Dictionary:
spi·nel spi·nelle (spĭ-nĕl')  |
[Italian spinella, diminutive of spina, thorn (from its sharply pointed crystals), from Latin spīna.]
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Any of a family of important AB2O4 oxide minerals, where A and B represent cations. Spinel minerals are widely distributed in the earth, in meteorites, and in rocks from the Moon. While the ideal spinel formula is MgAl2O4, some 30 elements, with valences from 1 to 6, are known to substitute in the A or B cation sites, resulting in well over 150 synthetic compounds having the spinel crystal structure. The term spinel is derived from spina (Latin, thorn) in reference to its pointed octahedral, crystal habit, and also to its dendritic snowflake form in rapidly chilled high-temperature slags and lavas.
The named spinel minerals that have so far been recorded in nature are oxides that occur as a matrix of A2+ versus B3+ cations. Three spinel series evolve from the classification: spinel, magnetite, and chromite. In addition to these spinels, there are other oxide, sulfur (thiospinels), silicate, and rare selenide-bearing spinels, all of which have relatively simple end-member compositions. See also Magnetite.
Aluminous spinels are highly refractory, vary from translucent to transparent, and vary from colorless to green, blue, brown, and black. All other oxide and thiospinels are opaque with metallic lusters. Mohs hardness varies from about 4.5 (linnaeite) to about 8 (spinel). Density is approximately 5 g/cm3 (3 oz/in.3). Magnetite and maghemite are ferrimagnetic, with high saturation magnetizations and Curie temperatures of 580°C (1076°F) and 675°C (1247°F), respectively. See also Curie temperature; Ferrimagnetism; Hardness scales.
Spinels are widely employed to deduce the evolutionary history of rocks because compositions are extremely sensitive to environmental conditions of formation. Emery, the abrasive, is magnetite + corundum. Chromite is the chief source of chromium. Iron, titanium and vanadium are derived from magnetite, and zinc is extracted from franklinite. Spinel also occurs as a semiprecious gem, and it is widely employed as a mechanically robust ceramic. The compass used in ancient times was a mixture of magnetite and maghemite; the entire fields of rock magnetism and paleomagnetism as well as the recorded history of the Earth's magnetic field hinge on the magnetic properties of these inverse spinels. Maghemite is widely employed in magnetic recording tapes and magnetic colloids. See also Mineralogy; Paleomagnetism; Petrology.
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Environment
Plutonic, pegmatitic, and metamorphic rocks.
Crystal descriptionIn octahedrons, with cube and dodecahedron truncations rare. Often two halves are intergrown (twinned), one side rotated 180°, forming a flat triangle and creating re-entrant angles beneath each corner . Some large but drab Madagascar crystals show additional forms, but they are very rare. Also in irregular embedded grains, and coarsely granular.
Physical propertiesMultihued: black, dark green, red, blue, violet, orange-brown, lilac, or white. Luster glassy; hardness 7Ɖ-8; specific gravity 3.5-4.1; fracture conchoidal; cleavage none, but poor octahedral parting. Brittle; transparent to opaque; red and lilac varieties fluorescent red or yellow-green.
CompositionMagnesium aluminum oxide (28.2% MgO, 71.8% Al 2 O 3 ); but in this formula magnesium can be wholly or partly replaced by iron, zinc, or manganese, making a series of related minerals with different names. The zinc spinel (deep green gahnite) is the most common of these; hercynite, the iron spinel, and galaxite, the manganese spinel, are rarer.
TestsInfusible, insoluble.
Distinguishing characteristicsUsually distinguished by its crystal shape and hardness, and often by its color and twinning. Magnetite is magnetic, chromite is heavier, garnet is fusible, zircon and microlite are heavier. Most confusion is with ruby rhombohedrons.
