STACKING DISORDER IN KAOLINITE REVEALED BY HRTEM: A REVIEW

概要

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Although stacking disorder in kaolinite was extensively investigated for a long time, its real structure has not been understood sufficiently. Recently high-resolution transmission electron microscopy (HRTEM) studies of kaolinite were reported with sufficient resolution to determine the origin of the stacking disorder directly. Most common stacking faults in kaolinite are disorder of the alternative layer displacement, t_1 (approximately -a/3) and t_2 (a/6-b/6), between adjacent layers. The two layer displacements are related to each other by the pseudo-mirror plane passing the octahedral vacant site in the kaolinite unit layer. Furthermore, this type of disorder can occur as (1) isolated stacking faults (for instance, insertion of t_2 in the ordered matrix with t_1) as well as (2) interstratifications of packets in which the layer displacement is ordered. These two occurrence modes of the t_1/t_2 disorder probably reflect the growth process of kaolinite. Disorder by displacement of the octahedral vacancy sites in the dioctahedral 1:1 layer and/or layer rotation, is another stacking disorder found in kaolinite of sedimentary origin.