Spinel

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Spinel
Image:Spinelma.jpeg
General
Category	Mineral
Chemical formula	MgAl₂O₄
Identification
Colour	Various, red to blue to mauve. Dark green, brown. Black
Crystal habit	Cubic, octahedral
Crystal system	Isometric
Cleavage	Absent
Fracture	Conchoidal, uneven
Mohs Scale hardness	8.0
Luster	Vitreous
Refractive index	Transparent to opaque
Pleochroism	?
Streak	White
Specific gravity	3.6 - 3.7
Fusibility	?
Solubility	?
Major varieties
See main article

The spinels are any of a class of minerals which crystallize in the isometric system with an octahedral habit. The general formula is as (X²⁺)(Y³⁺)₂(O^2-)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.

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

Important members of the spinel group are:

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.

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.

Image:Spinel.jpgThe 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.

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.