Wadsleyite
| Wadsleyite | |
|---|---|
| General | |
| Category | Sorosilicate |
|
Formula (repeating unit) |
Mg2SiO4 |
| Strunz classification | 9.BE.02 |
| Crystal system | Orthorhombic (Horiuchi and Sawamoto, 1981) |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Space group | Imma |
| Unit cell | a = 5.7 Å, b = 11.71 Å c = 8.24 Å; Z = 8 |
| Identification | |
| Color | Dark green |
| Crystal habit | Microcrystalline aggregates |
| Diaphaneity | Transparent |
| Specific gravity | 3.84 calculated |
| Optical properties | Biaxial |
| Refractive index | n = 1.76 |
| References | |
Wadsleyite is a high-pressure phase of Mg2SiO4 and is polymorphous with the olivine phase forsterite. An orthorhombic mineral with the formula β-Mg2SiO4, it was first found in nature in the Peace River meteorite from Alberta, Canada. It is formed by a phase transformation from forsterite (α-Mg2SiO4) under increasing pressure and eventually transforms into spinel-structured ringwoodite (γ-Mg2SiO4) as pressure increases further. The structure can take up a limited amount of other bivalent cations instead of magnesium, but contrary to the α and γ structures, a β structure with the sum formula Fe2SiO4 is not thermodynamically stable. Its cell parameters are approximately a = 5.7 Å, b = 11.7 Å and c = 8.24 Å.
Wadsleyite is found to be stable in the upper part of the transition zone of the Earth’s upper mantle between 410–520 kilometres (250–320 mi) in depth. Because of oxygens not bound to silicon in the Si2O7 groups of wadsleyite, it leaves some oxygen atoms underbonded, and as a result, these oxygens are hydrated easily, allowing for high concentrations of hydrogen atoms in the mineral. Hydrous wadsleyite is considered a potential site for water storage in the Earth’s mantle due to the low electrostatic potential of the underbonded oxygen atoms. Although wadsleyite does not contain H in its chemical formula, it may contain more that 3 percent by weight H2O, and may coexist with a hydrous melt at transition zone pressure-temperature conditions. The solubility of water and the density of wadsleyite depend on the temperature and pressure in the Earth. Furthermore, the transformation resulting in wadsleyite is thought to occur also in the shock event when a meteorite impacts the Earth or another planet at very high velocity.
Wadsleyite was first identified by Ringwood and Major in 1966 and was confirmed to be a stable phase by Akimoto and Sato in 1968. The phase was originally known as β-Mg2SiO4 or “beta-phase”. Wadsleyite was named for mineralogist Arthur David Wadsley (1918-1969).
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