Wadsleyite

Wadsleyite is an orthorhombic mineral with the formula β-(Mg,Fe)₂SiO₄. It was first found in nature in the Peace River meteorite from Alberta, Canada. It is formed by a phase transformation from olivine (α-(Mg,Fe)₂SiO₄) under increasing pressure and eventually transforms into spinel-structured ringwoodite (γ-(Mg,Fe)₂SiO₄) 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 = 11.71 Å and c = 8.24 Å.

Wadsleyite is found to be stable in the upper part of the Transition Zone of the Earth's mantle between 410–520 kilometres (250–320 mi) in depth. Because of oxygen atoms not bound to silicon in the Si₂O₇ groups of wadsleyite, it leaves some oxygen atoms insufficiently bonded. Thus, 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 under bonded oxygen atoms. Although wadsleyite does not contain H in its chemical formula, it may contain more than 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. Even though their maximum water storage capabilities might be reduced to about 0.5–1 wt% along the normal geotherm, the transition zone which holds up to 60 vol% wadsleyite could still be a major water reservoir in the Earth's interior. 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).