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The high-pressure behavior of synthetic end-member IIb clinochlore, Mg5Al(Si3Al)O10(OH)8, has been studied by synchrotron X-ray powder diffraction to 27 GPa at 300 K. A non-quenchable, reversible transformation occurs between 9 and 10 GPa that is dominated by compression normal to the structural layering and has an associated small but significant shear of the β angle from 97.2 to 96.3°. The high-pressure chlorite is more compressible than the low-pressure phase. Diffraction patterns of the high-pressure chlorite are very similar from 10 to 27 GPa, indicating that it persists stably with no significant change in β to very high pressures: β is effectively locked at the transformation to the high-pressure structure. It is proposed that the transformation is not polytypic and that the distortion reflects reorganization of the interlayer hydrogen bonding, possibly involving novel proton behavior as adjacent sheets of cations of the brucite-like and tetrahedral layers close down on the sheet of interlayer protons. The transformation is likely driven by O atom close-packing requirements imposed by pressure. [ABSTRACT FROM AUTHOR]

Presents a study that examined the effect of pressure on hydrogen bonding in synthetic end-member clinochlore chlorite minerals using Raman spectroscopy. Methodology; Analysis of the Raman frequencies at ambient conditions; Impact of pressure on the Raman frequencies of the samples.

Presents a study which measured the Raman spectra of phase E in a diamond-anvil cell. Raman active frequencies of phase E at ambient conditions; Pressure dependence of the Raman frequencies of phase E; Discussion on the framework of Raman bands.

Raman spectra in the range 80 to 4000 cm-1 of wadsleyite II (Fo90 with 2.0 wt% H2O and Fo88 with 2.7 wt% H2O) have been measured in a diamond-anvil cell with solid rare-gas pressure-transmitting media to 51.4 GPa at room temperature. The ambient Raman spectrum of wadsleyite II is closely similar to wadsleyite modified with bands in frequency regions where the SiO4 tetrahedral and OH stretching vibrations of hydrous ringwoodite occur. The most intense, characteristic wadsleyite II modes at 709 and 911 cm-1 (Si2O7 and SiO3 symmetric stretching vibrations, respectively) shift continuously to 51.4 GPa showing no evidence for a change in the crystal structure. A striking feature in the high-pressure Raman spectra of wadsleyite II is a significant growth in intensity in the mid-frequency range (300-650 cm-1 at 10-4 GPa and 400-750 cm-1 at 51.4 GPa) under compression accompanied by the appearance of new Raman modes near 40 GPa, perhaps a result of resonance electronic Raman scattering. In the OH stretching frequency range, the Raman spectrum of wadsleyite II exhibits at least six modes and their high-pressure behavior agrees with that of Fo90 hydrous wadsleyite: OH stretching modes above 3530 cm-1 remain approximately constant up to at least 21.8 GPa whereas OH modes at frequencies <3530 cm-1 decrease with increasing pressure. The OH stretching modes are consistent with protonation of the non-silicate oxygen O2 and the O atoms surrounding the partially vacant tetrahedral site Si2, as suggested from X-ray diffraction data. [ABSTRACT FROM AUTHOR]