2005 年 12 巻 Supplement1 号 p. 6-12
The advent of modern transmission electron microscopes has revolutionized the classic field of clay mineralogy. Its application to the study of dioctahedral phyllosilicates, illite-smectite in particular, in the past two decades has contributed much of our understanding of the crystal chemical relations and the smectite to illite reaction mechanisms. These contributions are summarized in the following five areas. first, there are only 3 discrete clay minerals in a prograde sequence from smectite to regularly interstratified R1 I-S to illite with increasing diagenetic/metamorphic grade. These observations are in contrast to the conventionally accepted continuous sequence as determined by X-ray diffraction. Reasons for such a contrast are discussed. Second, the 1M polytype is not an intermediate between 1Md for smectite-rich I-S and 2M1 for mature illite of diagenetic/metamorphic grade. Rather, the 1M polytype has a different composition than either 1Md or 2M1, i.e., it is Mg rich as compared to 1Md or 2M1 polytypes. Third, smectite, interstratified I-S, and illite may form from fluids at the same time over a temperature (depth)-composition gradient, without one being necessarily preceded by another. Fourth, the smectite to illite reaction mechanism is via dissolution of reactants, transport of dissolved chemical ions, and crystallization of products. Different mechanisms, such as layer-by-layer replacement (or sometimes called solid-state transformation), may be different variations of the same mechanism, e.g., the layer-by-layer replacement mechanism may operate at the atomic scale, whereas dissolution-crystallization may be over dimensions of pore space or greater. finally, TEM data have made important contributions in providing evidence that are inconsistent with the fundamental particle theory, suggesting that natural, interstratified I-S sequences are not accumulations of fundamental particles.
