Hydrogen-Disordering Transformation and High-Temperature and High-Pressure Phase Diagram of Brucite: Insights from Raman Spectroscopy and Electrical Conductivity.

The structural and electrical transport properties of brucite [Mg(OH)2] were investigated by virtue of in situ Raman spectroscopy and alternating-current impedance spectroscopy under conditions of 0.5-20.2 GPa, 298-873 K, and different hydrostatic environments using a diamond anvil cell (DAC). Under the non-hydrostatic condition, the emergence of new Raman peaks and discontinuities in Raman shifts, FWHMs, as well as electrical conductivity well disclosed a hydrogen-disordering structural phase transition in brucite from the ordered (P3¯m1 symmetry)-disordered (P3¯ symmetry) structure at 5.7 GPa. Under hydrostatic condition, this transformation occurs at a lower pressure of 3.6 GPa using the 4:1 methanol-ethanol mixture (ME) as the pressure-transmitting medium (PTM), which can be attributed to the influence of deviatoric stress within the sample chamber. The reversibility of this transformation is confirmed by the recovery of Raman peaks and electrical conductivity upon decompression. Furthermore, the high-temperature and high-pressure electrical conductivity results clearly revealed a negative Clapeyron slope for the hydrogen-disordering transformation in brucite, and the corresponding high P-T phase diagram was established for the first time at pressures up to 7.0 GPa and temperatures up to 873 K, which can be expressed as P (GPa) = 8.664 (±1.511) - 0.008 (±0.002) T (K). These results provide direct experimental constraints on the high-pressure phase stability and structural phase transition of brucite and offer an important reference for understanding the behavior of other hydroxide minerals under extreme conditions.