Surface Layer Types of Kaolinite: A High-Resolution Transmission Electron Microscope Study

Ma Chi and Richard A. Eggleton
Cooperative Research Center for Landscape Evolution and Mineral Exploration, Department of Geology, Australian National University, Canberra, ACT 0200, Australia
Present address: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125.

Abstract: 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-Å surface layer as terminations. Type 2 has one 10-Å 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-Å 2:1 surface layer on one side of the crystal. The spacing between the 10-Å layer and the adjacent 7-Å layer is not expandable. Type 3 kaolinite has one or several 10-Å 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-Å 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.

Key Words: 2:1 Layer • Clays • HRTEM • Kaolinite • Pyrophyllite • Smectite • Surface Layer

Clays and Clay Minerals; April 1999 v. 47; no. 2; p. 181-191; DOI: 10.1346/CCMN.1999.0470208
© 1999, The Clay Minerals Society
Clay Minerals Society (www.clays.org)