Software for the assisted determination of the structural parameters of mixed-layer phyllosilicates
Résumé
The structural determination of a mixed-layer mineral is classically derived from modelling its X-ray diffraction (XRD) patterns. A synthetic X-ray diffraction (XRD) pattern is calculated for a given stacking model and a given set of structural and textural parameters, and is compared with the experimental pattern. The values of the parameters are then modified in order to improve the fit between the experimental and calculated patterns. This time-consuming trial and error method allows a precise determination of the structural parameters. The multispecimen method is the standard used to test the structural result for clays. Specimens are air-dried (AD), solvated with ethylene glycol (EG) or glycerol, and sometimes heated to induce complete dehydration. The EGandADfittingsmust lead to the same stacking parameters. Decreasing the time required to determine the structural parameters, while keeping the accuracy of the trial and error method, is the aim of the software presented here. This tool uses an interactive non-linear least-squares method with a graphical user interface. Educated user interaction is necessary to progress toward a meaningful solution. It is important to emphasize that this software, for at least two reasons, must be considered as a help tool and not as a universal tool which can be used without knowledge of the crystalchemistry of clays. The first is that when used by operators poorly informed about the crystal-chemistry of clay minerals, an automatic procedure may lead to unrealistic structural parameters. The second is that as natural samples are seldom monomineralic, but are generally complex mixtures of both discrete and interstratified phases, extending the use of the software to such mixtures requires care.