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.