THE LOCATION OF H IN THE HIGH-PRESSURE SYNTHETIC AL2SIO4(OH)2 TOPAZ ANALOG

The positions of H in Al2SiO4(OH)2, the fully hydrated high-pressure s ynthetic analogue of topaz [Al2SiO4(F,OH)2], have been determined from single-crystal X-ray diffraction. Two nonequivalent H positions, appr oximately 2.4 angstrom apart, were located. The H sites are significan tly displaced from the single one found in natural OH-bearing fluor-to paz and violate the mirror plane of space group Pbnm, at least locally . Although not resolved using X-ray data, the possibility of long-rang e order and the reduction of symmetry to Phn2(1) is supported by a mea surement of the second harmonic generation (SHG), which gave a positiv e value of 0.6 times that of quartz. Each H is associated with three O atoms in an irregular trifurcated H-bond arrangement. These results p rovide 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. T he structure of a natural fluor-topaz crystal was analyzed for compari son.