INFRARED SPECTRA OF INTERSTRATIFIED ILLITE/SMECTITE FROM HYDROTHERMALLY ALTERED TUFFS (SHINZAN, JAPAN) AND DIAGENETIC BENTONITES (KINNEKULLE, SWEDEN)

Abstract

Two series of interstratified illite/smectite (I/S) minerals from the Shinzan, Japan, hydrothermal alteration products (100-0% of smectite layers) and the Kinnekulle, Sweden, diagenetic bentonite beds (60-30% of smectite layers) have been examined by infrared (IR) spectroscopy in the 4000-250 cm^-1 frequency range as functions of percentage of smectite layers (%S), tetrahedral Al, and octahedral Fe+Mg contents. The band shift and intensity variation in the IR spectra of the I/S series did not show an additive nature as if interstratified minerals were a simple mixture of each component layer, but showed characteristic features at each %S and for different origins in some specific bands. The Shinzan I/S series can be classified into three groups taking into account the band shift and intensity variation of IR spectra and the other mineralogical properties: one I/S with 100-50%S, the second I/S with 50-30%S, and the third I/S with 20-0%S. Such a classification may be responsible for that I/S has different basic structures of silicate layers at each %S stage. The basic structure of silicate layer in the first group of I/S is fundamentally that of smectite, the third group is that of pure illite, and the second group is an intermediate. The Kinnekulle I/S did not follow the above classification. This may be due to differences in the basic structures of silicate layers and the chemical composition between the two I/S series. The Kinnekulle I/S is more phengitic and poorly ordered than the Shinzan I/S even at a given %S.