HIGH PRESSURE STUDIES OF HYDROUS MINERALS

Combined scattering and spectroscopic investigations reveal differences in the behavior of hydrous minerals which depend on the interplay between H-bond formation, H...H repulsion and the relative compressibility of the hydrated framework. High pressure neutron powder diffraction (NPD) investigations of minerals related to kalicinite (KHCO₃) and brucite (Mg(OH)₂), where H-bonds are respectively between molecules and rigid layers, reveal a critical H...H separation of between 1.8 and 2.0 Å. Below this distance disorder of hydrogen over partially occupied sites occurs and phase transitions may be induced. These structure types are also marked by a distinct difference in behavior for materials containing transition elements. For dense hydrous materials produced at high pressure and quenched to room pressure conditions, the mode of compressibility is dominated by the requirements of the close packed framework. In hexagonal (P6₃) Phase A, Mg₇Si₂O₈(OH)₆, NPD studies reveal pressure drives the packing to ideal hcp. Despite the domination by the framework, the weaker of the two unique H-bonding interactions is the more compressible and the crystal chemistry of the system is consistent with studies of other humite-related minerals. Diffraction studies give a bulk modulus, 106.2 (1.6) GPa for phase A, quite different from that reported in some of the literature.