1911 Encyclopædia Britannica/Spinel

by Frederick William Rudler

SPINEL, a name now given to a group of minerals, of which the typical member is a magnesium aluminate, sometimes used as a gem-stone, to which the term "spinel" was originally restricted. The name comes from the French spinelle (diminutive of Lat. spina), perhaps suggested by the sharp angles of the crystals. All spinels crystallize in the cubic system, usually in octahedra, and often twinned as in the accompanying figure, which is a form so characteristic as to be

called the "spinel twin." The hardness of spinel is about that of topaz (8) and its specific gravity near that of diamond. Professor A. H. Church gives the range in variously coloured spinels as 3.582 to 3.715. Pure spinel is colourless, but most varieties are coloured, no doubt in many cases with iron and probably in some with chromium. The deep red spinel is known as "spinel-ruby," or "ruby-spinel," and has often been taken for true ruby, from which it is distinguished, however, by being singly refracting and therefore not dichroic, as well as by its inferior hardness and density. The "balas ruby" is a rose-red spinel, said to derive its name from Balkh, the capital of Badakshan (Balaxia), where it occurs with rubies, and was formerly worked, chiefly in the Shighnan valley, in the upper Oxus basin. Rubicelle is a spinel in which the red colour tends to orange, whilst in almandine-spinel it passes into violet. Stones of the colour of vinegar are called vinegar- spinel. When the colour is blue the mineral is known as sapphire-spinel, and when green as chloro-spinel.

The spinels used in jewelry are found mostly in gem-gravels, where, however, the octahedral form is often well preserved. The chief localities are Ceylon, Siam and Upper Burma. In all these localities the spinels accompany the coloured corundums, and their close association with true rubies led Tavernier to Gall spinel "the mother of ruby." Formerly there was much confusion between the two minerals, and probably many stones described as monster rubies have been spinels. The great historic "ruby" set in the Maltese cross in front of the Imperial state crown of England is really a spinel. This fine stone was given to Edward the Black Prince by Pedro the Cruel, king of Castile, on the victory of Najera in 1367, and it was afterwards worn by Henry V. at the battle of Agincourt, when it narrowly escaped destruction. V. Ball described, in 1894, a spinel weighing 133½ carats, engraved with a Persian inscription, then in the possession of Lady Carew.

All the isomorphous minerals known as the group of spinellids, of which spinel is the type, crystallize in regular octahedra and have a composition conforming to the general formula R′′R₂′′′O₄ ( = R′′O·R₂′′′O₃). Ordinary spinel is MgAl₂O₄. A black opaque spinel in which Fe partly replaces Mg is known as pleonaste (πλεονασμός, abundant, from the number of faces on certain crystals) or ceylonite, from the island of Ceylon, but sometimes written ceylanite. It occurs in gneiss, often with cordierite, and is found also in the ejected blocks of Monte Somma, Vesuvius. Large crystals come from Warwick and Amity, Orange county, New York, U.S.A. The black spinels are generally green or brown when viewed in thin sections by transmitted light. In some cases spinel is evidently a result of contact metamorphism, whilst in others it has crystallized out of a molten magma, as illustrated by the experiments of J. Morozewicz. A chrome-spinel with the formula (Mg,Fe) (Al,Fe,Cr)₂C₄ is named picotite, after Picot de la Peyrouse, who described it. Picotite occurs in the form of black grains and crystals in certain olivine rocks and in serpentine. A black iron-spinel (FeAl₂O₄), found in the granulites of Saxony and Bohemia, is known as hercynite from the Hercynian Forest. A zinc-spinel (ZnAl₃O₄) , occurring in talcose slate near Falun in Sweden, is named gahnite, after its discoverer J. G. Gahn; whilst it has also been termed automolite from Gr. αὐτόςμολος, a deserter, in allusion to the occurrence of zinc in a mineral where it was unexpected. The group of spinellids includes, as its extreme members, magnetite (Fe′′Fe₂′′′O₄) and chromite (FeCr₂O₄) (q.v.).  (F. W. R.*)