Conversion reactions from dioctahedral smectite to trioctahedral chlorite and their structural simulations

Abstract The clay minerals are good indicators of rock history because their wide transformation variations are sensitive to the formation conditions, like temperature (T), pressure (P) and fluid composition. Accordingly, many geothermometers and fluid-rock interaction indicators based on mineral assemblages, illite polytypism and composition of chlorite have been proposed during the last 30 years. In this study, dioctahedral smectite dominated mudstone was tested hydrothermally in the 250–550 °C temperature range and 5–280 MPa pressure range, with reactions lasting 24 h, with a liquid/solid ratio of 1:1, and in saturated KCl solution. The mineralogical and chemical evolution of the clay minerals was characterized by using X-ray powder diffraction (XRD), X-ray fluorescence spectroscopy (XRF), field-emission scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and high resolution transmission electron microscopy (HRTEM). Molecular simulation was performed to determine the structure and behavior of interlayer cations and interlayer water changes of Na and Ca-rich smectite. We then obtained the transformation series as follows: from dioctahedral smectite to dioctahedral randomly interstratified illite/smectite to dioctahedral ordered illite/smectite to coexistence of dioctahedral and trioctahedral ordered illite/smectite and finally to trioctahedral chlorite and illite/smectite. This work illustrated the dioctahedral smectite alteration and transformation process and showed a continuous octahedral sheet transformation from dioctahedral smectite to trioctahedral chlorite. The series of clay minerals transformation provided an important experimental basis for understanding the geochemical conditions to which clay minerals were exposed to in the geological environment and will offer basic parameters for establishing a quantitative link between composition and formation conditions.