The location of H in the high-pressure synthetic Al₂SiO₄ (OH)₂ topaz analogue

The positions of H in Al₂SiO₄(OH)₂, the fully hydrated high-pressure synthetic analogue of topaz have been determined from single-crystal XRD. Two nonequivalent H positions, approximately 2.4 Å apart, were located. The H sites are significantly displaced from the single one found in natural OH-bearing fluor-topaz and violate the mirror plane of space group Pbnm, at least locally. Each H is associated with three O atoms in an irregular trifurcated H-bond arrangement. These results provide an explanation for observations of split OH-stretching bands in infrared spectra, as well as for the uniform dilatation observed for both the ^[6]Al and ^[4]Si polyhedra relative to those in fluor-topaz. -from Authors