Empirical study of the infrared lattice vibrations (1100–350 cm−1) of phlogopite

A detailed evaluation of the assignments given to the infrared (IR) vibrations in the lattice stretching region is presented here based on observations of the effects of various chemical substitutions in synthetic analogues of phlogopite, KMg₃(AlSi₃)O₁₀(OH)₂. As in previous studies, this study has confirmed that the 995, 960, and 460 cm^?1 vibrations are influenced by Si, the 822 and 760 cm^?1 vibrations by Al, the 915 and 725 cm^?1 vibrations by Al and Si, and the 592 cm^?1 vibration by OH. Contrary to previous studies, it is shown here that the 690, 495, and 375 cm^?1 vibrations are strongly linked with Mg and not just Si. The 655 cm^?1 band in phlogopite is attributed to an in-plane Al-O vibration rather than an Al-O-Si vibration. As a check on the band assignments made here, IR spectra were obtained for synthetic clintonite, CaMg₂Al(Al₃Si)O₁₀(OH)₂, as well as its chemical analogues and compared with the IR spectrum of phlogopite. The band intensities for the Si-O, Al-O, and Si-O-Mg vibrations changed in accord with the composition of clintonite. The most intense band in clintonite at 660 cm^?1 appears to be associated only with Al and is assigned here to a tetrahedral Al-O-Al vibration which must be present, if not dominant, in this mineral. The near coincidence of an in-plane Al-O vibration at 655 cm^?1 (phlogopite) and an in-plane Al-O-Al vibration at 660 cm^?1 (clintonite) makes the identification of tetrahedral Al-Si order-disorder in trioctahedral layered silicates by IR spectroscopy very difficult. The ratio of the 822/995 cm^?1 bands may, however, prove to be very useful for discerning the amount of tetrahedrally coordinated Al in these types of minerals.