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The vibrational properties of hydroxyl groups in pure and Fe-bearing brucite have been investigated under high-pressure and -temperature conditions relevant to subduction zones. Using externally heated diamond-anvil cell techniques combined with in situ Raman spectroscopy, we observed that the hydroxyl stretching modes in pure brucite change from blueshift to redshift at ∼600 K and 1.2 GPa. This indicates a sublattice proton order-disorder transition associated with decreasing interlayer spacing in brucite. Moreover, the Fe-Mg substitution in brucite leads to a secondary hydroxyl stretching band at 3644.6 cm −1  for Mg 0.9 Fe 0.1 (OH) 2 , while the primary Raman band is changed to 3636.7 cm −1 . This new Raman mode persists with increasing temperature at atmospheric pressure but disappears at about 3.0 GPa. The decomposition temperature of this Fe-bearing brucite sample is about 130 K lower compared to pure brucite under high oxygen fugacity conditions. These results suggest that the sublattice transition and Fe-Mg substitution may influence the stability of brucite, providing insight into the deep-water cycle at shallow subduction zones.

Sublattice disorder and Fe-Mg substitution in brucite: Implications for the subduction-zone water cycle