Phase Stability and Hydroxyl Vibration of Brucite Mg(OH)_2 at High Pressure and High Temperature

Brucite Mg(OH)_2 is an archetypal hydrous mineral and it has attracted a great deal of attention. However, little is known about the evolution of hydroxyl groups in brucite with respect to subduction fluids. We carried out Raman measurements up to 15.4 GPa and 874 K via an externally heated diamond anvil cell, investigating the stability of brucite under the conditions relevant to subducting slabs. The hydroxyl vibration mode A_\rm1_g(I) of brucite is weakened under simultaneous high pressure-temperature conditions. Meanwhile, the presence of carbonated solution can destabilize the hydroxyl groups of brucite at low pressure. Our results suggest that brucite releases water when reacting with hydrogen carbonate ion to form magnesite MgCO_3 in subduction zones. This implies that the global water cycle is largely coupled with the deep carbon cycle in Earth's interior.