Spinel
in mineralogy: (1) The type species of a group of minerals called the spinel group, crystallizing in the isometric system, and being compounds of protoxides and sesquioxides with the typical formula ROR₂O₃. (2) A mineral ocurring in crystals of octahedral habit, and very rarely massive. Hardness, 8.0; sp. gr., 3.5-4.1; luster, vitreous to splendent, sometimes dull; color, many shades of red, also blue, green, yellow, brown, and black; sometimes nearly white, or colorless; transparent to opaque; fracture, conchoidal. Com- SPINEL position: When pure, alumina, 72.0; magnesia, 28.0=100, corresponding with the formula, MgO Al2O3; but the magnesia is often partly replaced by other protoxides, and the alumina by sesquioxides, giving rise to many varieties. Dana thus distinguishes them: (1) Ruby or magnesia-spinel; with sp. gr. 3.52-3.58; (a) spinel-ruby, deep red: (b) balas-ruby, rose-red; (c) rubicelle, yellow or orange-red; (d) almandine, violet. (2) Ceylonite, or iron-magnesia spinel = pleonaste, containing much iron; color, dark green to black. (3) Magnesia-lime-spinel; green. color, (4) Chlorospinel; color, grass-green, with the iron constituent as sesquioxide. (5) Picotite, containing over 7 per cent. of oxide of chromium.