Infrared (IR) absorption spectra of antigorite were measured up to 27 GPa and 320 °C using synchrotron IR radiation to elucidate OH group behavior under high-pressure (HP) and high-temperature (HT) conditions. The absorption bands attributable to the OH stretching modes of outer OH groups (OHouter) and inner OH groups (OHinner) show positive pressure dependencies. The shift rate of the OHinner band is almost constant at all pressure ranges. In contrast, that of the OHouter band increases slightly at about 6 GPa. This discontinuous change of the shift rate is consistent with the anomalous behavior of the OHouter upon compression, which was predicted in the previous first-principle calculation study. Specifically, the pressure dependence of the OHouter band shows that the hydrogen ion of an OHouter interacts not only with the nearest basal oxygen ion of the SiO4 tetrahedron but also with the second nearest two basal oxygen ions upon compression. The latter interaction becomes dominant over the former interaction at about 6 GPa.
Pressure-induced amorphization was indicated from IR spectra measured at 300 °C and 25.6 GPa. This P-T condition is out of the thermodynamic stability field of antigorite. A broad absorption band, which is close to the broad band attributable to natural hydrous silicate glass, appeared after amorphization, which suggests that the pressure-induced amorphization of antigorite does not induce dehydration. Hydrogen atoms are retained in amorphized antigorite as OH groups.