Experimental study of the hydrothermal formation of smectite in the beidellite-saponite series

Abstract

Kaolinite and dioctahedral and trioctahedral smectite were hydrothermally synthesized using silicate gels with variable Al/Fe/Mg ratios (0.9/0.05/0.05 to 0.05/0.05/0.9) at temperatures of 175 degrees , 200 degrees , and 225 degrees C and time periods of 192, 360, 720, and 1440 hrs. The solids were studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential thermal analysis and thermogravimetry (DTA-TG), scanning electron microscopy coupled with an energy dispersive spectrometer (SEM-EDS), and transmission and analytical electron microscopy (TEM-AEM). Kaolinite and smectite were the only mineral phases found in the products. EDS and AEM analyses of individual smectite particles show that many of them have compositions intermediate between dioctahedral and trioctahedral smectite. Nevertheless, XRD and FTIR results show that the two structures are segregated from each other. We interpreted those particles of intermediate composition to be due to topotactic growth of dioctahedral smectite on trioctahedral smectite lamellae. Kaolinite and trioctahedral smectite seem to grow readily from the starting amorphous materials with compositions moderately similar to these minerals. In some of the experiments, kaolinite started to dissolve after approx 15 days, seemingly favoring the formation of dioctahedral smectite. Trioctahedral smectite formation seems to have been enhanced by the presence of brucite domains in the original gel. Dioctahedral smectite always coexisted with kaolinite (Al-rich gels) or trioctahedral smectite (intermediate Al/Mg gels), seemingly needing an appropriate substrate to nucleate (kaolinite or trioctahedral smectite in this study) and/or a specific gel composition to grow at a sufficient rate. The formation rates of the three phases were modeled assuming that kaolinite formed from the gel and that dioctahedral and trioctahedral smectite formation occurred through an intermediate phase.