|Color||Various, red to blue to mauve. Dark green, brown. Black|
|Crystal habit||Cubic, octahedral|
|Mohs Scale hardness||8.0|
The spinels are an important group of oxide minerals with similar crystalline structures. Their general chemical formula may be written as (X)(Y)2O4, in which X and Y represent cations of various metals. They are named after the representative mineral spinel, which corresponds to magnesium aluminum oxide (MgAl2O4).
This group consists of more than 20 minerals, including valuable ores such as chromite, a source of chromium; magnetite, a source of iron; and franklinite, a source of manganese and zinc. The representative member, spinel, produces gem-quality stones. In particular, spinel-rubies are red spinels that resemble rubies and are used as gemstones.
The spinel group includes the following minerals:
Spinel is found as a metamorphic mineral and as a primary mineral in basic rocks. In such magmas, the absence of alkalis prevents the formation of feldspars, and any aluminum oxide present will form corundum (aluminum oxide) or combine with magnesia (magnesium oxide) to form spinel. For this reason, spinel and ruby are often found together.
As noted above, the general chemical formula for the spinel group is (X)(Y)2O4. In this formula, "X" and "Y" represent various divalent, trivalent, and quadrivalent cations, including the cations of magnesium, zinc, iron, manganese, aluminum, chromium, and titanium. Also, "O" represents the oxide anion.
Spinels crystallize in the isometric (cubic) system, usually with an "octahedral habit"—that is, outwardly shaped as octahedrons. The crystals are most often twinned. The oxide anions are arranged in a cubic close-packed structure, the X cations occupy tetrahedral sites, and the Y cations, octahedral sites. For "inverse spinels," half the Y cations occupy the tetrahedral sites, and both X and Y cations occupy octahedral sites.
Spinels have an imperfect octahedral cleavage and a conchoidal fracture. Their hardness is about eight on the Mohs scale, and their specific gravity is 3.5-4.1. They range from transparent to opaque, with a vitreous to dull luster. Some spinels are colorless, but they are usually in various shades of red, blue, green, yellow, brown, or black. A unique white spinel, now lost, surfaced briefly in Sri Lanka.
Several spinels—such as chromite, magnetite, and franklinite—are important ores of metals. True spinel is the only member of the group that provides gemstones. The transparent red spinels are called spinel-rubies or balas-rubies and were often confused with actual rubies in ancient times. Yellow to orange-red spinel is called rubicelle. The famous Black Prince's Ruby in the British Crown Jewels is a spinel.
All links retrieved October 16, 2015.
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