IN-SITU INFRARED SPECTRA OF HYDROXYL IN AMPHIBOLES AT HIGH PRESSURE

The infrared spectra of natural C2/m amphibole samples on the tremolite (Ca₂Mg₅Si₈O₂₂(OH)₂) – actinolite (Ca₂(Mg_,Fe)₅Si₈O₂₂(OH)₂) join were collected at room temperature at pressures up to 60 GPa. Amphiboles contain several weight percent water and are probable carriers of water into subduction zones. The dehydration of amphiboles may extend to depths exceeding 300 km, depending on local variations in the subducting slab’s geotherm (Schmidt and Poli, 1998). The effect of this dehydration is significant, as water (OH^-) within the crystalline structure of deep Earth minerals influences melting temperatures, rheology, electrical conductivity, and atomic diffusivity.

Infrared spectra were collected at atmospheric pressure, at regularly increasing pressure intervals, and upon decompression, allowing for the simultaneous collection of spectra centered on the typical OH-stretching region at 3650 cm^-1 and the Si-O stretching modes around 1150 cm^-1as they evolved with pressure. The Si-O and OH-stretching bands in all samples broadened and shifted near-linearly to higher frequency with pressure, and returned to their original positions on decompression. Tremolite had one OH-stretching band while actinolite exhibited two prominent OH-stretching bands. These two OH-stretching bands are interpreted as nonequivalent stretching positions, indicating the accommodation of a secondary OH site through the substitution of Mg with Fe.

Unlike previous studies of naturally occurring C2/m or synthetic P2₁/m amphiboles, no symmetry transformation occurred with increasing pressure, as has been interpreted from the merging or bifurcating of OH-stretching bands with pressure. Our study also utilized two methods of sample preparation, allowing for direct comparison between the quasi-hydrostatic conditions of neon loaded samples versus the non-hydrostatic conditions achieved with a KBr pressure medium. Samples loaded in Ne produced sharper peaks, allowing greater spectral resolution especially at high pressure. Our newer, quasi-hydrostatic loading method may prove valuable to tracing OH-stretching behavior to higher pressures than previously obtainable.