High-pressure synthesis and properties of OH-rich topaz

Members of the (F,OH)-topaz solid solution series, Al 2 SiO 4 F (sub 2-x) (OH) x were synthesized in high-pressure experiments in the system Al 2 O 3 -SiO 2 -H 2 O-AlF 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 3 ], r 2 = -0.947. The OH content of topaz increases with decreasing temperature, increasing pressure and increasing H 2 O/F ratio of the starting material, respectively. In experiments at low temperatures and high H 2 O/F ratios X OH of topaz exceeded 0.5, as determined by X-ray powder diffraction. Topaz with the lowest F content (X 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 OH >0.5. For topaz of lower X OH values, absorption bands related to topaz-OH are not visible in the IR spectra.