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The spinels are any of a class of
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...
s of general formulation which
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
lise in the cubic (isometric) crystal system, with the X anions (typically
chalcogen The chalcogens (ore forming) ( ) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioact ...
s, like
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
and
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
) arranged in a cubic
close-packed In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occu ...
lattice Lattice may refer to: Arts and design * Latticework, an ornamental criss-crossed framework, an arrangement of crossing laths or other thin strips of material * Lattice (music), an organized grid model of pitch ratios * Lattice (pastry), an orna ...
and the cations A and B occupying some or all of the
octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...
and
tetrahedral In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the o ...
sites in the lattice.H-J Meyer
Festkörperchemie
in: H-J Meyer (ed.), ''Riedel Moderne Anorganische Chemie'', Walter de Gruyter, 2012, . Retrieved 15 April 2018.
Although the charges of A and B in the prototypical spinel structure are +2 and +3, respectively (), other combinations incorporating
divalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Description The combining capacity, or affinity of an ...
, trivalent, or tetravalent
cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s, including
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
,
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
,
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
,
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
,
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
,
chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...
,
titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
, and
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
, are also possible. The anion is normally oxygen; when other
chalcogen The chalcogens (ore forming) ( ) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioact ...
ides constitute the anion sublattice the structure is referred to as a
thiospinel The thiospinel group is a group of sulfide minerals with a general formula where A is nominally a +2 metal, B is a +3 metal and X is -2 sulfide or similar anion (selenide or telluride). ''Thio'' refers to sulfur and ''spinel'' indicates their iso ...
. A and B can also be the same metal with different valences, as is the case with magnetite, (as ), which is the most abundant member of the spinel group. Spinels are grouped in series by the B cation. The group is named for
spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...
(), which was once known as "spinel
ruby A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sa ...
". (Today the term ''ruby'' is used only for
corundum Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the pres ...
.)


