Spinel

The spinels are any of a class of minerals of general formulation XY₂O₄ which crystallize in the cubic (isometric) crystal system, with the oxide anions arranged in a cubic close-packed lattice and the cations X and Y occupying some or all of the octahedral and tetrahedral sites in the lattice. X and Y can be divalent, trivalent, or quadrivalent cations, including magnesium, zinc, iron, manganese, aluminium, chromium, titanium, and silicon. Although the anion is normally oxide, structures are also known for the rest of the chalcogenides.

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Types of Spinel

Important members of the spinel group include:

• Spinel – MgAl₂O₄, after which this class of minerals is named
• Gahnite - ZnAl₂O₄
• Franklinite - (Fe,Mn,Zn)(Fe,Mn)₂O₄
• Chromite - (Fe·Mg)Cr₂O₄
• Magnetite - Fe₃O₄
• Hercynite - FeAl₂O₄
• Ulvöspinel - TiFe₂O₄
• Jacobsite - MnFe₂O₄
• Trevorite - NiFe₂O₄
• Ringwoodite - SiMg₂O₄, an abundant olivine polymorph within the Earth's mantle from about 520 to 660 km depth, and a rare mineral in meteorites

Properties of true spinel

Spinel crystallizes in the isometric system; common crystal forms are octahedra, 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 Samarian Spinel is the largest known 500 carat spinel in the world.

The transparent red spinels are 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.

Occurrence

True spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestones of Myanmar and Thailand. Spinel is found as a metamorphic mineral, and also as a primary mineral in rare mafic igneous rocks; in these igneous rocks, the magmas are relatively deficient in alkalis relative to aluminum, and aluminium oxide may form as the mineral corundum or may combine with magnesia to form spinel. This is why spinel and ruby are often found together.

Spinel, (Mg,Fe)(Al,Cr)₂O₄, is common in peridotite in the uppermost Earth's mantle, between the Mohorovicic discontinuity (the Moho) and a depth of 70 kilometers or so; below that depth, the spinel (if present) becomes increasingly rich in chromium, as with increasing depth, pyrope-rich garnet becomes the more stable aluminous mineral in peridotite. At depths significantly shallower than the Moho, calcic plagioclase is the more stable aluminous mineral in peridotite.

Spinel, (Mg,Fe)Al₂O₄, is a common mineral in the Ca-Al-rich inclusions (CAIs) in some chondritic meteorites.

The Spinel structure

In the so-called normal spinel structure, X cations occupy the tetrahedral sites of the oxide lattice, and Y cations the octahedral sites. For inverse spinels, half the Y cations occupy the tetrahedral sites, and both X and Y cations occupy the octahedral sites. For many years, crystal field theory was invoked to explain the distribution of the cations within the spinels. As the octahedral and tetrahedral sites in the lattice generate different amounts of crystal field stabilisation energy (CFSE), it was argued that the arrangement of the two types of cation that generated the most CFSE would be the most stable. However, this idea was challenged by Burdett and co-workers, who showed that a better treatment used the relative sizes of the s and p atomic orbitals of the two types of atom to determine their site preference.^[1] This is because the dominant stabilising interaction in the solids is not the crystal field stabilisation energy generated by the interaction of the ligands with the d-electrons, but the σ-type interactions between the metal cations and the oxide anions. This rationale can explain anomalies in the spinel structures that crystal-field theory cannot, such as the marked preference of Al³⁺ cations for octahedral sites or of Zn²⁺ for tetrahedral sites - using crystal field theory would predict that both have no site preference. Only in cases where this size-based approach indicates no preference for one structure over another do crystal field effects make any difference - in effect they are just a small perturbation that can sometimes make a difference, but which often do not.

References

1. ^ J.K. Burdett, G.L. Price and S.L. Price, J. Am. Chem. Soc., 104 (1982), 92-95

See also

• Deer, Howie and Zussman (1966) An Introduction to the Rock Forming Minerals, Longman, pp.424-433, ISBN 0-582-44210-9
• Shumann, Walter (2006) Gemstones of the World 3rd edition, Sterling, pp.116-117.
• Spinel structure by Steven Dutch
• Spinel structure
• http://www.gemstone.org/gem-by-gem/english/spinel.html
• http://mineral.galleries.com/minerals/oxides/spinel/spinel.htm
• http://www.mindat.org/min-3729.html
• http://www.webmineral.com/data/Spinel.shtml
• The story of the worlds most famous Red Spinel, the Black Princes Ruby