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Spinel

 
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Spinel



 
 
The spinels are any of a class of mineral
Mineral

A mineral is a naturally occurring solid formed through Geology processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties....
s of general formulation A2+B23+O
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
42- which crystal
Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions....
lise in the cubic
Cubic crystal system

The cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
 (isometric) crystal system, with the oxide anions arranged in a cubic close-packed
Close-packing

In geometry, close-packing of spheres is the construction of an infinite regular arrangement of identical spheres so that they take up the greatest possible fraction of an infinite 3-dimensional space ....
 lattice
Bravais lattice

In geometry and crystallography, a Bravais lattice, named after Auguste Bravais, is an infinite set of points generated by a set of discrete translation operations....
 and the cations A and B occupying some or all of the octahedral
Octahedral molecular geometry

In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron....
 and tetrahedral
Tetrahedral molecular geometry

In a Tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron....
 sites in the lattice. A and B can be divalent, trivalent, or quadrivalent cations, including magnesium
Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, atomic weight 24.3050 and common oxidation number +2.Magnesium, an alkaline earth metal, is the ninth most abundance of the chemical elements in the universe by mass....
, zinc
Zinc

Zinc is a metallic chemical element with the symbol Zn and atomic number 30. It is a first-row transition metal of the group 12 element of the periodic table....
, iron
Iron

Iron is a chemical element with the symbol Fe and atomic number 26. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
, manganese
Manganese

Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a Oxidation state in nature , and in many minerals....
, aluminium
Aluminium

Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13....
, chromium
Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is a steely-gray, Lustre , hard metal that takes a high polish and has a high melting point....
, titanium
Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the ?space age metal?, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver colour....
, and silicon
Silicon

Silicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. The atomic mass is 28.0855....
.






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Encyclopedia


The spinels are any of a class of mineral
Mineral

A mineral is a naturally occurring solid formed through Geology processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties....
s of general formulation A2+B23+O
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
42- which crystal
Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions....
lise in the cubic
Cubic crystal system

The cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
 (isometric) crystal system, with the oxide anions arranged in a cubic close-packed
Close-packing

In geometry, close-packing of spheres is the construction of an infinite regular arrangement of identical spheres so that they take up the greatest possible fraction of an infinite 3-dimensional space ....
 lattice
Bravais lattice

In geometry and crystallography, a Bravais lattice, named after Auguste Bravais, is an infinite set of points generated by a set of discrete translation operations....
 and the cations A and B occupying some or all of the octahedral
Octahedral molecular geometry

In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron....
 and tetrahedral
Tetrahedral molecular geometry

In a Tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron....
 sites in the lattice. A and B can be divalent, trivalent, or quadrivalent cations, including magnesium
Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, atomic weight 24.3050 and common oxidation number +2.Magnesium, an alkaline earth metal, is the ninth most abundance of the chemical elements in the universe by mass....
, zinc
Zinc

Zinc is a metallic chemical element with the symbol Zn and atomic number 30. It is a first-row transition metal of the group 12 element of the periodic table....
, iron
Iron

Iron is a chemical element with the symbol Fe and atomic number 26. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
, manganese
Manganese

Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a Oxidation state in nature , and in many minerals....
, aluminium
Aluminium

Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13....
, chromium
Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is a steely-gray, Lustre , hard metal that takes a high polish and has a high melting point....
, titanium
Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the ?space age metal?, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver colour....
, and silicon
Silicon

Silicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. The atomic mass is 28.0855....
. Although the anion is normally oxide, structures are also known for the rest of the chalcogen
Chalcogen

The chalcogens are the Chemical element in periodic table group 16 of the periodic table. It is sometimes known as the oxygen family. It consists of the chemical element oxygen , sulfur , selenium , tellurium , the radioactive element polonium , and the synthetic element ununhexium ....
ides. A and B can also be the same metal under different charges, such as the case in Fe3O4 (as Fe2+Fe23+O42-).

Types of spinel

Important members of the spinel group include:

  • Spinel – MgAl2O4, after which this class of minerals is named
  • Gahnite
    Gahnite

    Gahnite, ZnAl2O4, is a rare mineral belonging to the spinel group. It forms octahedral crystals which may be green, blue, yellow, brown or grey....
     - ZnAl2O4
  • Franklinite - (Fe,Mn,Zn)(Fe,Mn)2O4
  • Chromite
    Chromite

    Chromite is iron magnesium chromium oxide: Cr2O4. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts; also, aluminium and ferric iron commonly substitute for chromium....
     - (Fe·Mg)Cr2O4
  • Magnetite
    Magnetite

    Magnetite is a ferrimagnetism mineral with chemical formula Iron3Oxygen4, one of several iron oxides and a member of the spinel group....
     - Fe3O4
  • Hercynite - FeAl2O4
  • Ulvöspinel
    Ulvöspinel

    Ulv?spinel or ulvospinel is an iron titanium oxide mineral with formula: iron2titaniumoxygen4. It forms brown to black metallic Cubic crystals with a Mohs hardness of 5.5 to 6....
     - TiFe2O4
  • Jacobsite
    Jacobsite