Spinel group members

Members of the spinel group include: *Aluminium spinels: **
Spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...
: , after which this class of minerals is named **
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. It often forms as an alteration product of sphalerite in altered massive sulphide deposits such as at ...
: **
Hercynite Hercynite is a spinel mineral with the formula FeAl2O4. It occurs in high-grade metamorphosed iron rich argillaceous sediments as well as in mafic and ultramafic igneous rocks. Due to its hardness it also is found in placers. It was first descr ...
: **
Galaxite Galaxite, also known as 'mangan-spinel' is an isometric mineral belonging to the spinel group of oxides with the ideal chemical formula . Galaxite is the manganese (Mn) rich endmember of the aluminium (Al) series of the spinel group. Divalent ...
: ** Pleonaste: *Iron spinels: **
Cuprospinel Cuprospinel is a mineral. Cuprospinel is an inverse spinel with the chemical formula CuFe2O4, where copper substitutes some of the iron cations in the structure. Its structure is similar to that of magnetite, Fe3O4, yet with slightly differe ...
: **
Franklinite Franklinite is an oxide mineral belonging to the normal spinel subgroup's iron (Fe) series, with the formula ZnFe3+2O4. As with another spinel member magnetite, both ferrous (2+) and ferric (3+) iron may be present in Franklinite samples. Di ...
: **
Jacobsite Jacobsite is a manganese iron oxide mineral. It is in the spinel group and forms a solid solution series with magnetite. The chemical formula is (Mn,Mg)Fe2O4 or with oxidation states and substitutions: (Mn2+,Fe2+,Mg)(Fe3+,Mn3+)2O4.
: **
Magnesioferrite Magnesioferrite is a magnesium iron oxide mineral, a member of the magnetite series of spinels. Magnesioferrite crystallizes as black metallic octahedral crystals. It is named after its chemical composition of magnesium and ferric iron. The dens ...
: **
Magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
: , where one Fe is +2 and two Fe's are +3, respectively. **
Trevorite Trevorite is a rare nickel iron oxide mineral belonging to the spinel group. It has the chemical formula NiFe3+2O4. It is a black mineral with the typical spinel properties of crystallising in the cubic system, black streaked, infusible and insolu ...
: **
Ulvöspinel Ulvöspinel or ulvite is an iron titanium oxide mineral with formula: iron, Fe2titanium, Tioxygen, O4 or TiFe2+2O4. It forms brown to black metallic Cubic (crystal system), isometric crystals with a Mohs hardness of 5.5 to 6. It belongs to the sp ...
: **
Zinc ferrite Zinc ferrites are a series of synthetic inorganic compounds of zinc and iron (ferrite) with the general formula of ZnxFe3−xO4. Zinc ferrite compounds can be prepared by aging solutions of Zn(NO3)2, Fe(NO3)3, and triethanolamine in the presence ...
: *Chromium spinels: **
Chromite Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can s ...
: **
Magnesiochromite Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can s ...
: ** Zincochromite: *Cobalt spinels: ** Manganesecobaltite: *Vanadium spinels: ** Coulsonite: ** Magnesiocoulsonite: *Others with the spinel structure: **
Ringwoodite Ringwoodite is a high-pressure phase of Mg2SiO4 (magnesium silicate) formed at high temperatures and pressures of the Earth's mantle between depth. It may also contain iron and hydrogen. It is polymorphous with the olivine phase forsterite (a ma ...
: , an abundant
olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
polymorph within the
Earth's mantle Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01 × 1024 kg and thus makes up 67% of the mass of Earth. It has a thickness of making up about 84% of Earth's volume. It is predominantly sol ...
from about 520 to 660 km depth, and a rare mineral in meteorites **
Taaffeite Taaffeite (; BeMgAl4O8) is a mineral, named after its discoverer Richard Taaffe (1898–1967) who found the first sample, a cut and polished gem, in October 1945 in a jeweler's shop in Dublin, Ireland.Dept. Mineralogy, British Museum, June 7 195 ...
: with an
empirical formula In chemistry, the empirical formula of a chemical compound is the simplest whole number ratio of atoms present in a compound. A simple example of this concept is that the empirical formula of sulfur monoxide, or SO, would simply be SO, as is th ...
as a spinel, but the
chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
is four times as large. **
Musgravite Musgravite or magnesiotaaffeite-6N’3S is a rare oxide mineral used as a gemstone. Its type locality is the Ernabella Mission, Musgrave Ranges, South Australia, for which it was named following its discovery in 1967. It is a member of the taaf ...
: a type of "multi-spinel". There are many more compounds with a spinel structure, e.g. the
thiospinel The thiospinel group is a group of sulfide minerals with a general formula where A is nominally a +2 metal, B is a +3 metal and X is -2 sulfide or similar anion (selenide or telluride). ''Thio'' refers to sulfur and ''spinel'' indicates their iso ...
s and selenospinels, that can be synthesized in the lab or in some cases occur as minerals. The heterogeneity of spinel group members varies based on composition with ferrous and magnesium based members varying greatly as in
solid solution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word ...
, which requires similarly sized cations. However, ferric and aluminium based spinels are almost entirely homogeneous due to their large size difference.


