Surface layer types of kaolinite: A high-resolution transmission electron microscope study

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-Angstrom surface layer as terminations . Type 2 has one 10-Angstrom pyrophyllite-like (or low-charge beidellite-li ke) layer as the surface layer on one side of a kaolinite particle (i.e., t he layer sequence is TOTOTO... TOTOTOT, where T stands for tetrahedral shee t, O for octahedral sheet). Some industrial-grade highly-ordered kaolinites have such a lo-A 2: 1 surface layer on one side of the crystal. The spacin g between the 10-Angstrom layer and the adjacent 7-Angstrom layer is not ex pandable. Type 3 kaolinite has one or several 10-Angstrom collapsed smectit e-like layers at one or both sides of a stack, i.e., (TOT)TOTO...TOTOTOT(TO T), forming a special kind of kaolinite-smectite interstratification. This type has only been recognized in some poorly ordered kaolinites. The surfac e smectite layer(s) contribute to higher cation exchange capacity (CEC) val ues. These 10-Angstrom surface layers were not detectable by X-ray diffract ion (XRD). HRTEM and electron diffraction examination also revealed the str uctural features of individual kaolinite crystals. All kaolinites (from var ious origins and sources) studied show C-face-centering of non-hydrogen ato ms. Defects within the layer structure are common in both well-ordered kaol inite and poorly-ordered kaolinite.