Raman investigation of hydrostatic and nonhydrostatic compressions of OH- and F-apophyllites up to 8 GPa

Layer silicates F- and OH-apophyllites, KCa4Si8O20(F, OH)center dot 8H2O, have been investigated by Raman spectroscopy at hydrostatic and nonhydrostatic pressures up to similar to 8GPa in diamond anvil cells using a 4:1 methanolethanol mix as pressure-transmitting medium. Our experiments show that at hydrostatic compression, apophyllites retain their crystalline states (i.e. no amorphization) up to 5?GPa. The wavenumbers of most bands exhibit linear dependences on pressure, except for a few ones, e.g. at 162 and 3565cm1 in OH-form (160.5 and 3558?cm1 in F-form) that show nonlinear dependences. Nonhydrostatic compression with additional uniaxial loading induces amorphization of apophyllites. The majority of the bands in OH-apophyllite decreases markedly in intensity and shows considerable broadening under nonhydrostatic compression up to 36GPa. In addition, the wavenumbers of several bands at nonhydrostatic compression exhibit considerable nonlinear dependences on pressure with strong hysteresis. These bands are mainly associated with vibrations of the interlayer ions and molecules and also of stretching and bending silicate sheets, hence being highly sensitive to the interlayer distance. Finally, we have calculated the lattice dynamics of F-apophyllite and interpreted the majority of bands, and these data are used to explain the complex baric behavior of the bands. Copyright (C) 2011 John Wiley & Sons, Ltd.