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Analysis of H2O in silicate glass using attenuated total reflectance (ATR) micro-FTIR spectroscopy

December 19, 2013

We present a calibration for attenuated total reflectance (ATR) micro-FTIR for analysis of H2O in hydrous glass. A Ge ATR accessory was used to measure evanescent wave absorption by H2O within hydrous rhyolite and other standards. Absorbance at 3450 cm−1 (representing total H2O or H2Ot) and 1630 cm−1 (molecular H2O or H2Om) showed high correlation with measured H2O in the glasses as determined by transmission FTIR spectroscopy and manometry. For rhyolite,

wt%H2O=245(±9)×A3450-0.22(±0.03)

and

wt%H2Om=235(±11)×A1630-0.20(±0.03)

where A3450 and A1630 represent the ATR absorption at the relevant infrared wavelengths. The calibration permits determination of volatiles in singly polished glass samples with spot size down to ~5 μm (for H2O-rich samples) and detection limits of ~0.1 wt% H2O. Basaltic, basaltic andesite and dacitic glasses of known H2O concentrations fall along a density-adjusted calibration, indicating that ATR is relatively insensitive to glass composition, at least for calc-alkaline glasses. The following equation allows quantification of H2O in silicate glasses that range in composition from basalt to rhyolite:

wt%H2O=(ω×A3450/ρ)+b

where ω = 550 ± 21, b = −0.19 ± 0.03, ρ = density, in g/cm3, and A3450 is the ATR absorbance at 3450 cm−1.

The ATR micro-FTIR technique is less sensitive than transmission FTIR, but requires only a singly polished sample for quantitative results, thus minimizing time for sample preparation. Compared with specular reflectance, it is more sensitive and better suited for imaging of H2O variations in heterogeneous samples such as melt inclusions. One drawback is that the technique can damage fragile samples and we therefore recommend mounting of unknowns in epoxy prior to polishing. Our calibration should hold for any Ge ATR crystals with the same incident angle (31°). Use of a different crystal type or geometry would require measurement of several H2O-bearing standards to provide a crystal-specific calibration.

Citation Information

Publication Year 2013
Title Analysis of H<sub>2</sub>O in silicate glass using attenuated total reflectance (ATR) micro-FTIR spectroscopy
DOI 10.2138/am.2013.4466
Authors Jacob B. Lowenstern, Bradley W. Pitcher
Publication Type Article
Publication Subtype Journal Article
Series Title American Mineralogist
Series Number
Index ID 70059177
Record Source USGS Publications Warehouse
USGS Organization Volcano Science Center

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