5 cm (2 in) across.
The spectral signature of hematite was seen on the planetMars by the infraredspectrometer on theNASAMars Global Surveyor[20] and2001 Mars Odyssey[21] spacecraft in orbit around Mars. The mineral was seen in abundance at two sites[22] on the planet, theTerra Meridiani site, near the Martian equator at 0° longitude, and theAram Chaos site near theValles Marineris.[23] Several other sites also showed hematite, such asAureum Chaos.[24] 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 twoMars Exploration Rovers was sent to a site in the Terra Meridiani region designatedMeridiani Planum. In-situ investigations by theOpportunity rover showed a significant amount of hematite, much of it in the form of small "Martian spherules" that were informally named "blueberries" by the science team. Analysis indicates that thesespherules are apparentlyconcretions 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".[25]
Hematite is often shaped into beads, tumbling stones, and other jewellery components.[26] Hematite was once used as mourning jewelry.[27][7] Certain types of hematite- or iron-oxide-rich clay, especiallyArmenian bole, have been used ingilding. Hematite is also used in art such as in the creation ofintaglio engraved gems.Hematine is a synthetic material sold asmagnetic hematite.[28]
Hematite has been sourced to make pigments since earlier origins of human pictorial depictions, such as on cave linings and other surfaces, and has been employed continually in artwork through the eras. In Roman times, the pigment obtained by finely grinding hematite was known assil atticum. Other names for the mineral when used in painting includecolcotar andcaput mortuum. In Spanish, it is calledalmagre oralmagra, from the Arabical-maghrah, red earth, which passed into English and Portuguese. Other ancient names for the pigment includeochra hispanica,sil atticum antiquorum, andSpanish brown.[29] It forms the basis for red, purple, and brown iron-oxide pigments, as well as being an important component of ochre, sienna, and umber pigments.[30] The main producer of hematite for the pigment industry is India, followed distantly by Spain.
As mentioned earlier, hematite is an important mineral for iron ore. The physical properties of hematite are also employed in the areas of medical equipment, shipping industries, and coal production. Having high density and capable as an effective barrier against X-ray passage, it often is incorporated into radiation shielding. As with other iron ores, it often is a component of ship ballasts because of its density and economy. In the coal industry, it can be formed into a high specific density solution, to help separate coal powder from impurities.[31]
^Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (eds.)."Hematite"(PDF).Handbook of Mineralogy. Vol. III. Chantilly, VA: Mineralogical Society of America.ISBN978-0962209727. RetrievedDecember 22, 2018.
^Benvenuti, M.; Dini, A.; D'Orazio, M.; Chiarantini, L.; Corretti, A.; Costagliola, P. (June 2013). "The tungsten and tin signature of iron ores from Elba Island (Italy)".Archaeometry.55 (3):479–506.doi:10.1111/j.1475-4754.2012.00692.x.
^Dang, M.-Z.; Rancourt, D. G.; Dutrizac, J. E.; Lamarche, G.; Provencher, R. (1998). "Interplay of surface conditions, particle size, stoichiometry, cell parameters, and magnetism in synthetic hematite-like materials".Hyperfine Interactions.117 (1–4):271–319.Bibcode:1998HyInt.117..271D.doi:10.1023/A:1012655729417.S2CID94031594.
^Calvo Rebollar, Miguel (2009).Minerales y Minas de España. Vol. 4. Óxidos e hidróxidos [Minerals and mines of Spain Vol 4. Oxides and Hidroxides] (in Spanish). Madrid, Spain: Escuela Técnica Superior de Ingenieros de Minas de Madrid. Fundación Gómez Pardo.ISBN978-84-95063-99-1.
