A schematic diagram showing the concentrations of various defects as a function of water content in olivine (modified from Karato et al. 2006). As the water content change, the dominant charged defects ferric iron |${\bf{Fe}}_{\bf{M}}^ \cdot $| and M-site vacancy |${\bf{V}}_{\bf{M}}^{^{\prime\prime}}$| at low-water content become |${\bf{Fe}}_{\bf{M}}^ \cdot $| and |${\bf{H}}_{\bf{M}}^{\prime}$| at high-water content.