Near-Infrared Spectroscopic Study of Biotite–Phlogopite (Mg# = 30~99): OH-Stretching Modes and Mg# Content Prediction Equation

Biotite–phlogopite minerals are a complete Mg–Fe isomorphism series of phyllosilicates. A Fourier transform infrared spectroscopy (FTIR) and electron microprobe analysis (EMPA) were conducted on end-member phlogopite, Mg–biotite, and annite samples. In the mid-infrared region, absorption peaks were observed at 460, 1000, 3680, and 3710 cm−1 in the biotite group. Samples with higher Mg content exhibited stronger peaks assigned to OH vibrations and a weak absorption peak at 810 cm−1. In the near-infrared region, combination peaks were observed near 4200, 4300, and 4450 cm−1, with wavenumbers showing a linear inverse relationship with the Mg# [=100 × Mg/(Mg + FeT)] value. For annite, combination peaks occurred at 4173, 4292, and 4439 cm−1, decreasing by 10–15 cm−1 compared to end-member phlogopite. Judging the fundamental peaks of the combination band contributes to identifying suitable near-infrared characteristic peaks for quantitative research. The 4300 cm−1 absorption peak in biotite–phlogopite was assigned to OH-bending and -stretching vibrations, making it suitable for mineral identification and Mg# estimations across all biotite groups. The 4450 cm−1 characteristic peak, assigned to Al–O-stretching vibrations and OH-stretching vibrations, is suitable for accurately predicting Mg# values in high AlVI samples. The first overtones of biotite–phlogopite appeared at 7250 cm−1, with an average factor of 1.955 between the fundamental and corresponding overtones. This study aims to refine the patterns of OH-stretching vibrations and characteristic peak shifts in the near-infrared spectral region of phyllosilicate minerals, providing data references for planetary spectroscopy and ore deposit studies.