Spinel

The spinels are any of a class of minerals of general formulation A²⁺B³⁺₂O^2-₄ whichcrystallise in the cubic (isometric) crystal system, with the oxide anions arranged in a cubicclose-packed lattice and the cations A and B occupying some or all of the octahedral andtetrahedral sites in the lattice. A and B 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 thechalcogenides. A and B can also be the same metal under different charges, such as the case in Fe₃O₄ (as Fe²⁺Fe³⁺₂O^2-₄).

Spinel
General
Category	Mineral
Chemical formula	MgAl2O4
Identification
Color	Various, red to blue to mauve. Dark green, brown. Black
Crystal habit	Cubic, octahedral
Crystal system	Isometric
Cleavage	Indistinct
Fracture	Conchoidal, uneven
Mohs Scalehardness	8.0
Luster	Vitreous
Streak	White
Specific gravity	3.54-3.63
Refractive index	1.712-1.762
Pleochroism	Absent

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 - Mg₂SiO₄, 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 crystallises 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. Some spinels are among the most famous gemstones: Among them is the Black Prince's Ruby and the 'Timur ruby' in the British Crown Jewels, and the 'cote de Bretagne' formerly from the French Crown jewels. The Samarian Spinel is the largest known spinel in the world, weighing 500 carats (100 g).

The transparent red spinels were called spinel-rubies or balas-rubies. In the past, before the arrival of modern science, spinels and rubies were equally known as rubies. After the 18th century the word ruby was only used for the red gem variety of the mineral corundum and the word spinel became used. "Balas" is derived from Balascia, the ancient name forBadakhshan, a region in central Asia situated in the upper valley of the Kokcha River, one of the principal tributaries of the Oxus River. The Badakshan province was for centuries the main source for red and pink spinels.

Occurrence

True spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestonesof the Badakshan province in nowadays Tajikistan and of Mogok in Burma. Recently gem quality spinels were also found in the marbles of Luc Yen (Vietnam), Mahenge and Matombo (Tanzania), Tsavo (Kenya) and in the gravels of Tunduru (Tanzania) and Ilakaka (Madagascar). 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 toaluminium, 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 450km (where olivine is metamorphosed to spinel) to 670km kilometers or so; below that depth, the spinel is oxidised. At depths significantly shallower than the Moho, calcicplagioclase is the more stable aluminous mineral in peridotite.

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

Cut Spinel

The spinel structure

In normal spinel structures, they are usually cubic closed-packed oxides with one octahedral and two tetrahedral sites per oxide. The tetrahedral points are smaller than the Octahedral points. B³⁺ ions occupy the octahedral holes because of a charge factor, but can only occupy half of the octahedral holes. A²⁺ ions occupy 1/8th of the tetrahedral holes. This maximises the lattice energy if the ions are similar in size. A common example of a normal spinel is MgAl₂O₄.

Inverse spinel structures however are slightly different in that you must take into account the Crystal Field Stabilisation Energies (CFSE) of the Transition metals present. Some ions may have a distinct preference on the octahedral site which is dependent on the d-electron count. If the A²⁺ ions have a strong preference for the octahedral site, they will force their way into it and displace half of the B³⁺ ions from the octahedral sites to the tetrahedral sites. If the B³⁺ions have a low or zero Octahedral Site Stabilisation Energy (OSSE), then they have no preference and will adopt the tetrahedral site. A common example of an inverse spinel is Fe₃O₄, if the Fe²⁺ (A²⁺) ions are d⁶ high-spin and the Fe³⁺ (B³⁺) ions are d⁵ high-spin.

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 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-typeinteractions 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.

Synthetic Spinel

Synthetic Spinel has been accidentally produced in the middle of the 18th century, and has been more recently described in scientific publications in 2000 and 2004.^[2]