THE INFRARED-SPECTROSCOPY OF VESUVIANITE IN THE OH REGION

Many important substitutions in vesuvianite involve variable H, and th ose that do not still perturb the local environment of the OH- anions, Consequently, infrared spectroscopy of the OH fundamental and overton e regions is an important probe of local order. We have examined a ser ies of vesuvianite crystals carefully characterized (by electron-micro probe 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 ba nds (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 d ifferent local cation and anion configurations at nearest-neighbor and next-nearest-neighbor sites, and (3) a low-energy electronic absorpti on band. Boron is incorporated into the vesuvianite structure primaril y via the substitution B + Mg reversible arrow 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. Alth ough lacking the fine detail of the principal-stretching region, the o vertone 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 spec tra: (1) the band width increases significantly with decreasing band-f requency (from similar to 20 cm(-1) at 3670 cm(-1) to similar to 120 c m(-1) at 3060 cm(-1)), and (2) the band intensity is (nonlinearly) cor related with band frequency (in addition to H content). These two feat ures are of significance in quantitatively fitting spectra by numerica l techniques, and in relating band intensities to compositional featur es.