The infrared spectroscopy of vesuvianite in the OH region

Abstract

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 + A1. 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.