An infrared spectroscopic study of hydrogen feldspar (HA1Si3O8)

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Abstract
Hydrogen in H-feldspar obtained by ion-exchange was studied in the spectral range 1000–5500 cm−1 by single crystal IR microspectroscopy. Spectra were almost identical for H-feldspars prepared either from sanidine or from adularia. Two bands in the middle-infrared were identified by D/H exchange as OH vibration modes. One broad band with a maximum at 3000 cm−1 and shoulders at 2800, 3200 and 3500 cm−1 confirms previous work. An additional OH absorption band with a maximum at 2485 cm−1 was observed for the first time in feldspars. The pleochroism of the OH absorption bands suggests that the H-feldspar is composed of two phases, an amorphous phase and a feldspathic phase. The proportion of the amorphous phase is increased by heating, producing a shift of the maximum of the band at 3000 cm−1 towards higher wavenumber and a decrease of the intensity of the band at 2485 cm−1. Near-infrared spectroscopy showed that hydrogen is present as hydroxyl groups bound to tetrahedral cations in both phases. Molecular water was not detected. The experimental results imply that hydrogen is incorporated in the H-feldspars as protons attached to bridging oxygen as well as to non-bridging oxygen. The complex structure of the IR spectra implies that the protons are distributed over a large number of sites in the cation cavity of the feldspars.