Mineralogy, Geochemistry, and Genesis of Kaolinitic Claystone Deposits in the Datong Coalfield, Northern China
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Abstract The use of field emission electron microscopy for the study of clay mineral petrography in mudrocks and sandstones is discussed. The methodology including sample preparation is outlined and three examples of the application of the technique are described: the formation of authigenic illite in mudrocks, the multiple generation of authigenic illites in sandstones and the effects of KCl drilling muds on shale fabrics. In the study of authigenic illite formation in Palaeocene mudrocks from the North Sea, the FESEM analyses have demonstrated the formation of illites with increasing burial depth that conventional SEM and XRD analyses had failed to show. The FESEM analyses of the authigenic illites in Carboniferous sandstones from the southern North Sea revealed at least three different habits representing different generation episodes rather than one illite formation event.This has important repercussions with regard to the interpretation of stable isotope and dating data for the illites. Significant petrographic changes in shales after treatment with KCl drilling muds have been observed from FESEM analyses, suggesting reactivity between the shales and the KCl muds.
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To understand clay mineral transformations in hydromorphic conditions in the red earth sediments in Xuancheng, south China, clay mineralogy was investigated using X‑ray diffraction (XRD) and highresolution transmission electron microscopy (HRTEM). The XRD results indicated that clay minerals in the hydromorphic soils were illite, kaolinite, smectite, vermiculite, and mixed-layer illite/smectite and illite/smectite/kaolinite. Changes of the kaolinitic reflections under the various conditions suggested that the kaolinitic phase is a mixed-layer structure having kaolinite layers randomly interstratified with illite and smectite layers. HRTEM observation showed that 10 Å illite layers interstratified with both 15 Å smectite layers and 7 Å kaolinite layers in clay particles, confirming the occurrence of illite/smectite/kaolinite (I/S/K) three-component mixed-layer clays. The lattice fringes of the I/S/K clays appeared corrugated and vanishing, and also exhibited variable thickness along a lattice fringe, which were consistent with changes from illite to smectite, from smectite to kaolinite, and from illite to kaolinite, respectively. Hydromorphic conditions in the Xuancheng soils led simultaneously to the direct transformation of illite to kaolinite and the transformation of illite to smectite to kaolinite in the pedogenic processes, and the formation of I/S/K three-component mixed-layer clays as intermediate products of these processes.
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