Quantification of dissolved H2O in silicate glasses using confocal microRaman spectroscopy

Application of confocal microRaman spectroscopy for quantification of total water content and water speciation was tested for hydrous glasses of various compositions (haplogranite, albite, tonalite, peralkaline rhyolite, dacite, andesite, basalt). Glasses with water contents between 0.5 and 11 wt.% were synthesized in internally heated gas pressure vessels. Total water contents of the glasses were measured by Karl–Fischer titration and IR spectroscopy. To quantify the total water content (CH2Ot) by Raman spectroscopy either the integrated intensity of the OH stretching vibration band at 3550 cm− 1 (A⁎3550) was used directly or A⁎3550 was scaled to low wavenumber Raman bands. A very high reproducibility of A⁎3550 could be achieved with a stand alone Raman microscope LabRam 010. A single regression line reproduces the data of all samples with a 2σ variation of 1.2 wt.%. However, slightly different trends are observed for polymerized compositions (haplogranite, albite, tonalite), intermediate compositions (dacite, andesite) and depolymerized compositions (basalt) in plots of A⁎3550 vs. water content, implying that the Raman cross section of dissolved hydrous species depends on glass composition.In measurements with a Raman spectrometer T64000, A⁎3550 depends largely on measurement conditions. A good reproducibility could be achieved only by normalizing the OH band intensity with low wavenumber bands. Such an approach has the advantage to eliminate at least partially effects of specific measurement conditions and, hence, opening the possibility to apply the Raman calibration to another spectrometer. For silicic glasses scaling to the area of T–O–T bending band near 500 cm− 1 (A⁎500) yields the best results. Data for albite, haplogranite and dacite (11 samples, 37 spectra) covering a range of 0.8–7.3 wt.% H2O are reproduced with a 2σ standard deviation of 0.16 wt.% H2O byCH2Ot(wt.%)=0.28+5.135·A3550⁎A500⁎For intermediate and depolymerized compositions the T–O stretching band near 1000 cm− 1 is more intensive than the T–O–T bending band near 500 cm− 1 and, hence, it is more convenient for calibration. Data for andesite and basalt (14 samples, 22 spectra) covering a range of 0.5 to 4.7 wt.% H2O are reproduced with a 2σ stand deviation of 0.18 wt.% H2O byCH2Ot(wt.%)=3.66·A3550⁎A1000⁎Attempts to determine the relative abundance of OH groups and H2O molecules in the glasses by decomposing the OH stretching Raman band into 3–5 Gaussians are not consistent with water speciation data derived from near-infrared spectroscopy. We suggest that the intensity growth with water content in the low wavenumber region of the 3550 cm− 1 band reflects only enhanced hydrogen bonding but is not related to the relative abundance of water species.