Distinguishing Features and Identification Criteria for K-Dioctahedral 1M Micas (Illite-Aluminoceladonite and Illite-Glauconite-Celadonite Series) from Middle-Infrared Spectroscopy Data

A representative collection of K-dioctahedral 1M micas ranging in composition from (Mg, Fe)-poor illites to aluminoceladonites through Mg-rich illites (Fe-poor varieties) and from Fe-bearing, Mg-rich illites to celadonites through Fe-illites, Al-glauconites and glauconites (Fe-bearing varieties) was studied by Fourier-transform infrared (FTIR) spectroscopy in the middle-infrared region. Analysis and comparison of the relationships between the band positions and cation compositions of Fe-poor and Fe-bearing K-dioctahedral micas provided a generalized set of FTIR identification criteria that include the band positions and profiles in the regions of Si–O bending, Si–O stretching, and OH-stretching vibrations. FTIR data allow unambiguous identification of illites, aluminoceladonites, and celadonites, as well as distinction between Fe-illites and illites proper, as well as between Al-glauconites and glauconites. Specifically, a sharp maximum from the AlOHMg stretching vibration at ~3600 cm⁻¹, the presence of a MgOHMg stretching vibration at 3583–3585 cm⁻¹, as well as characteristic band positions in the Si–O bending (435–439, 468–472 and 509–520 cm⁻¹) and stretching regions (985–1012 and 1090–1112 cm⁻¹) are clearly indicative of aluminoceladonite. The distinction between Fe-illites and Al-glauconites, which have similar FTIR features, requires data on cation composition and unit-cell parameters.