High pressure polymorph of olivine, β-Mg2SiO4, found on Earth and in some meteorites. It is thought to make up 50% or more of Earth’s mantle between depths of 400 and 525 km. Wadsleyite transforms into ringwoodite at high pressure, but the exact pressure depends strongly on composition. At lower pressures, it transforms into olivine. This mineral was synthesized in the laboratory and subsequently observed as inclusions in diamond. It has an unusual structure in which one oxygen ion is incompletely bonded, allowing it to be easily protonated. The structure can accept up to 3 wt. % H2O
as hydroxyl at this site, so wadsleyite may constitute a vast reservoir of mantle water.

Image source: http://bowfell.geol.ucl.ac.uk/~lidunka/GlobalGeophysics/Geophysics7%20-%20Deep%20Earth/Earth%20Structure.htm.