High-pressure synthesis and properties of OH-rich topaz
High-pressure synthesis and properties of OH-rich topaz
European Journal of Mineralogy (October 1999) 11 (5): 803-813
Members of the (F,OH)-topaz solid solution series, Al (sub 2) SiO (sub 4) F (sub 2-x) (OH) (sub x) were synthesized in high-pressure experiments in the system Al (sub 2) O (sub 3) -SiO (sub 2) -H (sub 2) O-AlF (sub 3) at temperatures between 400 and 900 degrees C and pressures up to 30 kbar. Topaz composition was calculated by the modified unit cell volume regression equation wt.% F = 564.7-1.586 V [Aa (super 3) ], r (super 2) = -0.947. The OH content of topaz increases with decreasing temperature, increasing pressure and increasing H (sub 2) O/F ratio of the starting material, respectively. In experiments at low temperatures and high H (sub 2) O/F ratios X (sub OH) of topaz exceeded 0.5, as determined by X-ray powder diffraction. Topaz with the lowest F content (X (sub OH) = 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 (sub OH) >0.5. For topaz of lower X (sub OH) values, absorption bands related to topaz-OH are not visible in the IR spectra.