High-pressure synthesis and properties of OH-rich topaz
Wunder, Bernd; Andrut, Michael; Wirth, Richard
European Journal of Mineralogy Volume 11 Number 5 (1999), p. 803 - 814
published: Sep 30, 1999
manuscript accepted: Mar 24, 1999
manuscript received: Nov 17, 1998
ArtNo. ESP147051105002, Price: 29.00 €
Abstract Members of the (F,OH)-topaz solid solution series, Al2SiO4F2-x(OH)x were synthesized in highpressure experiments in the system Al2O3-SiO2-H2O-AlF3 at temperatures between 400 and 900 °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 [A3], r2 = -0.947. The OH content of topaz increases with decreasing temperature, increasing pressure and increasing H2O/F ratio of the starting material, respectively. In experiments at low temperatures and high H2O/F ratios XOH 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°C. IR measurements reveal the existence of topaz-OH components within the structure of F-containing topaz with XOH › 0.5. For topaz of lower XOH values, absorption bands related to topaz-OH are not visible in the IR spectra.