Spinels


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Spinels

 

a group of minerals of the class of multiple oxides with the general formula AB2O4 or A(A, B)O4, where A is Ms, Zn, Mn, Fe2+, Co, or Ni and B is Al, Fe3+, Cr, Mn, Ti4+, or V3+.

Spinels are systems of solid solutions, with extensively developed isomorphism of the A and B cations. They are classified according to the predominant B cation as aluminum spinels, ferrospinels, chrome spinellids, titanium spinels, and vanadium spinels. Aluminum spinels include spinel, MgAl2O4; hercynite, FeAl2O4; galaxite, (Mn, Fe)Al2O4; and gahnite, ZnFe2O4, Ferrospinels include magnesioferrite, MgAl2O4; magnetite (seeMAGNETITE); jacobsite, MnFe2O4; franklinite, ZnFe2O4; and trevorite (seeTREVORITE). Titanium spinels include uivospinel (seeULVOSPINEL) and the magnesium analog of uivospinel, MgTiO4. Vanadium spinels include coulsonite, FeV2O4. (See alsoCHROME SPINELLIDS.) There is nearly complete substitution among the minerals within each isomorphic series, and there is limited substitution among the members of different series.

Spinels crystallize in the isometric system, forming primarily octahedral crystals. In the unit cell of a spinel structure, 32 anions of oxygen form a closest cubic packing, with 64 tetrahedral vacancies (eight occupied by cations) and 32 octahedral vacancies (16 occupied by cations). Spinels are grouped according to the distribution of cations in the occupied tetrahedral and octahedral positions of the structure into normal spinels (eight tetrahedrons occupied by A2+ cations and 16 octahedrons by B3+ cations), inverse (eight tetrahedrons occupied by B3+ and 16 octahedrons occupied by eight B3+ and eight A2+ cations, with both statistical and ordered distribution of B3+A2+ cations in the octahedral vacancies being possible), and intermediate spinels. Spinel, gahnite, hercynite, and galaxite are among the spinels with normal structure, while magnetite, jacobsite, and ulvospinel are among those with inverse structure. A large number of minerals with the intermediate type of structure are known. The structures of certain sulfides with the composition RX2S4, where R2+ is Co, Ni, Fe, or Cu and X3+ is Co, Ni, or Cr, are classified with the structural type of spinels. Maghemite, γ – Fe2O3, has an abnormal spinel structure.

All the spinel minerals are characterized by considerable hardness (5–8 on Mohs’ scale) and by chemical and thermal stability. Spinels are the primary carriers of the magnetic properties of rocks. Their density, reflective capacity, hardness, size of the unit cell, and magnetic and electrical properties depend significantly on the composition and nature of distribution of the cations and vary noticeably within each group. Spinels are typically formed at high temperatures. They are resistant to weathering and last well in placers. Many spinels are important ores of chromium, iron, manganese, titanium, and zinc. They are used in the production of ceramics, refractory materials, and heat-resistant paints.

Many synthetic spinels are known that contain, in addition to the cations typical of natural spinels, ions of lithium, indium, calcium, cadmium, copper, tungsten, gallium, silver, antimony, niobium, and germanium. Being variations of ferrites (ferrospinels), synthetic spinels are the basis of many magnetic materials and are used extensively in instrument-making, radio engineering, and the ceramics industry.

REFERENCES

Blasse, G. Kristallokhimiia ferroshpinelei. Moscow, 1968. (Translated from English.)
Mineraly: Spravochnik, vol. 2, fasc. 3. Moscow, 1967.
Trukhin, V. I. Vvedenie v magnetizm gornykh porod. Moscow, 1973.

G. P. KUDRIAVTSEVA

References in periodicals archive ?
Total Aluminium Magnesium Inclusion Oxide Oxide Count ([Al.sub.2] (MgO) [O.sub.3]) Ladle Treatment ([mm.sup.2]kg) (%) (%) ID Condition X5 before 0.146 1% -- after 0.146 9% -- X7 before 0.054 3% 97% after 0.006 95% -- X9 before 0.037 2% 98% after 0.084 2% 3% Spinel Titanium Aluminum (MgO.
On prolonged exposure the protective [Cr.sub.2][O.sub.3] (Chromia) will react with other metal oxides (metal 'M' may be Ni, Fe or Co) to form spinel (for example Nichromite-Ni[Cr.sub.2][O.sub.4]) and partial dissolution occurs with molten salt at elevated temperature [18-21].
Considering the presence of Fe-Cr rich spinel oxide layer, the oxidation rates of magnetite layer and Fe-Cr rich spinel oxide layer are calculated simultaneously.
For example, the price of a 10 ct spinel of vibrant red colour has increased six-fold since 2005, while a brownish red spinel of the same size saw only a three-fold price increase during the same period.
"Generally, the more spinels, the more hydrogen," she added.
Thermal treatment of co-precipitated precursors is proven to be the most promising method in preparing cobaltite spinels [12-15].
Dunnell is partial to spinels, the gemstone in her ring, describing the normally multi-coloured stones as under appreciated.
The ultramafic cumulates in the WO include wherlite (olivine, diopside with enstatite, magnetite and chrome spinel as accessories), harzburgite (olivine, enstatite, with minor accessroes of diopside and chrome spinel), and dunite (olivine with minor clinopyroxene); wherlite being the most common and dunite being the least.
The x-ray diffraction analysis on the composition of the surface layer of the corrosion products, formed on the specimens of the ZhS-32 alloy after testing in synthetic sol at both temperatures show that the main phase components are the chromium spinel NiO x [Cr.sub.2][O.sub.3] and the nickel oxide NiO and also (in small quantities), the nickel oxides Ni[Al.sub.2][O.sub.4], Ni[Cr.sub.2][O.sub.4], chromium oxide [Cr.sub.2][O.sub.3], tantalum oxide [Ta.sub.2][O.sub.5] and nickel sulphide [Ni.sub.3][S.sub.2].
The An Phu mine near An Phu village in the Luc Yen mining district, Vietnam has been the source of attractive specimens of ruby corundum and large purple to red spinel crystals, as well as phlogopite, pyrite, dravite and green edenite crystals embedded in white marble matrix.
Spinel (chromite) is an early crystallizing phase in oceanic island basalts (OIB).
In this regard, Ni[Co.sub.2][O.sub.4] as a cobaltite spinel has shown excellent electrocatalytic properties, because the mixed oxidation states of its cations favor the adsorption of water and hydroxide ions.