Unique crystal chemistry of two polymorphs of topaz-OH: A multi-nuclear NMR and Raman study

A new polymorph of topaz-OH (denoted as topaz-OH H) was recently discovered at higher P-T conditions than has been known thus far (denoted as topaz-OH I). High-resolution H-1, Si-29, and Al-27 nuclear magnetic resonance (NMR) and micro-Raman spectroscopy are applied to shed light on the crystal chemistry of both polymorphs. Topaz-OH I, synthesized at 7 GPa and 640 degrees C, is stoichiometric (Si/Al = 0.5) with a largely ordered local structure. Higher P-T topaz-OH I synthesized at conditions close to the polymorphic phase transition boundary, on the other hand, shows lower Si/Al ratios (0.44-0.45) and greater local structural disorder [including a small fraction (similar to 3%) of octahedral Si with a unique Si-29 chemical shift near 133 ppm]. The latter may be accounted for by the development of defects (Si/Al in normally vacant octahedral sites and vacancies in the tetrahedral sites) at higher P-T conditions. Topaz-OH II synthesized at 13.5 similar to 14 GPa and 1300 similar to 1400 degrees C similarly exhibits low Si/Al ratios (0.41-0.46). The NMR and Raman spectra for these topaz-OH II are, in general, broader and revealed a substantial fraction (33-37%) of octahedral Si with a range of Si-29 chemical shifts (-130 to -190 ppm), a small fraction (2-3%) of tetrahedral Al, and a range of (and overall shorter) hydrogen-bonding distances than topaz-OH I. Therefore, the phase transition from topaz-OH I to II is characterized by both a significant increase in the occupied octahedral/tetrahedral site ratio as well as disordering of cation distribution, which is unique from the viewpoint of crystal chemistry.