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Inverse spinel structures have a different cation distribution in that all of the A cations and half of the B cations occupy octahedral sites, while the other half of the B cations occupy tetrahedral sites. An example of an inverse spinel is Fe 3 O 4, if the Fe 2+ (A 2+) ions are d 6 high-spin and the Fe 3+ (B 3+) ions are d 5 high-spin.
Cation distribution [ edit ] There is experimental [ 8 ] and theoretical [ 9 ] evidence that Fe(III) cations and vacancies tend to be ordered in the octahedral sites, in a way that maximizes the homogeneity of the distribution and therefore minimizes the electrostatic energy of the crystal .
Polyhedral representation of spinel MgAl 2 O 4. Spinel (/ s p ɪ ˈ n ɛ l, ˈ s p ɪ n əl / [7]) is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula MgAl 2 O 4 in the cubic crystal system. Its name comes from the Latin word spinella, a diminutive form of spine, in reference to its pointed crystals. [5]
For typical ionic solids, the cations are smaller than the anions, and each cation is surrounded by coordinated anions which form a polyhedron.The sum of the ionic radii determines the cation-anion distance, while the cation-anion radius ratio + / (or /) determines the coordination number (C.N.) of the cation, as well as the shape of the coordinated polyhedron of anions.
The structure is inverse spinel, with O 2-ions forming a face-centered cubic lattice and iron cations occupying interstitial sites. Half of the Fe 3+ cations occupy tetrahedral sites while the other half, along with Fe 2+ cations, occupy octahedral sites. The unit cell consists of thirty-two O 2-ions and unit cell length is a = 0.839 nm. [15] [16]
Asterism is generated by reflections of light from twin-lamellae or from extremely fine needle-shaped acicular inclusions within the stone's crystal structure. [1] A common cause is oriented sub-microscopic crystals of rutile within the gem mineral. It occurs in rubies, sapphires, garnet, diopside, and spinel when a cabochon is cut from a ...
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The mineral ceylonite has the chemical composition of (Mg, Fe 2+) Al 2 O 4, putting it into a group of minerals known as the spinel group, or the oxide spinels. The oxide spinels have a formula of the model [A][B] 2 O 4; where [A] is commonly Fe 2+, Mg 2+, or Mn 2+, and [B] is Fe 3+, Al 3+, or Cr 3+. It is an iron-rich variety of the spinel ...