CRYSTALLOCHEMICAL PROPERTIES VARIATION DURING A SMECTITE-ILLITE TRANSITION

The characterization of clays in mudrocks is based on well known controlled techniques. The most common method is the X-ray diffraction (XRD) that allows identification and quantification of mineral phases present in the samples. Difficulties arise when mixed layered minerals (MLM) are presents because the results are more difficult to interpret. Interpretation of XRD is performed by simulating the diffraction patterns using numerical models constrained by properties measured or estimated. Some properties that depend on the instrumentation and method of preparation can be obtained indirectly and are not the main source of uncertainty. The transition from smectite to illite is often encountered when studies attempt to understand the changes associated with burial diagenesis of sediments. The latest published data describe the reactions of minerals by considering macroscopic measurements obtained from a large amount of sample and characterized by XRD. The chemical composition and crystalline properties, the main source of uncertainty, used in numerical modeling are often derived from assumptions that make the results equivocal. In this study, we investigated the transformation of smectite to illite using two types of measurements. First, the samples were characterized using a conventional method of interpretation of diffraction patterns. Secondly, the samples were observed using electron diffraction (SAED) and energy dispersive spectrometry (EDS) using a transmission electron microscope. To simplify the interpretations and obtain the most accurate results, samples have been altered during laboratory experiments. The initial smectite were synthesized to obtain a higher chemical homogeneity. The comparison of results made at the scale of clay particles (average chemical compositions and electronic diffraction patterns) with the results obtained from whole rocks (MLM identification) allow us to propose a reaction path for the transition from smectite to illite.