Comparative Compressibility of Smectite Group under Anhydrous and Hydrous Environments.

High-pressure synchrotron X-ray powder diffraction studies of smectite group minerals (beidellite, montmorillonite, and nontronite) reveal comparative volumetric changes in the presence of different fluids, as pressure transmitting media (PTM) of silicone oil and distilled water for anhydrous and hydrous environments at room temperature. Using silicone oil PTM, all minerals show gradual contraction of unit-cell volumes and atomistic interplane distances. They, however, show abrupt collapse near 1.0 GPa under distilled water conditions due to hydrostatic to quasi-hydrostatic environmental changes of water PTM around samples concomitant with the transition from liquid to ICE-VI and ICE-VII. The degrees of volume contractions of beidellite, montmorillonite, and nontronite up to ca. 3 GPa are ca. 6.6%, 8.9%, and 7.5% with bulk moduli of ca. 38(1) GPa, 31(2) GPa, and 26(1) GPa under silicone oil pressure, whereas 13(1) GPa, 13(2) GPa, and 17(2) GPa, and 17(1) GPa, 20(1) GPa, and 21(1) GPa under hydrostatic and quasi-hydrostatic environments before and after 1.50 GPa, respectively.