NEAR-INFRARED SPECTROSCOPIC DETERMINATION OF WATER SPECIES IN GLASSESOF SYSTEM MALSI(3)O(8) (M=LI, NA, K) - AN INTERLABORATORY STUDY

The near-infrared absorption bands at 5200 cm(-1), assigned to molecul ar water, and at 4500 cm(-1), assigned to hydroxyl groups bonded to ne twork forming cations, were used to specify concentrations of water sp ecies in glasses of alkali feldspar composition MAlSi(3)O(8) (M = Li, Na, K). To allow an accurate quantitative evaluation of the water spec ies 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 isob arically to avoid desorption of water during cooling, Water contents o f 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 attribu ted to the specific measurement conditions (e.g., magnification of the objectives, spectral ranges of the systems, characteristics of the de tectors) applied in the laboratories. However, an unambiguous explanat ion of the differences is not possible due to the complexity of FTIR s pectroscopy. In order to reduce the uncertainty in determination of wa ter species and total water by FTIR spectroscopy a calibration of spec trometers against a reference system should be performed. For evaluati on of linear and integral molar extinction coefficients we have chosen the FTIR spectrometer Bruker(R) IFS88 of Hannover as the reference sy stem. All spectroscopic data were recalculated on the basis of this sp ectrometer. For compositions of the system MAlSi(3)O(8) the extinction coefficients are strongly dependent on the alkali cation and vary non -linearly along the binary joins of the system. The extinction coeffic ients of both the 4500- and the 5200-cm(-1) bands are significantly lo wer for a strong peralkaline glass than for glasses with feldspar comp ositions. This is attributed to strong H bonding of water species to a djacent non-bridging oxygens in the peralkaline glass, Probably, for t he peralkaline composition only a part of the water species contribute to the NIR absorption bands. Variation of species concentration for g lasses of the system MAlSi(3)O(8) with same water content are attribut ed to differences in the fictive temperature of the glass which depend s on cooling rate, water content and anhydrous composition of glass.