#### What is description of spinel? What are the characteristics of spinal group mineral, formulas, structure and information.

SPINEL, a mineral group of metallic oxides or any member of that group.

The Group. The spinel group has the general formula $\displaystyle A{{B}_{2}}{{O}_{4}}$, in which A is a divalent element ( + 2) and B is trivalent ( + 3). The A position may be occupied by magnesium, ferrous iron, zinc, or manganese, and the B position by aluminum, ferric iron, or chromium. One of the A elements may freely substitute for another in a given mineral series, but the B elements are less free to change. The principal members of the group are listed below in columns and rows that give their A and B content.

The Mineral. Spinel, or magnesium aluminum oxide, is a common metamorphic mineral that is sometimes used to make relatively inexpensive gems. Crystals of spinel are transparent to opaque and have a glassy or dull luster. Commonly they are black, but they may also be red, violet, blue, or yellow. The mineral occurs in metamorphosed limestones and in gneiss, or as an accessory mineral in basic igneous rocks. It is also found as pebbles in stream sands.

Transparent red spinel is the variety used most often for gems, which sometimes are called ruby spinel, balas ruby, or sapphirine. Buby spinel is found in deposits in Ceylon, Burma, and Thailand. It may also be synthesized, using the same process by which synthetic rubies and sapphires are produced. The synthetic material is also used as a refractory.

Composition, $\displaystyle MgA{{l}_{2}}{{O}_{4}}$; specific gravity, 3.5-4.1; hardness, 7.5-8; crystal system, isometric.

mavi

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