Detection of surface hydroxyl species on quartz, γ-alumina, and feldspars using diffuse reflectance infrared spectroscopy

Abstract

Surface hydroxyl groups were detected on silica, γ-alumina, quartz, and feldspar powders using diffuse reflectance Fourier transform infrared (IR)-spectroscopy. Spectra were collected on all samples before and after heating in air to remove physisorbed H 2O. On both natural and synthetic samples of quartz and feldspars, this treatment reveals a single IR-band at 3742–3745 cm −1, which is attributable to terminal >SiOH groups at the mineral surface. Many previous studies have suggested that the speciation of H on feldspar surfaces in water can be modeled based on the speciation of H on γ-alumina and silica surfaces. However, no definitive evidence of >AlOH groups analogous to those found on γ-alumina was detected on the feldspars measured in this study. This strongly suggests that γ-alumina surface hydroxyl groups should not be used as guides to the surface chemistry of feldspars. The IR spectra observed on the feldspars in this study cannot rule out the existence of either >AlOH 2 0 or >AlOHSi< surface groups on feldspars in air, but such groups were not definitively observed in the IR-spectra. Models of surface protonation and dissolution rates in aqueous solution might be improved by inclusion of terms describing either >AlOH 2 0, >AlOH − and >AlO −2, or >AlOHSi< species, rather than >AlOH 0, >AlOH 2 + , or >AlO − groups.