SPINEL, in mineralogy, the name given to a group of minerals, of the general composition where R"=Mg, Fe, Mn, Zn, and R"'=A1, Fen Cr"'. The typical member is a magnesium aluminate to which the name (Fr. spinelle, from the Latin "spina," perhaps in allusion to the sharp angles of the crystals) was originally restricted. All spinels crystallize in the cubic system, typically in octahedra. Twinning is common, the octahedral face being the twin plane, giving a characteristic form known as the spinel twin.
Cleavage is typically absent in these minerals with the exception of the zinc spinel (cleavage on II ), but an octahedral parting is observable in magnetite and franklinite. The hardness is variable, in ordinary spinel 7.5-8, but magnetite H=6 and chromite The sp. gr. varies with the composition, from 3.6 (magnesia-spinel) to 5.2 (magnetite). The light refraction shows a range from 1.718 (pure MgA1,04), hercynite 1.80, to picotite 2.05, chromite 2.10 and magnetite 2.42. Spinels vary much in colour. Magnesia spinels are pink, red and blue, and are used as gem-stones. The pure magnesia spinel is colourless, hercynite is dark green in thin slices and picotite and chromite are brown in the thinnest sec tions. Spinels are readily produced artificially. Magnesia spinel melts at 2135° C and between this compound and FeAl,a, there is complete miscibility as revealed in spinel analyses, but solid solution between (Mg, and magnetite is very limited.
The spinels used in jewellery are found mostly in gem gravels, the chief localities being Ceylon, Siam and Upper Burma.
Spinels occur both in igneous and metamorphic rocks. The home of the magnesia spinel is in thermally altered dolomites where it arises by reaction of alumina with dolomite. Here it is usually accompanied by calcite and forsterite. The iron-rich member, pleonaste, is common both in ultrabasic rocks such as dunites and in quartz-less argillaceous hornfelses and gneisses. In these latter it is almost universally accompanied by cordierite and frequently by sillimanite, andalusite or corundum. Hercynite is characteristic of the granulites of Saxony in association with garnet, sillimanite, etc. The chromiferous spinel, picotite, is rele gated to the ultrabasic rocks, as dunite, lherzolite and the serpen tines derived from them. Gahnite, the zinc spinel, occurs in schists associated with zinc ores, and in pegmatites in Finland and at Broken Hill, New South Wales; while franklinite is associated with zinc and manganese minerals in limestone at Franklin Fur nace, New Jersey. (See also CHROMITE ; MAGNETITE.) (C. E. T.)