The role of hydrogen bonds in hydrous minerals stable at lower mantle pressure conditions

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Over the past few decades, hydrous minerals were thought to be absent in the lower mantle, due to their instability at high-pressure conditions. Recently, however, hydrous phases including phase H (MgSiO ₄ 4 H ₂ 2 ), pyrite-type FeOOH, and δ δ -AlOOH have been discovered to be thermodynamically stable at lower mantle pressures. Investigations using ab initio calculations methods play a key role in identifying these novel phases and determining their geophysical properties (i.e., compressibility, elasticity, and sound velocities). These calculations suggest that the hydrous minerals which are stable at lower mantle pressure conditions (i.e., phase H, FeOOH, and AlOOH), have symmetric hydrogen bonds at these pressures. This indicates that hydrogen bond strength is closely connected to the stability and physical properties of hydrous minerals at extreme pressures. In this review, we summarize the theoretical and experimental studies of hydrous minerals stable at the high-pressure conditions of the Earth’s lower mantle in light of the role of hydrogen bonding.