INTERLAYER VACANCY CHARACTERIZATION OF SYNTHETIC PHLOGOPITIC MICAS BY IR SPECTROSCOPY

Abstract

Phlogopitic micas of the solid solution binaries KMg3[AlSi3O10](OH)2 (phlogopite) RbMg3[AlSi3O10](OH)2 (Rb-phlogopite), phlogopite – CsMg3[AlSi3O10](OH)2 (Cs-phlogopite), and phlogopite – BaMg3[Al2Si2O10](OH)2 (kinoshitalite) have been synthesized at temperatures of 700 and 800°C and pressures of 0.2 and 2.0 GPa. The run products have been investigated by optical microscopy, X-ray powder diffraction, electron microprobe, and infrared spectroscopy. All runs yielded between 81 and 100 wt.% of phlogopiticmicas, beside traces of quartz, sanidine, and in one run talc. Celsian and cymrite formed as additional phases in the runs of the (K-Ba)-series. The synthetic phlogopitic micas often consist of mixtures of the three polytypes 1M, 2M1 and 2M2, with 1M being the most abundant polytype. Based on electron microprobe analyses, interlayer vacancy concentrations of up to 0.29 (p.f.u.) were determined, indicating a significant talc component within the synthesized phlogopitic micas. In addition to the known characteristic phlogopite OH-stretching vibrational bands, the infrared spectra of the synthetic micas with incompletely filled interlayer sites exhibit a further OH-band, centered in the spectral range 3674 – 3678 cm-1. The intensity of this band is correlated with the amount of vacancies. The vacancy concentration of phlogopitic micas was determined quantitatively from the intensity of this infrared band by using the intensity of the principal OH-band of synthetic talc (Mg3[Si4O10](OH)2) as a standard. The vacancy concentration of the interlayer site as determined in such a way by infrared spectroscopy corresponds to those independently derived by electron microprobe analyses.