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Abstract
Rietveld method is a powerful tool in obtaining structural information of clay minerals by using of X-ray diffraction (XRD) patterns. However, the interstratified illite-smectites (I-S) show various stacking defects preventing the direct application of this method. It was shown that the Rietveld method combined with a recursive structure-factor calculation can be used for describing the stacking disorder of I-S. In this study, a series of samples with different stacking sequences and different proportions of layer types were chosen to verify the applicability of Rietveld method in determination of structural parameters of I-S. The Rietveld refinements were carried out on powder samples and oriented specimens in air-dry (AD) and ethylene glycol (EG) state. The structural information obtained by X-ray fluorescence (XRF) and thermal analysis were used as an independent test of the reliability of the refinements. The refined and experimental results were compared systematically and the relationship between structural parameter was discussed. For powder and oriented specimens, the refined results of occupancies of potassium and iron and the proportion of illitic layers showed consistent results. The refined value of cis-vacant layers was in good agreement with the experimental data. The reliability of the refinements increased with increasing proportion of illitic layers.
Highlights
Interstratified Illite-smectite (I-S) is one of the most common interstratified clay minerals found in sedimentary rocks
The chemical composition of Ca-saturated samples was calculated from the X-ray fluorescence (XRF) analysis using the method described by Klein [42] based on one formula unit (FU) [O10 (OH)2 ]
The chemical composition of Ca-saturated samples was calculated from the XRF analysis using the method described by Klein [42] based on one formula unit (FU) [O10(OH)2]
Summary
Interstratified Illite-smectite (I-S) is one of the most common interstratified clay minerals found in sedimentary rocks. The stacking sequences and the proportions of different layer types of I-S are important symbols for the diagenetic stage division and an important basis for determination oil formation and oil generation zone in oil and gas exploration and development [1,2,3]. The stacking sequences of I-S can be described by the term “Reichweite (R)” [4]. The Reichweite values R = 1, 2 or 3 mean that the stacking sequences of illitic and smectitic interlayers are periodic and the structure is ordered (for example, R1 means ISIS, R2 means IISIIS, R3 means IIISIIIS) [7]. The crystal structure of I-S or the stacking of illitic and smectitic interlayers can be described by the “McEwan crystallite” [6] or “fundamental particles”.
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