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.