Spinel

Spinel is a mineral composed of magnesium and aluminum oxides, because it contains magnesium, iron, zinc, manganese and other elements, they are divided into many kinds, such as aluminum spinel, iron spinel, zinc spinel, manganese spinel, chromium spinel and so on.

Spinel's English name is Spinel, meaning a crystal with sharp angles, is a magnesium aluminum oxide, chemical formula is (Mg, Fe, Zn, Mn)(Al, Cr, Fe)2O4, spinel chemical composition in the isomorphic substitution is common, often containing iron, zinc, chromium, manganese and so on. The composition of equiaxed crystals is more complex, including Aluminum spinel (Aluminum spinel) and chromium spinel (Picotite) and other subfamilies. The spinel subgroups containing iron are opaque magnetite and magnehematite. Most spinel is aluminum spinel subgroup, where Mg2+ and Fe2+ can be mixed in any proportion. Crystallization habit: often in octahedral crystal shape, sometimes octahedral and rhombohedral dodecahedron, cube into polyform, but also often with the octahedral surface as a double crystal and joint surface to form a double crystal [1]. Spinel group minerals, formed in nature when molten magma intrudes into impure limestone or dolomite by contact metamorphism, or crystallized by magma and produced in basic and ultrabasic igneous rocks, some appear in aluminum-rich basic magmatic rocks.

Spinel crystals, belonging to the equiaxial crystal system, are face-centered cubic, Z=8. The basic structure is the accumulation of oxygen in the ABC order perpendicular to (111). Tetrahedrons and octahedral layers are interphase, and the ratio of tetrahedrons to octahedrons is 2:1. The spinel structure can be seen as the closest cubic packing of oxygen ions, and then X ions occupy 1/8 of the 64 tetrahedral Spaces, that is, 8 A sites, and Y ions occupy 1/2 of the 32 octahedral Spaces, that is, 16 B sites. Thus, the general formula of spinel unit cell is X8Y16O32, which is often written as XY2O4 (1 ~ 5) after simplification.
The crystal of spinel includes normal spinel type structure and anti-spinel type structure. Normal spinel structure: the general structure is XY2O4, X is a divalent cation, Y is a trivalent cation, oxygen ions are cubic tightly packed, trivalent cation Y occupies the hexagonal coordination octahedral space, divalent cation X occupies the tetrahedral space of the quadric coordination. Antispinel structure: The structure of the general formula Y[XY]O4, bivalent cations and half of the trivalent cations occupy the octahedral space, the other half of the trivalent cations occupy the tetrahedral space, also known as inverted spinel structure, magnetite (Fe2+FeO4) structure is this type.

Spinel is often found in schist, serpentine and related rocks, and most gem-grade spinel is found in alluvial fans. It can be produced in marble (skarn type), with ruby, sapphire and other symbiosis, can also be produced in placer. The main producing areas of spinel are Myanmar, Sri Lanka, Kenya, Nigeria, Tanzania, Tajikistan, Vietnam, the United States and Afghanistan

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