Wadsleyite

Wadsleyite is a high-pressure polymorph of olivine, and is an orthorhombic mineral found in the Peace River meteorite in Alberta, Canada. In phase transformations with increasing pressure from Mg₂SiO₄-Fe₂SiO₄ (forsterite – fayalite), olivine is transformed to wadsleyite (β-Mg₂SiO₄) and then to a spinel-structured ringwoodite (γ-Mg₂SiO₄). This series of transformations is thought to occur during an extraterrestrial shock event in the meteorite prior to its fall on Earth. With a formula of (Mg,Fe2+)₂(SiO₄), its cell parameters are as follows: a = 5.7 Å, b = 11.7 Å and c = 8.24 Å. It is polymorphous with ringwoodite and is found to be stable in the transition zone of the Earth’s upper mantle. These regions are from 400–525 kilometres (250–326 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. As a result, there can be high concentrations of hydrogen atoms in the mineral. Hydrous wadsleyite is a 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 water solubility and density of wadsleyite are ultimately affected by the temperature and pressure inside of the Earth.

Wadsleyite was first identified by Ringwood and Major in 1966 and was confirmed to be a stable phase by Akimoto and Sato in 1968.(Horiuchi and Sawamoto, 1981) The phase was originally known as β-Mg₂SiO₄ or “beta-phase ” and is a polymorph of olivine, along with minerals ringwoodite. Wadsleyite was named for mineralogist Arthur David Wadsley (1918-1969).