From Wikipedia, the free encyclopedia

Magnetite is not to be confused with Magnesite or Magnemite.

Magnetite
Magnetite from the Kola Peninsula, Russia
General
Category	Mineral
Chemical formula	iron(II,III) oxide, Fe3O4
Identification
Color	Black, greyish
Crystal habit	Octahedral, fine granular to massive
Crystal system	Isometric
Cleavage	Indistinct
Fracture	Uneven
Mohs Scale hardness	5.5–6.5
Luster	Metallic
Refractive index	Opaque
Streak	Black
Specific gravity	5.17–5.18
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. Magnetite is the most magnetic of all the naturally occurring minerals on Earth, and these magnetic properties led to lodestone being 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. 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

Magnetite is sometimes found in large quantities in beach sand. Such mineral sands or iron sands or black 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 also are found in Kiruna, Sweden, the Pilbara region 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[1].

Biological occurrences

Crystals of magnetite have been found in some bacteria (e.g., Magnetospirillum magnetotacticum) and in the brains of bees, of termites, of some birds (e.g., the pigeon), and of humans. 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

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.

Mineralogy related

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0-471-80580-7
• Webmineral data
• Mineral galleries
• Powder X-Ray Diffraction (XRD) Pattern

Biology related

• Heinz A. Lowenstam and Stephen Weiner, On Biomineralization, Oxford University Press, USA (1989) ISBN 0-19-504977-2
• Shih-Bin Robin Chang' and Joseph Lynn Kirschvink, Magnetofossils, the Magnetization of Sediments, and the Evolution of Magnetite Biomineralization, Ann. Rev. Earth Planet. Sci. 1989. 17:169-95 PDF file
• Bio-magnetics
• Magnetic bacteria (Italian)

Mining related links

• History of Magnetite Mining in the NJ Highlands
• Magnetite mining in New Zealand
• Magnetite mining in Santa Cruz
• Peruvian sand dunes

From Wikipedia, the free encyclopedia

Chromium(III) oxide
Other names	Chromium sesquioxide Chromia Chrome green
Identifiers
CAS number	12018-34-7
Properties
Molecular formula	Cr2O3
Molar mass	151.99 g/mol
Melting point	2435 °C (4415 °F)
Boiling point	approx. 4000 °C (7250 °F)
Solubility in other solvents	Insoluble
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Chromium(III) oxide is the inorganic compound of the formula Cr₂O₃. It is one of principal oxides of chromium, it is widely used precursor to other chromium compounds.

Structure and properties

Cr₂O₃ adopts the corundum structure, consisting of cubic close packed oxides with 2/3 of the octahedral holes occupied by chromium. It is an amphoteric oxide, dissolving in acids to give chromium(III) salts and in molten alkali to give chromites.

Production

The Parisians Pannetier and Binet first prepared Cr₂O₃ in 1838 via a secret process. It is manufactured from the mineral chromite, Fe(CrO₂)₂, which is mined in southern Africa, Asia, Turkey, and Cuba. The conversion of chromite to chromia entails air oxidation to Na₂Cr₂O₇, which is subsequently reduced with carbon or sulfur.

Chromium oxide can be converted into elemental chromium metal through the thermite reaction, although this metal oxide is much less commonly used than Fe₂O₃ and Fe₃O₄. Unlike iron oxide thermites, chromium oxide thermite creates little or no sparks, smoke or sound, but it glows with a blinding light. Because of the very high melting point of chromium, chromium thermite casting is impractical.

Pigment

It is commonly called chrome green or institutional green when used as a pigment; however it was referred to as viridian when it was first discovered.

References

• A. F. Holleman and E. Wiberg "Inorganic Chemistry" Academic Press, 2001, New York.

External links

• National Pollutant Inventory: Chromium (III) compounds fact sheet