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Quantitative analysis of ammonium in biotite using infrared spectroscopy

¹ Laboratoire de Géochimie des Isotopes Stables, CNRS-UMR 7047, IPGP et Université Paris VII, 2 place Jussieu, 75252 Paris Cedex 05, France
² Laboratoire de Géosciences Marines, CNRS-UMR 7097, IPGP et Universités Paris VI et VII, 2 place Jussieu, 75252 Paris Cedex 05, France

Correspondence: ^* E-mail: busigny{at}ipgp.jussieu.fr

The present paper provides a calibration of the Beer-Lambert law allowing the determination of the ammonium (NH₄) content of biotite using infrared (IR) spectroscopy. Single biotite crystals were analyzed by Fourier Transform Infrared spectroscopy. Using a linear correlation between the NH₄ infrared absorption band intensity and the NH₄ content as determined by vacuum techniques, the NH4 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 A¹²⁴⁹, A¹⁴³⁰, and A²³⁹⁵ are absorbances corresponding to wavenumbers 1249 cm^–1 (Si-O vibration peak), 1430 cm^–1 (NH₄ 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 NH₄ partitioning between muscovite and biotite coexisting in the same rocks shows good agreement with results of previous studies and further validates the method.

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