AN INFRARED SPECTROSCOPIC STUDY OF HYDROGEN FELDSPAR (HALSI3O8)

Hydrogen in H-feldspar obtained by ion-exchange was studied in the spe ctral range 1000-5500 cm(-1) by single crystal IR microspectroscopy. S pectra were almost identical for H-feldspars prepared either from sani dine or from adularia. Two bands in the middle-infrared were identifie d 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 p revious work. An additional OH absorption band with a maximum at 2485 cm(-1) was observed for the first time in feldspars. The pleochroism o f 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 s pectroscopy showed that hydrogen is present as hydroxyl groups bound t o 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-b ridging oxygen. The complex structure of the IR spectra implies that t he protons are distributed over a large number of sites in the cation cavity of the feldspars.