Spinel
The spinels are any of a class of
minerals which
crystallize in the isometric system with an octahedral habit. The general formula is as 24, with X representing cations occupying tetrahedral sites and Y cations occupying octahedral sites. Divalent, trivalent, and quadrivalent cations can occupy the X and Y sites, and they include
Mg,
Zn,
Fe,
Mn,
Al,
Cr,
Ti, and
Si.
The oxygen anions are arranged in a cubic close-packed structure. In the normal spinel structure, X cations occupy the tetrahedral sites, and Y the octahedral sites.
Encyclopedia
| Spinel | | 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
24, with X representing cations occupying tetrahedral sites and Y cations occupying octahedral sites. Divalent, trivalent, and quadrivalent cations can occupy the X and Y sites, and they include
Mg,
Zn,
Fe,
Mn,
Al,
Cr,
Ti, and
Si.
The oxygen anions are arranged in a cubic close-packed structure. In the normal spinel structure, X cations occupy the tetrahedral sites, and Y the octahedral sites. For
inverse spinels, half the Y cations occupy the tetrahedral sites, and both X and Y cations occupy the octahedral sites.
Important members of the spinel group include:
- spinel – MgAl2O4, after which this class of minerals is named
- gahnite - ZnAl2O4
- franklinite - 2O4
- chromite - Cr2O4
- magnetite - Fe3O4
- hercynite - FeAl2O4
- ulvospinel - TiFe2O4
- ringwoodite - SiMg2O4, an abundant olivine polymorph within the Earth's mantle from about 520 to 660 km depth, and a rare mineral in meteorites
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.
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.
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.
The spinel structure type is named after the spinel structure Al2MgO4 but is adopted by a range of species which do not necessarily include oxygen. Sn4N4 for example adopts this structure with Sn in both the octahedral and the tetrahedral holes.
See also:
List of mineralsExternal links
