Quantitative analysis of ammonium in biotite using infrared spectroscopy

The present paper provides a calibration of the Beer-Lambert law allowing the determination of the ammonium (NH 4 ) content of biotite using infrared (IR) spectroscopy. Single biotite crystals were analyzed by Fourier Transform Infrared spectroscopy. Using a linear correlation between the NH 4 infrared absorption band intensity and the NH 4 content as determined by vacuum techniques, the NH 4 molar absorption coefficient at 1430 cm -1 was found to be 441 ± 31 L/mol·cm. After having calibrated the biotite thickness to Si-O absorption band, the NH4 content of biotite can be calculated directly from its IR spectrum by the relation: where A 1249 , A 1430 , and A 2395 are absorbances corresponding to wavenumbers 1249 cm -1 (Si-O vibration peak), 1430 cm -1 (NH 4 bending), and 2395 cm -1 (spectrum baseline), respectively. The analysis of biotites having different chemical compositions suggests that, to a first approximation, the calibration is independent of biotite chemical composition. An infrared determination of NH 4 partitioning between muscovite and biotite coexisting in the same rocks shows good agreement with results of previous studies and further validates the method.