Magnetite: Wikis

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Magnetite
Magnetite exposed on the ground. The mineral is black and irregularly smooth. Individual chunks jut at angles characteristic of the crystal habit.
Magnetite from the Kola Peninsula, Russia
General
Category Mineral
Chemical formula iron(II,III) oxide, Fe3O4
Identification
Color Black, grayish
Crystal habit Octahedral, fine granular to massive
Crystal system Isometric
Cleavage Indistinct
Fracture Uneven
Mohs scale hardness 5.5–6.5
Luster Metallic
Streak Black
Specific gravity 5.17–5.18
Refractive index Opaque
Major varieties
Lodestone Magnetic with definite north and south poles

Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron(II,III) oxide and the common chemical name ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part wüstite (FeO) and one part hematite (Fe2O3). This refers to the different oxidation states of the iron in one structure, not a solid solution. The Curie temperature of magnetite is 858 K.

Contents

Properties

Magnetite is the most magnetic of all the naturally occurring minerals on Earth.[1] Naturally magnetized pieces of magnetite, called lodestone, will attract small pieces of iron, and this was how ancient man first discovered the property of magnetism. Lodestone was used as an early form of magnetic compass. Magnetite typically carries the dominant magnetic signature in rocks, and so it has been a critical tool in paleomagnetism, a science important in discovering and understanding plate tectonics and as historic data for magnetohydrodynamics and other scientific fields. The relationships between magnetite and other iron-rich oxide minerals such as ilmenite, hematite, and ulvospinel have been much studied, as the complicated reactions between these minerals and oxygen influence how and when magnetite preserves records of the Earth's magnetic field.

Magnetite has been very important in understanding the conditions under which rocks form and evolve. Magnetite reacts with oxygen to produce hematite, and the mineral pair forms a buffer that can control oxygen fugacity. Commonly igneous rocks contain grains of two solid solutions, one between magnetite and ulvospinel and the other between ilmenite and hematite. Compositions of the mineral pairs are used to calculate how oxidizing was the magma (i.e., the oxygen fugacity of the magma): a range of oxidizing conditions are found in magmas and the oxidation state helps to determine how the magmas might evolve by fractional crystallization.

Small grains of magnetite occur in almost all igneous rocks and metamorphic rocks. Magnetite also occurs in many sedimentary rocks, including banded iron formations. In many igneous rocks, magnetite-rich and ilmenite-rich grains occur that precipitated together from magma. Magnetite also is produced from peridotites and dunites by serpentinization.

Magnetite is a valuable source of iron ore. It dissolves slowly in hydrochloric acid.

Distribution of deposits

A fine textured sample, ~5cm across
Magnetite and other heavy minerals (dark) in a quartz beach sand (Chennai, India).

Magnetite is sometimes found in large quantities in beach sand. Such black sands (mineral sands or iron sands) are found in various places such as California and the west coast of New Zealand. The magnetite is carried to the beach via rivers from erosion and is concentrated via wave action and currents.

Huge deposits have been found in banded iron formations. These sedimentary rocks have been used to infer changes in the oxygen content of the atmosphere of the Earth.

Large deposits of magnetite are also found in the Atacama region of Chile, Kiruna, Sweden, the Pilbara, Midwest and Northern Goldfields regions in Western Australia, and in the Adirondack region of New York in the United States. Deposits are also found in Norway, Germany, Italy, Switzerland, South Africa, India, Mexico, and in Oregon, New Jersey, Pennsylvania, North Carolina, Virginia, New Mexico, Utah, and Colorado in the United States. Recently, in June 2005, an exploration company, Cardero Resources, discovered a vast deposit of magnetite-bearing sand dunes in Peru. The dune field covers 250 square kilometers (100 sq mi), with the highest dune at over 2,000 meters (6,560 ft) above the desert floor. The sand contains 10% magnetite.[2]

Biological occurrences

Crystals of magnetite have been found in some bacteria (e.g., Magnetospirillum magnetotacticum) and in the brains of bees, of termites, fish, some birds (e.g., the pigeon) and humans.[3] These crystals are thought to be involved in magnetoreception, the ability to sense the polarity or the inclination of the Earth's magnetic field, and to be involved in navigation. Also, chitons have teeth made of magnetite on their radula making them unique among animals. This means they have an exceptionally abrasive tongue with which to scrape food from rocks.

The study of biomagnetism began with the discoveries of Caltech paleoecologist Heinz Lowenstam in the 1960s.

Preparation as a ferrofluid

Crystal structure of magnetite

Magnetite can be prepared in the laboratory as a ferrofluid in the Massart method by mixing iron(II) chloride and iron(III) chloride in the presence of sodium hydroxide.