OccurrenceGemmy spinel, like ruby corundum, is a mineral of metamorphosed, generally calcareous gneisses, impure marbles, and low-silica pegmatites, and consequently it is commonly associated with corundum. A significant gemstone, with Myanmar (Burma) and Sri Lanka (Ceylon) almost the sole important sources of gemmy material. Fine large brownish to black crystals with additional forms have come from Madagascar.
The largest American crystals, which are over 4 in. (10 cm) on an edge, came from a lost locality near Amity, New York. Spinel is common in the metamorphosed limestones of the New York--New Jersey highlands belt, with corundum, diopside, graphite, chondrodite, and phlogopite. Fine blue crystals are found near Helena, Montana.
Gahnite, dark green zinc spinel, occurs with garnet at Charlemont, Massachusetts, in good crystals decorated with triangular markings. Also found at Spruce Pine, North Carolina, where it sometimes forms transparent, bright green but very flat crystals in the mica plates. Gahnite is also found at Franklin, New Jersey, and in Brazil's brazilianite pegmatite. Galaxite (iron spinel) forms small black grains with garnets near Galax in North Carolina, on Bald Knob.
Spinel can also separate early from a magma and form phenocrysts in lava. Bronze asteriated octahedrons have been found in northern Mexico.
RemarksRed spinel, though less famous, is a valuable jewelry stone, often confused with ruby. A famous crown jewel of Great Britain, the Black Prince's Ruby, is such a spinel. Like corundum, the spinel series of minerals melt congruently and recrystallize instantly on cooling, so they are easily synthesized by the Verneuil method. Many common synthetic gemstones are spinels; the spinel lattice seems to accept pigmenting elements more readily to give hues unobtainable in corundum synthesis. Most synthetic "sapphires" are spinel, as are synthetic "aquamarines," "peridots," and "rose zircons."
| Wikipedia: Spinel |
| Spinel | |
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| General | |
| Category | Oxide mineral Spinel group |
| Chemical formula | MgAl2O4 |
| Identification | |
| Color | Various, red to blue to mauve. Dark green, brown. Black |
| Crystal habit | Cubic, octahedral |
| Crystal system | Isometric |
| Cleavage | Indistinct |
| Fracture | Conchoidal, uneven |
| Mohs scale hardness | 7.5 - 8.0 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.6 - 4.1 |
| Optical properties | Isotropic |
| Refractive index | 1.719 |
| Pleochroism | Absent |
| Solubility | none |
| Other characteristics | Nonmagnetic, non-radioactive, sometimes fluorescent (red) |
| References | [1][2] |
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4.[1]
Contents |
The spinels are any of a class of minerals of general formulation A2+B23+O42- which crystallise in the cubic (isometric) crystal system, with the oxide anions arranged in a cubic close-packed lattice and the cations A and B occupying some or all of the octahedral and tetrahedral sites in the lattice. A and B can be divalent, trivalent, or quadrivalent cations, including magnesium, zinc, iron, manganese, aluminium, chromium, titanium, and silicon. Although the anion is normally oxide, structures are also known for the rest of the chalcogenides. A and B can also be the same metal under different charges, such as the case in Fe3O4 (as Fe2+Fe23+O42-).
Members of the spinel group include:[3]
Spinel crystallises in the isometric system; common crystal forms are octahedra, usually twinned. It has an imperfect octahedral cleavage and a conchoidal fracture. Its hardness is 8, its specific gravity is 3.5-4.1 and it is transparent to opaque with a vitreous to dull lustre. It may be colorless, but is usually various shades of red, blue, green, yellow, brown or black. There is a unique natural white spinel, now lost, that surfaced briefly in what is now Sri Lanka. Some spinels are among the most famous gemstones: Among them is the Black Prince's Ruby and the 'Timur ruby' in the British Crown Jewels, and the 'cote de Bretagne' formerly from the French Crown jewels. The Samarian Spinel is the largest known spinel in the world, weighing 500 carats (100 g).
