Spinel

The spinels are any of class of minerals which crystallize in the isometric system with an octahedral habit. The general formula is as (X2+)(Y3+)2(O2-)4, with X representing a divalent cation and Y a trivalent cation. The bivalent cations may be Mg, Zn, Fe and Mn, and the trivalent oxides may be Al, Fe, Mn and Cr.

Related Topics:
Mineral - Crystal - O - Mg - Zn - Fe - Mn - Al - Cr

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The oxygen anions are arranged in a cubic close-packed structure. All of the octahedral holes are filled with trivalent cations, and the divalent cations are found in half of the tetrahedral holes. There are also inverse spinels, where half of the octahedral holes are filled with divalent cations and half of the tetrahedral holes are filled with trivalent cations.

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Important members of the spinel group are:

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• spinel ? MgAl₂O₄, after which this class of minerals is named
• gahnite (zinc spinel) - ZnAl₂O₄
• franklinite - (Fe,Mn,Zn)(Fe,Mn)₂O₄
• chromite - (Fe·Mg)Cr₂O₄
• magnetite - Fe₃O₄

True spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestones of Myanmar and Thailand.

Related Topics:
Gemstone - Sri Lanka - Limestone - Myanmar - Thailand

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Spinel usually occurs in isometric crystals, octahedrons, usually twinned. It has an imperfect octahedral cleavage and a conchoidal fracture. Its hardness is 8, its specific gravity is 3.5-4.1 and it is transparent to opaque with a vitreous to dull lustre. It may be colorless, but is usually various shades of red, blue, green, yellow, brown or black. There is a unique natural white spinel, now lost, that surfaced briefly in what is now Sri Lanka. Another famous spinel is the Black Prince's Ruby in the British Crown Jewels.

Related Topics:
Hardness - Specific gravity - Red - Blue - Green - Yellow - Brown - Black - Black Prince's Ruby - Crown Jewels

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The name "spinel" is derived from the Greek word for spark, in reference to the fiery red color of spinels often used for gems. The transparent red spinels are also called spinel-rubies or balas-rubies and were often confused with actual rubies in ancient times. "Balas" is derived from Balascia, the ancient name for Badakhshan, a region in central Asia situated in the upper valley of the Kokcha river, one of the principal tributaries of the Oxus river. Yellow spinel is called rubicelle and violet-colored manganese-bearing spinel is called almandine.

Related Topics:
Greek - Rubies - Badakhshan - Asia - Kokcha river - Oxus river

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Spinel is found as a metamorphic mineral, and also as a primary mineral in basic rocks, because in such magmas the absence of alkalis prevents the formation of feldspars and any aluminium oxide present will form corundum or combine with magnesia to form spinel. This is why spinel and ruby are often found together.

Related Topics:
Metamorphic mineral - Magma - Alkali - Feldspar - Corundum

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See also: List of minerals

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