The spinel structure

The
space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...
for a spinel group mineral may be Fdm (the same as for diamond), but in some cases (such as spinel itself, , beyond 452.6 K) it is actually the tetrahedral F3m. Normal spinel structures are usually cubic close-packed oxides with eight tetrahedral and four octahedral sites per formula unit. The tetrahedral spaces are smaller than the octahedral spaces. B ions occupy half the octahedral holes, while A ions occupy one-eighth of the tetrahedral holes. The mineral
spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...
has a normal spinel structure. In a normal spinel structure, the ions are in the following positions (where i, j, and k are arbitrary integers and δ, ε, and ζ are small real numbers): X: (1/4-δ, δ, δ ) + ((i+j)/2, (j+k)/2, (i+k)/2) ( δ, 1/4-δ, δ ) + ((i+j)/2, (j+k)/2, (i+k)/2) ( δ, δ, 1/4-δ) + ((i+j)/2, (j+k)/2, (i+k)/2) (1/4-δ, 1/4-δ, 1/4-δ) + ((i+j)/2, (j+k)/2, (i+k)/2) (3/4+ε, 1/2-ε, 1/2-ε) + ((i+j)/2, (j+k)/2, (i+k)/2) (1-ε, 1/4+ε, 1/2-ε) + ((i+j)/2, (j+k)/2, (i+k)/2) (1-ε, 1/2-ε, 1/4+ε) + ((i+j)/2, (j+k)/2, (i+k)/2) (3/4+ε, 1/4+ε, 1/4+ε) + ((i+j)/2, (j+k)/2, (i+k)/2) A: (1/8, 1/8, 1/8) + ((i+j)/2, (j+k)/2, (i+k)/2) (7/8, 3/8, 3/8) + ((i+j)/2, (j+k)/2, (i+k)/2) B: (1/2+ζ, ζ, ζ ) + ((i+j)/2, (j+k)/2, (i+k)/2) (1/2+ζ, 1/4-ζ, 1/4-ζ) + ((i+j)/2, (j+k)/2, (i+k)/2) (3/4-ζ, 1/4-ζ, ζ ) + ((i+j)/2, (j+k)/2, (i+k)/2) (3/4-ζ, ζ, 1/4-ζ) + ((i+j)/2, (j+k)/2, (i+k)/2) The first four X positions form a tetrahedron around the first A position, and the last four form one around the second A position. When the space group is Fdm then δ=ε and ζ=0. In this case, a three-fold
improper rotation In geometry, an improper rotation,. also called rotation-reflection, rotoreflection, rotary reflection,. or rotoinversion is an isometry in Euclidean space that is a combination of a rotation about an axis and a reflection in a plane perpendicula ...
with axis in the 111 direction is centred on the point (0, 0, 0) (where there is no ion) and can also be centred on the B ion at (1/2, 1/2, 1/2), and in fact every B ion is the centre of an improper three-fold rotation. Under this space group the two A positions are equivalent. If the space group is F3m then the improper three-fold rotations become proper three-fold rotations because the inversion disappears, and the two A positions are no longer equivalent. Every ion is on at least three mirror planes and at least one three-fold rotation axis. The structure has tetrahedral symmetry around each A ion, and the A ions are arranged just like the carbon atoms in
diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the Chemical stability, chemically stable form of car ...
. Inverse spinel structures have a different cation distribution in that all of the A cations and half of the B cations occupy octahedral sites, while the other half of the B cations occupy tetrahedral sites. An example of an inverse spinel is , if the Fe2+ (A2+) ions are d6 high-spin and the Fe3+ (B3+) ions are d5 high-spin. In addition, intermediate cases exist where the cation distribution can be described as (A1−''x''B''x'') B1−sub>2O4, where parentheses () and brackets [] are used to denote tetrahedral and octahedral sites, respectively. The so-called inversion degree, ''x'', adopts values between 0 (normal) and 1 (inverse), and is equal to for a completely random cation distribution. The cation distribution in spinel structures are related to the crystal field stabilization energies (CFSE) of the constituent transition metals. Some ions may have a distinct preference for the octahedral site depending on the ''d-electron'' count. If the A2+ ions have a strong preference for the octahedral site, they will displace half of the B3+ ions from the octahedral sites to tetrahedral sites. Similarly, if the B3+ ions have a low or zero ''octahedral site stabilization energy'' (OSSE), then they will occupy tetrahedral sites, leaving octahedral sites for the A2+ ions. Burdett and co-workers proposed an alternative treatment of the problem of spinel inversion, using the relative sizes of the s and p
atomic orbital In atomic theory and quantum mechanics, an atomic orbital is a function describing the location and 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 spe ...
s of the two types of atom to determine their site preferences. This is because the dominant stabilizing interaction in the solids is not the crystal field stabilization 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, which crystal field theory would predict neither has a 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 or perturb may refer to: * Perturbation theory, mathematical methods that give approximate solutions to problems that cannot be solved exactly * Perturbation (geology), changes in the nature of alluvial deposits over time * Perturbat ...
that can sometimes affect the relative preferences, but which often do not.


Common uses in industry and technology

Spinels commonly form in high temperature processes. Either native
oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
scales of
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
s, or intentional deposition of
spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...
coating A coating is a covering that is applied to the surface of an object, usually referred to as the Substrate (materials science), substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquid ...
s can be used to protect base metals from
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
or
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
. The presence of spinels may hereby serve as thin (few
micrometer Micrometer can mean: * Micrometer (device), used for accurate measurements by means of a calibrated screw * American spelling of micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...
thick) functional layers, that prevent the
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
of oxygen (or other atmospheric) ions or specific metal ions such as
chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...
, which otherwise exhibits a fast diffusion process at high temperatures.


Further reading

*


References

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