Location and quantitative analysis of OH in coesite

The incorporation of hydrogen (deuterium) into the coesite structure was in vestigated at pressures from 3.1 to 7.5 GPa and temperatures of 700, 800, a nd 1100 degreesC. Hydrogen could only be incorporated into the coesite stru cture at pressures greater 5.0 GPa and 1100 degreesC. No correlation betwee n the concentration of trace elements such as Al and B and the hydrogen con tent was observed based on ion probe analysis (1335 +/- 16 H ppm and 17 +/- I Al ppm at 7.5 GPa, 1100 degreesC. The FTIR spectra show three I relative ly kintense bands at 3575, 3516, and 3459 cm(-1) (v(1) to v(3), respectivel y) and two very weak bands at 3296 and 3210 cm(-1) (v(4) and v(5), respecti vely). The band at 3516 cm-1 is strongly asymmetric and can be resolved int o two bands, 3528 (v(2a)) and 3508 (v(2b)) cm(-1), with nearly identical ar eas. 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 dipol es in the structure and to calibrate the IR spectroscopy for quantitative a nalysis of OH in coesite 190 000 +/- 30 0001 mol(H2O)(-1) cm(-2)). The pola rized spectra revealed a strong pleocliroism of the OH bands. High-pressure FTIR spectra at pressures up to 8 GPa were performed in a diamond-anvil ce ll to gain further insight into incorporation mechanism of OH in coesite. T he peak positions of the v(1), v(2), and v(3) bands decrease linearly with pressure. The mode Gruneisen parameters for v(1), v(2), and v(3) are -0.074 , -0.144 and -0.398., respectively. There is a linear increase of the press ure derivatives with band position which follows the trend proposed by Hofm eister et al. (1999). The full widths at half maximum (FWHM) of the v(1), v (2), and v(3) bands increase from 35, 21, and 28 cm(-1) in the spectra at a mbient 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 c oesite was developed: the OH defects are introduced into the structure by t he substitution Si4+(Si2) + 4O(2-) = (4)square ((Si2)) + 4OH(-), which give s rise to four vibrations. v(1), v(2b). and v(3). Because the OH(D)-bearing samples do contain traces of Al and B, the bands v(4) and v(5) may be coup led to Al and/or B substitution.