OH group behavior and pressure-induced amorphization of antigorite examined under high pressure and temperature using synchrotron infrared spectroscopy

Infrared (IR) absorption spectra of antigorite were measured up to 27 GPa and 320 degrees 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 (OH (sub outer) ) and inner OH groups (OH (sub inner) ) show positive pressure dependencies. The shift rate of the OH (sub inner) band is almost constant at all pressure ranges. In contrast, that of the OH (sub outer) band increases slightly at about 6 GPa. This discontinuous change of the shift rate is consistent with the anomalous behavior of the OH (sub outer) upon compression, which was predicted in the previous first-principle calculation study. Specifically, the pressure dependence of the OH (sub outer) band shows that the hydrogen ion of an OH (sub outer) interacts not only with the nearest basal oxygen ion of the SiO (sub 4) 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 degrees 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.