The transparent red spinels were called spinel-rubies or balas-rubies. In the past, before the arrival of modern science, spinels and rubies were equally known as rubies. After the 18th century the word ruby was only used for the red gem variety of the mineral corundum and the word spinel became used. "Balas" is derived from Balascia, the ancient name for Badakhshan, a region in central Asia situated in the upper valley of the Kokcha River, one of the principal tributaries of the Oxus River. The Badakshan province was for centuries the main source for red and pink spinels.
True spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestones of the Badakshan province in nowadays Tajikistan and of Mogok in Burma. Recently gem quality spinels were also found in the marbles of Luc Yen (Vietnam), Mahenge and Matombo (Tanzania), Tsavo (Kenya) and in the gravels of Tunduru (Tanzania) and Ilakaka (Madagascar). Spinel is found as a metamorphic mineral, and also as a primary mineral in rare mafic igneous rocks; in these igneous rocks, the magmas are relatively deficient in alkalis relative to aluminium, and aluminium oxide may form as the mineral corundum or may combine with magnesia to form spinel. This is why spinel and ruby are often found together.
Spinel, (Mg,Fe)(Al,Cr)2O4, is common in peridotite in the uppermost Earth's mantle, between 450 km (where olivine is metamorphosed to spinel) to 670 km kilometers or so; below that depth, the spinel is oxidised. At depths significantly shallower than the Moho, calcic plagioclase is the more stable aluminous mineral in peridotite.
Spinel, (Mg,Fe)Al2O4, is a common mineral in the Ca-Al-rich inclusions (CAIs) in some chondritic meteorites.
Normal spinel structures are usually cubic closed-packed oxides with one octahedral and two tetrahedral sites per oxide. The tetrahedral points are smaller than the Octahedral points. B3+ ions occupy the octahedral holes because of a charge factor, but can only occupy half of the octahedral holes. A2+ ions occupy 1/8th of the tetrahedral holes. This maximises the lattice energy if the ions are similar in size. A common example of a normal spinel is MgAl2O4.
Inverse spinel structures however are slightly different in that one must take into account the crystal field stabilisation energies (CFSE) of the transition metals present. Some ions may have a distinct preference on the octahedral site which is dependent on the d-electron count. If the A2+ ions have a strong preference for the octahedral site, they will force their way into it and displace half of the B3+ ions from the octahedral sites to the tetrahedral sites. If the B3+ ions have a low or zero octahedral site stabilisation energy (OSSE), then they have no preference and will adopt the tetrahedral site. A common example of an inverse spinel is Fe3O4, if the Fe2+ (A2+) ions are d6 high-spin and the Fe3+ (B3+) ions are d5 high-spin.
For many years, crystal field theory was invoked to explain the distribution of the cations within the spinels. As the octahedral and tetrahedral sites in the lattice generate different amounts of CFSE, it was argued that the arrangement of the two types of cation that generated the most CFSE would be the most stable. However, this idea was challenged by Burdett and co-workers, who showed that a better treatment used the relative sizes of the s and p atomic orbitals of the two types of atom to determine their site preference.[4] This is because the dominant stabilising interaction in the solids is not the crystal field stabilisation energy generated by the interaction of the ligands with the d-electrons, but the σ-type interactions between the metal cations and the oxide anions. This rationale can explain anomalies in the spinel structures that crystal-field theory cannot, such as the marked preference of Al3+ cations for octahedral sites or of Zn2+ for tetrahedral sites - using crystal field theory would predict that both have no site preference. Only in cases where this size-based approach indicates no preference for one structure over another do crystal field effects make any difference — in effect they are just a small perturbation that can sometimes make a difference, but which often do not.
Synthetic Spinel was accidentally produced in the middle of the 18th century, and has been more recently described in scientific publications in 2000 and 2004.[5]
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved. Read more |
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| Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/. Read more |
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| Rock & Mineral Guide. Peterson Field Guide to Rocks and Minerals, by Frederick H. Pough. Copyright © 1998 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved. Read more |
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