High-pressure synthesis and properties of OH-rich topaz

Members of the (F,OH)-topaz solid solution series, Al2SiO4F2-x(OH)(x) were synthesized in high-pressure experiments in the system Al2O3-SiO2-H2O-AlF3 at temperatures between 400 and 900 degrees C and pressures up to 30 kbar. Topaz composition was calculated by the modified unit cell volume regressio n equation wt.% F = 563.7 - 1.586 V [Angstrom(3)], r(2) = -0.947. The OH co ntent of topaz increases with decreasing temperature, increasing pressure a nd increasing H2O/F ratio of the starting material, respectively. In experi ments at low temperatures and high H2O/F ratios X-OH Of topaz exceeded 0.5, as determined by X-ray powder diffraction. Topaz with the lowest F content (XOH = 0.82) formed at 15.7 kbar and 400 degrees C. IR measurements reveal the existence of topaz-OH components within the structure of F-containing topaz with X-OH > 0.5 For topaz of lower X-OH values, absorption bands rela ted to topaz-OH are not visible in the IR spectra.