Hematite
| Hematite | |
|---|---|
Brazilian trigonal hematite crystal
| |
| General | |
| Category | Oxide minerals |
| Formula (repeating unit) | iron(III) oxide, Fe2O3, α-Fe2O3[1] |
| Strunz classification | 04.CB.05 |
| Crystal system | Trigonal |
| Unit cell | a = 5.038(2) Å, c = 13.772(12) Å; Z = 6 |
| Identification | |
| Color | Metallic gray, dull to bright red |
| Crystal habit | Tabular to thick crystals; micaceous or platy, commonly in rosettes; radiating fibrous, reniform, botryoidal or stalactitic masses, columnar; earthy, granular, oolitic |
| Crystal symmetry | Trigonal hexagonal scalenohedral H-M symbol: (32/m) Space group: R3c |
| Twinning | Penetration and lamellar |
| Cleavage | None, may show partings on {0001} and {1011} |
| Fracture | Uneven to sub-conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 5.5–6.5 |
| Luster | Metallic to splendent |
| Streak | Bright red to dark red |
| Diaphaneity | Opaque |
| Specific gravity | 5.26 |
| Optical properties | Uniaxial (-) |
| Refractive index | nω = 3.150–3.220, nε = 2.870–2.940 |
| Birefringence | δ = 0.280 |
| Pleochroism | O = brownish red; E = yellowish red |
| References | [2][3][4] |
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. Gray hematite is typically found in places that can have still 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
Main article: Ochre
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 around the 15th century, which originated from Ancient Greek: αἱματίτης λίθος (haimatitēs lithos, "blood-red stone").Ochre is a clay that is colored by varying amounts of hematite, varying between 20% and 70%.[5] 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.[5]
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.[6] Hematite residues are also found in old graveyards from 80,000 years ago. Near Rydno in Poland and Lovas in Hungary, palaeolithic red chalk mines have been found that are from 5000 BC, belonging to the Linear Pottery culture at the Upper Rhine.[citation needed]
Rich deposits of hematite have been found on the island of Elba that have been mined since the time of the Etruscans.
Magnetism
Hematite is an antiferromagnetic material below the Morin transition at 250 kelvin (K) or -9.7 degrees Fahrenheit (°F), 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 (0.002 µB). 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.[7][8] 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.[9] Two other end-members are referred to as protohematite and hydrohematite.
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 favoured.[10]
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.[11] Falu red is a pigment used in traditional Swedish house paints. Originally, it was made from tailings of the Falu mine.[12]Mars
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