Sesquioxides -- Hematite Group, Corundum Composition
The Hematite Group includes the closely related rhombohedral minerals, corundum, A12O3, hematite, Fe2O3, and ilmenite (Fe,Ti)2O3.
Hematite – Group
Corundum
Composition. Aluminum oxide, A12O3 = Oxygen 47.1, aluminum 52.9
Crystallization. Hexagonal-rhombohedral. Crystals usually prismatic in habit ir tapering hexagonal pyramids (Figs. 254 and 255). Often rounded into barrel shapes (Fig. 256). Frequently with deep horizontal striations. At times shows rhombohedral and pyramidal faces.
Structure. Rudely crystallized or massive with parting planes nearly cubic in angle; coarse or fine granular.
Physical Properties. Parting basal and rhombohedral, the latter giving nearly cubic blocks. H . = 9 (next to the diamond in hardness). G. = 3.95 – 4.1 (unusually high for a nonmetallic mineral). Adamantine to vitreous luster. Color various. Usually some shade of brown, pink or blue. May be white, gray, green, ruby-red or sapphire-blue. May be white, gray, green, ruby –red or sapphire-blue transparent to opaque.
Test. infusible. Insoluble. Finely pulverized material moistened with cobalt nitrate and intensely ignited assumes a blue color (aluminum). Characterizes by its great hardness, adamantine luster and high specific gravity.
Varieties. Ordinary Corundum. In translucent to opaque masses showing often the nearly cubical parting; also granular to compact.
Gem Corundum. When transparent and finely colored, corundum furnishes various gem stones. The ruby is deep red corundum; sapphire is blue corundum. Stones of other colors are sometimes spoken of as yellow, violet, etc., sapphires, or are designated by prefixing the word oriental to the name of some other mineral similar in color; thus, oriental topaz, is a brownish yellow corundum; oriental amethyst, a reddish violet corundum, etc.
Emery. Is a fin-grained corundum mixed with other minerals, chiefly magnetite.
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