    Jacobsite is a manganese iron oxide mineral. It is a magnetite spinel.It was first described in 1869 and named for the location of Jakobsberg, Sweden....
     - MnFe2O4
  • Trevorite
    Trevorite

    Trevorite is a very rare nickel mineral belonging to the spinel group. It has the chemical formula NiFe3+O4. It is a black mineral with the typical spinel properties of crystallising in the Cubic crystal system, black streaked, infusible and insoluble in most acids....
     - NiFe2O4
  • Ringwoodite
    Ringwoodite

    Ringwoodite is a high-pressure polymorph of olivine, and it is stable at high temperatures and pressures like those in the Earth's mantle near 600 km depth....
     - SiMg2O4, an abundant olivine
    Olivine

    The mineral olivine is a magnesium iron Silicate minerals with the formula 2siliconoxygen4. It is one of the most common minerals on Earth, and has also been identified in meteorites and on the Moon, Mars, and comet Wild 2....
     polymorph
    Polymorphism (materials science)

    Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements....
     within the Earth's mantle from about 520 to 660 km depth, and a rare mineral in meteorites


Properties of true spinel

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
Mohs scale of mineral hardness

Not to be confused with Siemens_#Mho, a unit of electric conductance.The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material....
 is 8, its specific gravity
Specific gravity

Specific gravity is defined as the ratio of the density of a given solid or liquid substance to the density of water at a specific temperature and pressure, typically at 4?C and , making it a dimensionless quantity ....
 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
Red

Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 625?740 Nanometer....
, blue
Blue

Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440?490 Nanometre....
, green
Green

Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520?570-Nanometre....
, yellow
Yellow

Yellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, but does not significantly stimulate the S cone cells; that is, light with much red and green but not very much blue....
, brown
Brown

Brown, when used as a general term, is a color that is a dark yellow, orange , or red, of low luminance relative to lighter or white colored objects....
 or black
Black

Black is the color of objects that do not emit or reflection light in any part of the visible spectrum; they absorb all such frequencies of light....
. 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
Black Prince's Ruby

The Black Prince's Ruby is actually a bead-shaped spinel weighing roughly , approximately the size of a chicken egg. It is currently set in the cross patt?e above the Cullinan II in the front of the Imperial State Crown....
 and the 'Timur ruby' in the British Crown Jewels
Crown Jewels of the United Kingdom

The collective term Crown Jewels denotes the regalia and vestments worn by the sovereign of the United Kingdom during the coronation ceremony and at various other state functions....
, and the 'cote de Bretagne' formerly from the French Crown jewels. The Samarian Spinel
Samarian spinel

The Samarian Spinel is a 500 carat spinel gemstone that is the largest of its kind in the world. It is part of the Iranian Crown Jewels....
 is the largest known spinel in the world, weighing .

The transparent red spinels were called spinel-rubies or balas-rubies. In the past, before the arrival of modern science, spinels and rubies were equaly known as rubies. After the XVIII 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
Badakhshan

Badakhshan is a region comprising parts of northeastern Afghanistan and southeastern Tajikistan. Badakhshan Province is one of the thirty-four provinces of Afghanistan....
, a region in central Asia
Asia

Asia is the world's largest and most populous continent. It covers 8.6% of the Earth's total surface area and, with over 4 billion people, it contains more than 60% of the world's current human population....
 situated in the upper valley of the Kokcha river
Kokcha River

The Kokcha River is a river of northeastern Afghanistan. A tributary of the Amu Darya river, it flows through Badakhshan Province in the Hindu Kush range of Afghan Turkestan....
, one of the principal tributaries of the Oxus river. The Badakshan province was for centuries the main source for red and pink spinels.

Occurrence

True spinel has long been found in the gemstone
Gemstone

A gemstone or gem, also called a precious or semi-precious stone, is a piece of attractive mineral, which — when cut and polished — is used to make jewellery or other adornments....
-bearing gravel of Sri Lanka
Sri Lanka

Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka is an island country in South Asia, located about off the southern coast of India....
 and in limestone
Limestone

File:Limestone Formation In Waitomo.jpgLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geology record....
s of the Badakshan province in nowadays Tajikistan
Tajikistan

Tajikistan , officially the Republic of Tajikistan , is a mountainous landlocked country in Central Asia. Afghanistan borders to the south, Uzbekistan to the west, Kyrgyzstan to the north, and People's Republic of China to the east....
 and of Mogok in Burma. Recently gem quality spinels were also found in the marbles of Luc Yen (Vietnam
Vietnam

Vietnam , officially the Socialist Republic of Vietnam , is the easternmost country on the Indochina Peninsula in Southeast Asia. It is bordered by People's Republic of China to the north, Laos to the northwest, Cambodia to the southwest, and the South China Sea to the east....
), Mahenge and Matombo (Tanzania
Tanzania

Tanzania , officially the United Republic of Tanzania , is a country in East Africa that is bordered by Kenya and Uganda on the north, Rwanda, Burundi and the Democratic Republic of the Congo on the west, and Zambia, Malawi and Mozambique on the south....
), Tsavo (Kenya
Kenya

