Abstract:
The near-infrared absorption bands at 5200 cm−1, assigned to molecular water, and at 4500 cm−1, assigned to hydroxyl groups bonded to network forming cations, were used to specify concentrations of water species in glasses of alkali feldspar composition MAlSi3O8 (M = Li, Na, K). To allow an accurate quantitative evaluation of the water species in hydrous glasses, we have determined the composition dependence of the density and the linear and integral extinction coefficients of the glasses. For each feldspar composition, 8–28 samples with various amounts of water have been synthesized. All samples were quenched isobarically to avoid desorption of water during cooling. Water contents of the glasses were analyzed by Karl-Fischer titration. Spectra of the same sample collected by four different FTIR micro spectrometers vary by up to 10% relative in peak intensities. The differences are attributed to the specific measurement conditions (e.g., magnification of the objectives, spectral ranges of the systems, characteristics of the detectors) applied in the laboratories. However, an unambiguous explanation of the differences is not possible due to the complexity of FTIR spectroscopy. In order to reduce the uncertainty in determination of water species and total water by FTIR spectroscopy a calibration of spectrometers against a reference system should be performed. For evaluation of linear and integral molar extinction coefficients we have chosen the FTIR spectrometer Bruker® IFS88 of Hannover as the reference system. All spectroscopic data were recalculated on the basis of this spectrometer. For compositions of the system MAlSi3O8 the extinction coefficients are strongly dependent on the alkali cation and vary non-linearly along the binary joins of the system. The extinction coefficients of both the 4500- and the 5200-cm− 1 bands are significantly lower for a strong peralkaline glass than for glasses with feldspar compositions. This is attributed to strong H bonding of water species to adjacent non-bridging oxygens in the peralkaline glass. Probably, for the peralkaline composition only a part of the water species contribute to the NIR absorption bands. Variation of species concentration for glasses of the system MAlSi3O8 with same water content are attributed to differences in the fictive temperature of the glass which depends on cooling rate, water content and anhydrous composition of glass.