Decomposition of X-Ray Diffraction Patterns: A Convenient Way to Describe Complex I/S Diagenetic Evolution

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Abstract
--Decomposition of complex,X-ray diffraction profiles is used,on well characterized,(image analysis of transmission electron micrographs, X-ray fluorescence chemical analyses) diagenetic samples from,the Paris basin. The simultaneous,occurrence,of three "illitic" phases,(mixed-layer illite/smectite or I/S, poorly crystallized illite, and mica-like phase) is shown on the various diffraction peaks of the 2- 50 ~ CuKa (44-1.8 ]k) range. However, because of theoretical and experimental constraints, it is easier to perform the decomposition routine in the 5-11 ~ CuKa (17.6-8.0 ~) range. The identification (i.e., illite content and mean,coherent,scattering domain,size) of the various phases is performed,by comparing the associated elementary peak characteristics (position, full width at half maximum intensity) with simulated X-ray patterns. When available, the characteristics obtained from the various angular regions are mutually consistent; however, the precise structures of smectite and illite end-members, on the one hand, and the structure of I/S crystallites, on the other hand, are not well known. Consequently, on some angular regions, there is a discrepancy between the characteristics obtained on experimental and calculated X-ray profiles. The definition of more realistic simulation hypotheses for I/S minerals, and for other interstratified clay minerals, would make this powerful and reliable tool to describe X-ray patterns a precise and sensitive identification tool even for complex,clay parageneses. Key Words--Decomposition, Diagenesis, Illite, Illite/smectite, Mixed-layering, Simulation, X-ray powder diffraction.