The Republic of Kenya is a country in East Africa. It is bordered by Ethiopia to the north, Somalia to the northeast, Tanzania to the south, Uganda to the west, and Sudan to the northwest, with the Indian Ocean running along the southeast border....
) and in the gravels of Tunduru (Tanzania
Tanzania

Tanzania , officially the United Republic of Tanzania , is a country in East Africa that is bordered by Kenya and Uganda on the north, Rwanda, Burundi and the Democratic Republic of the Congo on the west, and Zambia, Malawi and Mozambique on the south....
) and Ilakaka (Madagascar
Madagascar

Madagascar, or Republic of Madagascar , is an island nation in the Indian Ocean off the southeastern coast of Africa. The main island, also called Madagascar, is the List of islands by area, and is home to 5% of the world's plant and animal species, of which more than 80% are Endemism to Madagascar....
). Spinel is found as a metamorphic mineral
Metamorphic rock

Metamorphic rock is the result of the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form"....
, and also as a primary mineral in rare mafic igneous rock
Igneous rock

Igneous rock is one of the three main Rock types . Igneous rock is formed by magma being cooled and becoming solid . They may form with or without crystallization, either below the surface as Intrusion rocks or on the surface as extrusive rocks....
s; in these igneous rock
Igneous rock

Igneous rock is one of the three main Rock types . Igneous rock is formed by magma being cooled and becoming solid . They may form with or without crystallization, either below the surface as Intrusion rocks or on the surface as extrusive rocks....
s, the magma
Magma

Magma is molten Rock that is found beneath the surface of the Earth, and may also exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and gas bubbles....
s are relatively deficient in alkali
Alkali

In chemistry, an alkali is a Base , Ionic compound salt of an alkali metal or alkaline earth metal Chemical element. Alkalis are best known for being Base s that dissolve in water....
s relative to aluminium
Aluminium

Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13....
, and aluminium oxide may form as the mineral corundum
Corundum

Corundum is a crystalline form of aluminium oxide and is one of the rock -forming minerals. It is naturally clear, but can have different colors when impurities are present....
 or may combine with magnesia to form spinel. This is why spinel and ruby
Ruby

A ruby is a pink to blood-red gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium....
 are often found together.

Spinel, (Mg,Fe)(Al,Cr)2O4, is common in peridotite
Peridotite

A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic and ultrabasic, as the rock contains less than 45% silica....
 in the uppermost Earth's mantle, between the Mohorovicic discontinuity
Mohorovicic discontinuity

The Mohorovicic discontinuity, usually referred to as the Moho, is the boundary between the Earth's Crust and the Mantle . The Moho serves to separate both oceanic crust and continental crust from underlying mantle....
 (the Moho) and a depth of 70 kilometers or so; below that depth, the spinel (if present) becomes increasingly rich in chromium
Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is a steely-gray, Lustre , hard metal that takes a high polish and has a high melting point....
, as with increasing depth, pyrope
Pyrope

The mineral pyrope is a member of the garnet group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek for fire and eye....
-rich garnet
Garnet

The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" comes from the Latin language granatus , possibly a reference to the Punica granatum , a plant with red seeds similar in shape, size, and color to some garnet crystals....
 becomes the more stable aluminous mineral in peridotite. At depths significantly shallower than the Moho, calcic plagioclase
Plagioclase

Plagioclase is a very important series of Silicate minerals minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series ....
 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 meteorite
Chondrite

Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids....
s.

The spinel structure


In normal spinel structures, they 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 you 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
Crystal field theory

Crystal field theory is a model that describes the electronic structure of transition metal compounds, all of which can be considered complex ....
 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 orbital
Atomic orbital

An atomic orbital is a mathematical function that describes the wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus....
s of the two types of atom to determine their site preference. 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 s-type
Sigma bond

In chemistry, sigma bonds are the strongest type of covalent bond chemical bond. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups....
 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
Perturbation theory (quantum mechanics)

In quantum mechanics, perturbation theory is a set of approximation schemes directly related to mathematical perturbation theory for describing a complicated quantum system in terms of a simpler one....
 that can sometimes make a difference, but which often do not.

Synthetic Spinel

Synthetic Spinel has been accidentally produced in the middle of the 18th century, and has been more recently described in scientific publications in 2000 and 2004. . With the appreciation of fine quality, red spinels, new flux synthetics have recently been found in Bangkok.

See also

  • Ceylonite
    Ceylonite

    Ceylonite and Pleonaste are dingy blue or grey to black varieties of spinel. Ceylonite, apparently named for the island of Ceylon , is a ferroan spinel with Mg:Fe from 3:1 and 1:1, and little or no ferric iron....
  • Pleonaste
  • The Samarian Spinel: the largest known spinel in the world, part of the Iranian Crown Jewels
    Iranian Crown Jewels

    The Imperial Crown Jewels of Iran includes several elaborate crown and decorative Thrones, 30 tiaras and numerous aigrette, a dozen jewel laden swords and shields, a vast amount of precious unset gemstones, numerous plates and other dining services cast in precious metals and encrusted with gems and several other more unique items collec...


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