Utilisation du rayonnement synchrotron en diffusion aux petits angles pour l'etude du gonflement des smectites: II. Etude de différents systÈmes eau-smectites en fonction de la température

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.

Section: Discussionmentioning

confidence: 81%

Section: Introductionmentioning

confidence: 99%

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

Lanson

Velde

1992

“…Under stress, the uptake or loss of water by the clay occurs with a concomitant structural reorganization of the material at different scales (Millot 1964;Concaret 1967;Henin 1971;Monnier et al 1973;Pedro 1976;Pons et al 1981Pons et al , 1982Robertson et al 1982;Tessier 1984; Ben Rhaiem 1983; Ben Rhaiem et al 1986;Touret et al 1990). In order to characterize the structural changes of these clays under different stresses, it was necessary to get a continuous picture of the organization levels from the structural ordering of the layer stacks up to the size of the first aggregates (Pons et at.…”

Section: Introductionmentioning

confidence: 99%

Structural Organization of Na- and K-Montmorillonite Suspensions in Response to Osmotic and Thermal Stresses

Faisandier

1998

Abstract--In order to understand the influence of salt concentration and temperature on the behavior and properties of clays used in drilling muds, we studied montmorillonite supensions (4 g clay/100 g solution) in 0.1, 0.5 and 1 M NaC1 and KC1 solutions. A fraction of each sample was heated to 200 ~ in a closed vessel for 7 d, then cooled at room temperature (RT, 25 ~ Small-angle X-ray scattering (SAXS) spectra were recorded, for all the samples, at RT. The structure of the clay particles was determined by comparing the experimental intensity with the theoretical intensity computed from a model that took into account the number of layers per particle, the hydration state of the layers (0, l, 2, 3 or 4 water layers) and the order in the succession of these states. With this set of parameters, we can compute the mean statistical parameters ~/ (mean number of layers per particle), cl (mean interlayer distance) and g2/~ (parameter describing the disorder of the distribution of interlayer distances). The evolution of these parameters shows that: 1) At low concentration (0.1 M NaC1 or KC1), the samples do not consist of particles but of isolated layers (M = 1). The suspensions form gel-like structures. The difference between Na and K suspensions, or between heated and nonheated samples, is unnoticeable at the studied scale (5-500 ,~).2) An increase in salt concentration (from 0.1 to 0.5 M) brings the sample in a granular state: we notice the appearance of particles at 0.5 M (~7/ ~ 25). Differences appear between NaC1 and KC1 suspensions, and the temperature effect becomes visible. Thus, we noticed that in NaC1 suspensions, particles are composed of hydrated layers (1, 2, 3 or 4 water layers) and internal porositYo(d > 30 A), whereas suspensions in KC1 are characterized by the presence of interlayer distances of 10 A, that is, of collapsed layers. Particles in the KC1 suspensions are much thicker than in the NaCI corresponding ones, and also less hydrated at the interlayer level as well as at the internal porosity level. Further increase in salt concentration (0.5 to 1 M) amplifies this effect. As far as temperature is concerned, its effect is to promote the clay dispersion by breaking up the particles, dehydrating and disordering them. This effect is more important for low salt concentration, that is, when the system is less stressed.

“…A more realistic structural model includes the variation from the average structure and the presence of defects that affect their reactivity and growth. The presence of incomplete layers within the stacking sequence (Pevear et al 1991;Tsipursky et al 1992), the coexistence of various hydration states (Ports et al 1981 and1982) or the scatter of elementary layer thicknesses around a mean value (Kodama et al 1971;Srodofi 1980;Sato et al 1992) are some of the defects and strains that can modify XRD line profiles.…”

Section: Discussionmentioning

confidence: 99%

Decomposition of Experimental X-Ray Diffraction Patterns (Profile Fitting): A Convenient Way to Study Clay Minerals

Lanson

1997

212

141

Abstract--This paper thoroughly describes the decomposition procedure, using the example of DE-COMPXR (Lanson 1990). The steps of the decomposition procedure are: 1) preliminary data processing; 2) decomposition; 3) validation of results; and 4) use of the results. The use of decomposition is restricted to the separation of contributions from various phases. The effect of preliminary data processing steps (data smoothing, background stripping) on profile shape is shown to be limited and their implementation is detailed. Potential experimental limitations such as peak symmetry, experimental reproducibility or discrimination are equally minor. A logical decomposition process starts from the definition of the angular range to be fitted, proceeds with the determination of the number of elementary peaks to be fitted and ends with the check for results consistency.Numerical data processing is a powerful tool for the accurate identification of monophases, because of the additional parameters available to constrain XRD profile simulation. Ultimately, however, the match over the whole angular range of both the experimental and the simulated patterns remains the only valid way to characterize the phases present in the sample. Additionally, the decomposition procedure permits both the identification of complex clay mineral assemblages and the characterization of their evolution. This step constrains, and may help to determine, the reaction mechanisms of a transformation; and, as a consequence, to characterize and to model the kinetics of this transformation.