OH/F substitution in topaz studied by Raman spectroscopy

The Raman band related to the stretching mode of hydroxyl (OH-) centered at about 3650 cm(-1) is investigated in several natural specimens of topaz single crystals with a composition Al2SiO4 (OHxF(1-x))(2) within a wide range of OH mole fractions (x). The analysis of the OH band shape shows that its asymmetry is due to an unresolved splitting in two peaks, centered at 3639 and 3647 cm(-1), labeled as OHA and OHB, respectively. Although the asymmetry is drastically different for OH- rich and OH-poor topaz, we show that this is only caused by the change of the OHA/OHB intensity ratio with the total OH concentration. In order to explain the splitting of the OH band, we suggest a model which involves a reduction of the local symmetry from D-2h(16) to C-2v(9), resulting in two types of physically nonequivalent F sites (A and B) where the OH/F substitution can occur. Both sites have C-1 site symmetries and a multiplicity of 4.