X-RAY-DIFFRACTION IDENTIFICATION OF CLAY- MINERALS - IMPROVEMENTS INDUCED BY NUMERICAL DATA-PROCESSING

Clay mineral identification using X-ray diffraction is impaired by the small size of their coherent scattering domains and by the abundance of both crystal defects, and mixed-layer minerals. Due to such charact eristics, clay minerals usually exhibit very broad irrational 00l diff raction lines, with a slow variation of their first derivative. Conseq uently, mixtures of clay minerals with closely related crystallochemic al characteristics show complex diffraction bands combining several po orly individualized maxima. This effect is increased by the variabilit y of both clay mineral chemical composition and basal distance. As a r esult, comparison oi experimental diffraction peak positions with refe rence tables does not permit precise identification oi these minerals. The only satisfying procedure for clay mineral identification is to l it the experimental profile with a theoretical X-ray diffraction patte rn, calculated from a structural model. Owing to the high number oi ad justable parameters, even for a pure phase, this method is very time-c onsuming and cannot be applied routinely to a great number of natural samples. The recent development of computerized diffractometers combin ed with the increased availability of computers has enabled the develo pment of numerical methods for both experimental data processing and e xpert systems. These programs have been especially developed or modifi ed to make clay mineral identification easier, and in particular to pe rmit an accurate description of their crystallochemical structure, eve n for complex clay parageneses.