The Infrared Spectroscopy of Vesuvianite in the OH Region

Many important substitutions in vesuvianite involve variable H, and those that do not still perturb the local environment of the OH^- anions. Consequently, infrared spectroscopy of the OH fundamental and overtone regions is an important probe of local order. We have examined a series of vesuvianite crystals carefully characterized (by electron-microprobe analysis, wet-chemical analysis and crystal-structure refinement) by polarized single-crystal infrared spectroscopy. The crystals span the complete range of chemical variation reported in vesuvianite, and the spectra show tremendous variability. There are 13 recognizable bands (A-M) that can be divided into three types: (1) eight bands due to absorptions at the OH site; these result from different local cation and anion configurations at nearest-neighbor and next-nearest-neighbor sites; (2) four bands due to absorption at O(10); these result from different local cation and anion configurations at nearest-neighbor and next-nearest-neighbor sites, and (3) a low-energy electronic absorption band. Boron is incorporated into the vesuvianite structure primarily via the substitution B + Mg = 2H + Al. In boron-rich vesuvianite, the four bands J-M are not present, indicating that H has been completely replaced by B in the vicinity of the O(10) site. Although lacking the fine detail of the principal-stretching region , the overtone spectra are equally characteristic of this B=H substitution. The spectra in the principal OH-stretching region extend over a very wide spectral range (3700-3000 cm^-1) and show two features that are of general importance in the quantitative interpretation of such spectra: (1) the band width increases significantly with decreasing band-frequency (from ~20 cm^-1 at 3670 cm^-1 to ~120 cm^-1 at 3060 cm^-1), and (2) the band intensity is (nonlinearly) correlated with band frequency (in addition to H content). These two features are of significance in quantitatively fitting spectra by numerical techniques, and in relating band intensities to compositional features.