Hydrogen defects in feldspars: defect properties and implications for water solubility in feldspar

Abstract Hydrogen defects can strongly affect mechanical and chemical properties of feldspars. To get insight into the behavior such defects, alkali feldspar plagioclase igneous origin were studied combining IR spectroscopy with heating experiments under well-controlled conditions. Near-infrared spectra show that OH groups are predominant hydrous species in these feldspars but presence minor amounts molecular H 2 O cannot be excluded. Short-term annealing at 400–800 °C produces a small significant irreversible change stretching vibration band which is attributed to relaxation structure. Polarized mid-infrared sanidine, adularia, recorded situ temperatures up 600 reversible shifts maxima toward higher wavenumber an overall decrease integrated intensities. The pleochroic features bands, i.e., orientation dipoles along crystallographic axis all additional component perpendicular (010) plane sanidine still present high-temperature spectra. Different during long-term high temperature was found for plagioclases. At 900–1000 °C, Eifel sanidines rapidly lost about one quarter initial water content weakly bound hydrogen remaining very detectable 0.7–0.9 mm thick sections after 108 days 1000 air dried by phosphorus pentoxide. In contrast, 1-mm-thick section completely within 8 °C. After partial dehydration, absorption bands basically preserved except 3050 cm −1 oriented (010), becomes more pronounced than 3400 band, parallel direction. Annealing controlled pressures indicate equilibrium solubilities several tens ppm plagioclases 100 already 1 bar pressure. variation pressure spectroscopic observations indicates determined not only also existing defects. Vacancies on sites ( V A1 ) may accommodate molecules, possibly subsequent hydrolysis network bonds minimize local stress. A likely explanation coupled substitution + Al 3+ Si 4+ (AlOH defect) where protons located interstitial sites. This incorporation model supported complete recovery vapor previous proton/alkali exchange chloride