LOCATION AND QUANTITATIVE ANALYSIS OF OH IN COESITE

Abstract

The incorporation of hydrogen (deuterium) into the coesite structure was investigated at pressures from 3.1 to 7.5 GPa and temperatures of 700, 800, and 1100 °C. Hydrogen could only be incorporated into the coesite structure at pressures greater 5.0 GPa and 1100 °C . No correlation between the concentration of trace elements such as Al and B and the hydrogen content was observed based on ion probe analysis (1335 ± 16 H ppm and 17 ± 1 Al ppm at 7.5 GPa, 1100 °C). The FTIR spectra show three relatively intense bands at 3575, 3516, and 3459 cm-1font face=symbol>n1 ν3espectively) and two very weak bands at 3296 and 3210 cm-1font face=symbol>n4d ν5espectively). The band at 3516 cm-1 strongly asymmetric and can be resolved into two bands, 3528 (ν2and 3508 (ν2bm-1ith nearly identical areas. Polarized infrared absorption spectra of coesite single-crystal slabs, cut parallel to (0 1 0) and (1 0 0), were collected to locate the OH dipoles in the structure and to calibrate the IR spectroscopy for quantitative analysis of OH in coesite (εi5>,tot0 000 ± 30 000 l mol-1F>H2 cm-2).The polarized spectra revealed a strong pleochroism of the OH bands. High-pressure FTIR spectra at pressures up to 8 GPa were performed in a diamond-anvil cell to gain further insight into incorporation mechanism of OH in coesite. The peak positions of the ν1font face=symbol>n2nd ν3nds decrease linearly with pressure. The mode Gruneisen parameters for ν1font face=symbol>n2nd ν3e -0.074, -0.144 and -0.398, respectively. There is a linear increase of the pressure derivatives with band position which follows the trend proposed by Hofmeister et al. (1999). The full widths at half maximum (FWHM) of the ν1font face=symbol>n2nd ν3nds increase from 35, 21, and 28 cm-1 the spectra at ambient conditions to 71, 68, and 105 in the 8 GPa spectra, respectively. On the basis of these results, a model for the incorporation of hydrogen in coesite was developed: the OH defects are introduced into the structure by the substitution Si4+(Si2)2-[4]#(Si2)4OH-hich gives rise to four vibrations, ν1font face=symbol>n2afont face=symbol>n2bnd ν3ecause the OH(D)-bearing samples do contain traces of Al and B, the bands ν4d ν5y be coupled to Al and/or B substitution.