HIGH-PRESSURE INFRARED AND X-RAY POWDER STUDIES OF HYDROGARNET AND TOPAZ-OH

Ca-silicate garnets could be important water-bearing phases in eclogitic regions of the upper mantle. As part of a project to determine the high P-T behavior of hydrogarnets, high-pressure mid-infrared (IR) studies of synthetic Sr-hydrogarnet [Sr₃A1₂(O₄H₄)₃] (Sr₃A) and Ca-hydrogarnet with 50% grossular component [Ca₃A1₂(SiO₄)_1.5(O₄H₄)_1.5] (C₃ASi) were carried out at the U2A beam line at the National Synchrotron Light Source. The objectives of these experiments were to determine (1) if the phase transition in katoite [Ca₃A1₂(O₄H₄)₃] (C₃A) occurs in other hydrogarnets; and (2) if these transitions do occur, how is the pressure of the transition affected by substitutions at the X (dodecahedron) and Z (tetrahedron) sites. It has been proposed that H-H repulsion may initiate the phase transition in C₃A at 5 GPa. Previous experiments at the U2A beam line have shown that IR spectroscopy is very sensitive to the transition. IR spectra collected for Sr₃A clearly show a discontinuity in vibrational frequency (3662 cm^-1) at ~2.5 GPa. Both OH frequencies in C₃ASi also show a significant change in slope (from positive to negative) at ~6 GPa. However, based on these results, it is not possible to determine if H-H repulsion is the driving force of the transitions. The instability of the hydrogarnet structure could also be related to a size misfit between the X-site cation and the dodecahedral cavity, i.e., the H atoms play a passive role in the transition. Topazes [Al₂F_2-x(OH)_x(SiO₄)] found in ultrahigh-pressure metamorphic terrains are relatively OH-rich, with X_OH exceeding 0.5 in some cases. To learn more about the behavior of hydrogen in these materials at mantle pressures, high-pressure mid-IR and powder X-ray data were collected for topaz-OH (X = 2) to ~10 GPa at the U2A and X17C beam lines, respectively, using Ar (IR) and methanol:ethanol (X-ray) as pressure-transmitting media. The bulk modulus determined from a Birch-Murnaghan fit to energy-dispersive X-ray data is K₀ = 144.4 (4) GPa with K' = 4. Both OH frequencies decrease slightly (20-40 cm^-1) with pressure but at significantly different rates, consistent with results from a previous Raman study of the same phase. The hydrogen-bond geometry at high pressure in topazes may be controlled primarily by the compressional behavior of the surrounding cavity.