X-ray single-crystal structure refinement of an OH-rich topaz from Sulu UHP terrane (Eastern China) – Structural foundation of the correlation between cell parameters and fluorine content

An OH-rich topaz, Al₂SiO₄F_1.4OH_0.6, occurring in a kyanite quartzite of the Sulu UHP terrane (eastern China) has been optically and chemically (electron microprobe) characterised and its crystal structure refined by single-crystal X-ray diffraction data. Anisotropic refinement converged to R = 0.024 in the Pbnm space group with cell parameters a = 4.6696(8) Å, b = 8.8486(17) Å, c = 8.3915(22) Å. The position of the hydrogen atom has been detected; because the OH → F substitution is less than 50 %, the disorder of H does not lower the overall symmetry. The influence of the OH → F substitution on the lattice parameters is discussed in terms of crystal structure and shown to affect primarily the a and b cell edges; instead c is almost constant. A critical revision of earlier linear correlations of the F content (w_f) vs the lattice parameters and the availability of new data recently published, allowed to obtain improved regression lines for w_fvs a (r² = 0.982) and b (r² = 0.968) cell edges.