Spinel 29467 224905083 2008-07-10T22:52:31Z Melesse 200177 adding commons link {{Refimprove|date=May 2008}} {{Infobox mineral | name = Spinel | category = | boxwidth = | boxbgcolor = | image = Spinel2.jpg | caption = | formula = MgAl<sub>2</sub>O<sub>4</sub> | color = Various, red to blue to mauve. Dark green, brown. Black | habit = Cubic, octahedral | system = Isometric | twinning = | cleavage = Indistinct | fracture = Conchoidal, uneven | mohs = 8.0 | luster = Vitreous | refractive = 1.712-1.762 | opticalprop = | birefringence = | pleochroism = Absent | streak = White | gravity = 3.54-3.63 | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = | other = }} The '''spinels''' are any of a class of [[mineral]]s of general formulation XY<sub>2</sub>[[oxygen|O]]<sub>4</sub> which [[crystal]]lize in the [[cubic crystal system|cubic]] (isometric) crystal system, with the oxide anions arranged in a cubic [[close-packing|close-packed]] [[Bravais lattice|lattice]] and the cations X and Y occupying some or all of the [[octahedral molecular geometry|octahedral]] and [[tetrahedral molecular geometry|tetrahedral]] sites in the lattice. X and Y 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 [[chalcogen]]ides. ==Types of spinel== Important members of the spinel group include: *'''Spinel''' – MgAl<sub>2</sub>O<sub>4</sub>, after which this class of minerals is named *[[Gahnite]] - ZnAl<sub>2</sub>O<sub>4</sub> *[[Franklinite]] - (Fe,Mn,Zn)(Fe,Mn)<sub>2</sub>O<sub>4</sub> *[[Chromite]] - (Fe·Mg)Cr<sub>2</sub>O<sub>4</sub> *[[Magnetite]] - Fe<sub>3</sub>O<sub>4</sub> *[[Hercynite]] - FeAl<sub>2</sub>O<sub>4</sub> *[[Ulvöspinel]] - TiFe<sub>2</sub>O<sub>4</sub> *[[Jacobsite]] - MnFe<sub>2</sub>O<sub>4</sub> *[[Trevorite]] - NiFe<sub>2</sub>O<sub>4</sub> *[[Ringwoodite]] - SiMg<sub>2</sub>O<sub>4</sub>, an abundant [[olivine]] [[polymorphism (materials science)|polymorph]] within the [[Earth's mantle]] from about 520 to 660 km depth, and a rare mineral in meteorites ==Properties of true spinel== Spinel crystallizes in the isometric system; common crystal forms are octahedra, usually twinned. It has an imperfect octahedral cleavage and a conchoidal fracture. Its [[Mohs scale of mineral hardness|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. Another famous spinel is the [[Black Prince's Ruby]] in the British [[Crown Jewels of the United Kingdom|Crown Jewels]]. The [[Samarian Spinel]] is the largest known 500 carat spinel in the world. The transparent red spinels are called spinel-rubies or balas-rubies and were often confused with actual [[ruby|rubies]] in ancient times. "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]]. ==Occurrence== True spinel has long been found in the [[gemstone]]-bearing gravel of [[Sri Lanka]] and in [[limestone]]s of [[Burma]] and [[Thailand]]. Spinel is found as a [[metamorphic rock|metamorphic mineral]], and also as a primary mineral in rare mafic [[igneous rock]]s; in these [[igneous rock]]s, the [[magma]]s are relatively deficient in [[alkali]]s 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)<sub>2</sub>O<sub>4</sub>, is common in [[peridotite]] in the uppermost [[Earth's mantle]], between the [[Mohorovicic discontinuity]] (the Moho) and a depth of 70 kilometers or so; below that depth, the spinel (if present) becomes increasingly rich in [[chromium]], as with increasing depth, [[pyrope]]-rich [[garnet]] becomes the more stable aluminous mineral in peridotite. At depths significantly shallower than the Moho, calcic [[plagioclase]] is the more stable aluminous mineral in peridotite. Spinel, (Mg,Fe)Al<sub>2</sub>O<sub>4</sub>, is a common mineral in the [[Ca-Al-rich inclusion]]s (CAIs) in some [[chondrite|chondritic meteorite]]s. ==The spinel structure== In the so-called ''normal spinel structure'', X cations occupy the tetrahedral sites of the oxide lattice, and Y cations the octahedral sites. For ''inverse spinels'', half the Y cations occupy the tetrahedral sites, and both X and Y cations occupy the octahedral sites. 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 crystal field stabilisation energy (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 orbital]]s of the two types of atom to determine their site preference.<ref>[http://dx.doi.org/10.1021/ja00365a019 J.K. Burdett, G.L. Price and S.L. Price, ''J. Am. Chem. Soc.'', '''104''' (1982), 92-95]</ref> 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 [[sigma bond|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 Al<sup>3+</sup> cations for octahedral sites or of Zn<sup>2+</sup> 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 theory (quantum mechanics)|perturbation]] that can sometimes make a difference, but which often do not. ==See also== *[[Ceylonite]] *[[Pleonaste]] *[[The Samarian Spinel]]: the largest known spinel in the world, part of the [[Iranian Crown Jewels]] ==References== {{commonscat|Spinel}} *Deer, Howie and Zussman (1966) ''An Introduction to the Rock Forming Minerals'', Longman, pp.424-433, ISBN 0-582-44210-9 *Shumann, Walter (2006) ''Gemstones of the World'' 3rd edition, Sterling, pp.116-117. {{reflist}} ==External links== *[http://www.uwgb.edu/dutchs/PETROLGY/Spinel%20Structure.HTM Spinel structure by Steven Dutch] *[http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_2/basics/b2_1_6.html Spinel structure] *http://www.gemstone.org/gem-by-gem/english/spinel.html *http://mineral.galleries.com/minerals/oxides/spinel/spinel.htm *http://www.mindat.org/min-3729.html *http://www.webmineral.com/data/Spinel.shtml [[Category:Magnesium minerals]] [[Category:Aluminium minerals]] [[Category:Oxide minerals]] [[Category:Gemstones]] [[cs:Spinel]] [[de:Spinell]] [[es:Espinela (mineral)]] [[eu:Espinela]] [[fr:Spinelle]] [[it:Gruppo degli spinelli]] [[he:ספינל]] [[hu:Spinell]] [[nl:Spinel]] [[ja:スピネル]] [[pl:Spinel]] [[pt:Espinela]] [[ro:Spinel]] [[ru:Шпинель]] [[sr:Магнезијум алуминат]] [[fi:Spinelli]] [[sv:Spinell]] [[zh:尖晶石]]