High-resolution transmission electron microscopy (HRTEM) examinations have indicated that three types of surface layers may exist in natural kaolinite crystals. Type 1 has the expected 7-Aa surface layer as terminations. Type 2 has one 10-Aa pyrophyllite-like (or low-charge beidellite-like) layer as the surface layer on one side of a kaolinite particle (i.e., the layer sequence is TOTOTO...TOTOTOT, where T stands for tetrahedral sheet, O for octahedral sheet). Some industrial-grade highly-ordered kaolinites have such a 10-Aa 2:1 surface layer on one side of the crystal. The spacing between the 10-Aa layer and the adjacent 7-Aa layer is not expandable. Type 3 kaolinite has one or several 10-Aa collapsed smectite-like layers at one or both sides of a stack, i.e., (TOT)TOTO...TOTOTOT(TOT), forming a special kind of kaolinite-smectite interstratification. This type has only been recognized in some poorly-ordered kaolinites. The surface smectite layer(s) contribute to higher cation exchange capacity (CEC) values. These 10-Aa surface layers were not detectable by X-ray diffraction (XRD). HRTEM and electron diffraction examination also revealed the structural features of individual kaolinite crystals. All kaolinites (from various origins and sources) studied show C-face-centering of non-hydrogen atoms. Defects within the layer structure are common in both well-ordered kaolinite and poorly-ordered kaolinite.

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