Intermediate Oxides - Spinel Group, Spinel Composition & Structure

A group of oxides which in composition are combinations of a bivalent oxide with a trivalent oxide. The general formula being, R” O. R₂” “O₃. R”O may be MgO, ZnO, FeO, MnO, while R₂” “O₃ may be A1₂O₃, Fe₂O₃, Cr₂O₃. The chief members of the group are as follows:

• Spinel, MgO. A1₂O₃ or MgA1₂O₄.
• Gahnite, ZnO.A1₂O₃ or ZnA1₂O₄
• Magnetite, FeO.Fe₂O₃ or FeFe₂O₄
• Franklinite, (Fe, Mn, Zn) O. (Fe, Mn)₂O₃
  or (Fe,Mn,Zn) O. (Fe, Mn)₂O₄
• Chromite, (Fe, Mg) O.Cr₂O₃ or (Fe, Mg) Cr₂O₄

The crystalloid habit of all the members of the group is octahedral. The dodecahedron is sometimes present, but other forms are rare.

Spinel
Composition. MgA1₂O₄ or MgO.A1₂O₃ = Alumina 71.8, magnesia 28.2. The magnesium may be, in part, replaced by ferrous iron or manganese and the aluminum by ferric iron and chromium. 

Crystallization. Isometric. Habit strongly octahedral (Fig. 261). Sometimes in twinned octahedrons (spinel twins) (Fig. 262). Dodecahedron at times as small truncations (Fig. 263). Other forms rare.

Structure. Usually crystallized.

Physical Properties. H. 8 =. G. = 3.5-4.1. Nonmetallic. Vitreous luster. Color various, red, lavender, blue, green, brown, black, sometimes almost white. Streak white. Usually translucent to opaque, at times clear and transparent.

Test. infusible. The finely powdered mineral dissolves completely B. B. in the salt of phosphorus bead (proving the absence of silica). Recognized chiefly by its hardness (8), its octahedral crystals and vitreous luster.

Varieties. 

1. Ruby Spinel. Nearly pure magnesian spinel. Clear red; transparent to translucent. When rose-red known as baias ruby; yellow or orange – red; rubicelle; violet-red, almandine ruby.
2. Pleonate. Iron-magnesia spinel. Color dark green, brown to black. Opaque or nearly so.
3. Chlorospinel. Magnesia-iron spinel. Color grass-green owing to presence of copper.
4. Picotite, or Chrome Spinel. Contains chromium and has iron replacing magnesium. Color yellowish or greenish brown. Translucent to opaque.

Occurrence. A common metamorphic mineral occurring embedded in granular limestone, associated with calcite, serpentine, etc. Occurs also as an accessory mineral in many basic igneous rocks, as peridotites, etc. Spinel is frequently formed in contact zones between eruptive rocks and limestones, owing its to regain to pneumatolytic conditions. Found frequently as rolled pebbles in stream sands, where it as been preserved on account of its hardness. The ruby spinels are found in this wash, often associated with the corundum ruby, in the sands of Ceylon, Siam, Upper Burmanh, Madagascar, etc. Ordinary spinel is found in various localities in New York, and New Jersey.

Use. When transparent and finely colored is used as a gem. Usually red in color and known as the spinel ruby, balas ruby, etc. Some stones are blue in color. The largest cut stone known weighs in the neighborhood of 80 carts. The stones usually are comparatively inexpensive.