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

Abstract

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.

Experimental Determinations of the Coherent Scattering Domain Size Distribution of Natural Mica-Like Phases with the Warren-Averbach Technique

“…The Paris Basin samples (Figure 1) have been characterized thoroughly by image analysis of transmission electron micrographs, X-ray fluorescence chemical analysis (Lanson and Champion, 1991) and decomposition of XRD profiles (Lanson and Velde, 1992). The simultaneous occurrence of three "illitic" phases (i.e., highly illitic illite/smectite mixed-layer or I/S, poorly crystallized illite and well crystallized illite) has been shown on the various diffraction peaks (001,002, 003 and 005 bands) of the 2-50~ (44 to 1.8/~) range (Lanson and Velde, 1992).…”

“…The simultaneous occurrence of three "illitic" phases (i.e., highly illitic illite/smectite mixed-layer or I/S, poorly crystallized illite and well crystallized illite) has been shown on the various diffraction peaks (001,002, 003 and 005 bands) of the 2-50~ (44 to 1.8/~) range (Lanson and Velde, 1992). Because of the relative intensity (Table 1) of the well crystallized (CSDS > 20 layers) illite peak, one could expect the presence of some high CSDS values (above 20 nm) in the distribution determined with the WA technique.…”

“…These samples are Stephanian to Permian in age, and have been extensively described by Lanson and Champion (1991 ) and Lanson and Velde (1992). An additional ten samples came from the Upper Cretaceous Niger Delta mudstones described by Velde et al (1986).…”

“…All these samples contain mostly micaceous clays. However, some samples (e.g., from Paris basin) contain a slightly expandable (<10% smectite) illite/smectite mixed-layer coexisting with purely illitic clays (Lanson and Velde, 1992). The different size fractions (usually the <2.0 #m fraction) were obtained by centrifugation or sedimentation, and oriented slides were prepared according to the procedures described by the various authors listed above.…”

Influência da pressão e temperatura na cristalinidade da illita em sequências Proterozoicas: Norte do Distrito Federal e Goiás, Brasil

“…Os algoritmos usados no programa e suas limitações estão descritas nos trabalhos de Lanson & Champion (1991), Lanson & Besson (1992), Lanson & Velde (1992) e Lanson (1997), inclusive para estabelecer os valores do KI quando relacionado à influência tectônica (Lanson et al 1998).…”