Magnetite also can be prepared by chemical co-precipitation, which consist in a mixture of a solution 0.1 M of FeCl3·6H2O and FeCl2·4H2O with mechanic agitation of about 2000 rpm. The molar ratio of FeCl3:FeCl2 can be 2:1; heating this solution at 70 °C, and immediately the rpm is elevated to 7500 rpm and adding quickly a solution of NH4OH (10 volume %), immediately a dark precipitate will be formed, which consist of nanoparticles of magnetite.

Application as a sorbent

Magnetite powder efficiently removes As(III) and As(V) from water, and the efficiency of the removal increases ~200 times when the magnetite particle size decreases from 300 to 12 nm.[4] Arsenic(As)-contaminated drinking water is a major problem around the world, which can be solved using magnetite as a sorbent.

Jewelry

Magnetite ring

Magnetite is commonly used as the primary material in jewelry worn by adherents of the controversial[5] hypothesis of magnet therapy. When polished and made into jewelry magnetite has a dark, shiny finish with a smooth surface.

See also

References

  1. ^ Harrison, R. J. (2002). "Direct imaging of nanoscale magnetic interactions in minerals" (free-download pdf). Proceedings of the National Academy of Sciences 99: 16556. doi:10.1073/pnas.262514499. http://www.pnas.org/content/99/26/16556.full.pdf.  
  2. ^ Ferrous Nonsnotus
  3. ^ Baker, R R; J G Mather, J H Kennaugh (1983-01-06). "Magnetic bones in human sinuses". Nature 301 (5895): 79–80. PMID 6823284.  
  4. ^ J.T. Mayo et al. (2007). "The effect of nanocrystalline magnetite size on arsenic removal" (free download). Sci. Technol. Adv. Mater. 8: 71. doi:10.1016/j.stam.2006.10.005.  
  5. ^ Wanjek, Christopher (2003). Bad Medicine: misconceptions and misuses revealed from distance healing to vitamin O. Hoboken, New Jersey: John Wiley & Sons. pp. 1–253. ISBN 0-471-43499-X.  

Further reading

External links

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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

MAGNETITE, a mineral forming the natural magnet (see Magnetism), and important also as an iron-ore. It is an ironblack, opaque mineral, with metallic lustre; hardness about 6, sp. gr. 4.9 to 5.2. When scratched, it yields a black streak. It is an oxide of iron having the formula Fe 3 O 4, corresponding with 72.4% of metal, whence its great value as an ore. It may be regarded as a ferroso-ferric oxide, FeO.Fe 2 O 3, or as iron ferrate, Fe"Fe 2 '0 4. Titanium is often present, and occasionally the mineral contains magnesium, nickel, &c. It is always strongly magnetic. Magnetite crystallizes in the cubic system, usually in octahedra, less commonly in rhombic dodecahedra, and not infrequently in twins of the " spinel type " (fig. I). The rhombic faces of the dodecahedron are often striated parallel to the longer diagonal. There is no distinct cleavage, but imperfect parting may be obtained along octahedral planes.

Magnetite is a mineral of wide distribution, occurring as grains in many massive and volcanic rocks, like granite, diorite and dolerite. It appears to have crystallized from the magma at a very early period of con FIG. I. solidation. Its presence contributes to the dark colour of many basalts and other basic rocks, and may cause them to disturb the compass. Large ore-bodies of granular and compact magnetite occur as beds and lenticular masses in Archean gneiss and crystalline schists, in various parts of Norway, Sweden, Finland and the Urals; as also in the states of New York, New Jersey, Pennsylvania and Michigan, as well as in Canada. In some cases it appears to have segregated from a basic eruptive magma, and in other cases to have resulted from metamorphic action. Certain deposits appear to have been formed, directly or indirectly, by wet processes. Iron rust sometimes contains magnetite. An interesting deposit of oolitic magnetic ore occurs in the Dogger (Inferior Oolite) of Rosedale Abbey, in Yorkshire; and a somewhat similar pisolitic ore, of Jurassic age, is known on the continent as chamoisite, having been named from Chamoison (or Chamoson) in the Valais, Switzerland. Grains of magnetite occur in serpentine, as an alteration-product of the olivine. In emery, magnetite in a granular form is largely associated with the corundum; and in certain kinds of mica magnetite occurs as thin dendritic enclosures. Haematite is sometimes magnetic, and A. Liversidge has shown that magnetite is probably present. By deoxidation, haematite may be converted into magnetite, as proved by certain pseudomorphs; but on the other hand magnetite is sometimes altered to haematite. On weathering, magnetite commonly passes into limonite, the ferrous oxide having probably been removed by carbonated waters. Closely related to magnetite is the rare volcanic mineral from Vesuvius, called magnoferrite, or magnesioferrite, with the formula MgFe 2 O 4; and with this may be mentioned a mineral from Jakobsberg, in Vermland, Sweden, called jakobsite, containing MnFe204. (F. W. R.*)


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