is about 3·65. The following determinations of the specific gravity of choice cut specimens of spinel will be useful for reference:
Deep red | 3·582 | Rose-red | 3·631 |
Aurora-red | 3·590 | Dull purple | 3·637 |
Puce | 3·592 | Indigo | 3·675 |
Sky-blue | 3·615 | Deep indigo | 3·715 |
Where the specific gravity of a spinel is too near that of a garnet to allow of the species being thus distinguished, the superior hardness of the spinel enables the problem to be solved. It is scarcely necessary to say that the dichroiscope affords no criterion in such a case, since the spinel and garnet both belong to the monometric system and are necessarily moriochroic.
Spinel is essentially composed of one molecule of alumina and one of magnesia, or in 100 parts:
Alumina | 72 | Magnesia | 28 |
But in the coloured varieties decided traces of other oxides occur, such as those of chromium and iron in the spinel ruby; oxide of copper in the grass green spinel called chlorospinel; and the protoxide of iron in the darker and opaque varieties. Some specimens of pleonaste, the black spinel of Ceylon, have been found to contain over 20 per cent. of ferrous oxide, the protoxide of iron, which takes the place of its equivalent of magnesia.
Spinels in true crystals and very hard have been formed artificially, though not of fine quality and large size, by several different processes, such as heating alumina, magnesia, and boracic acid together to a very high temperature. The vapour of aluminium chloride passed over heated magnesia also produces spinel crystals, so does the strong heating together of magnesia and alumina. Some imitation blue spinels or sapphires have been made by the fusion together of alumina, lime, and a little cobalt. These ingredients have, however, given a blue glassy mass softer than true spinel, and merely enclosing here and there a few minute crystals of what may be termed a "lime spinel," the main mass being