| Research Abstract |
There are a number of interesting natural materials on the terrestrial surface, formed in water-abundant and oxidizing conditions at moderate temperatures. These materials are generally very fine-grained and do not have three-dimensional periodicity, unlike common crystalline minerals. From these characteristics, they may be called "inorganic macro-molecules". However, their atomic structures and textures are not well understood, owing to the very small particle size and non-periodic structure. Imogolite, smectite and some phyllosilicates with heavy stacking disorder are typical examples. The purpose of this work was to develop new methodologies and/or techniques to investigate these materials, and elucidate their structures and forming conditions on the earth.
At first, we tried to determine the atomic structure of imogolite, a natural nano-tube, from the intensity distribution in electron diffraction patterns. However, a plausible model that completely explains the electron diffractio
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n patterns was not obtained. This investigation must be continued as a future research. Meanwhile, Suzuki in AIST has made a progress in finding conditions to synthesize imogolite efficiently in a laboratory.
Smectite is a very fine-grained phyllosilicate, often regarded as "nanosheet". Its basic structure is a 2:1 layer with two tetrahedral sheets and one octahedral sheet between them, but the detail is not clear yet. Especially distribution of octahedral rations in the 2:1 layer is under discussion. We investigated possibility to determine the cation distribution by using high-resolution electron microscopy (HRTEM). However, it was found that intense electron beam during image recording dehydroxylates the 2:1 layer and this dehydroxylation induces cation migration. We need to find an idea to overcome this problem in future. On the other hand, we have found a novel 2:1 layer structure in the thermally dehydroxylated phase of a kind of mica, celadonite.
We have made a plenty of progress to elucidate the nature of stacking disorder in various phyllosilicates in this study, by using HRTEM and simulation of powder X-ray diffraction patterns. The investigated phyllosilicates include kaolin group minerals, pyrophyllite, talc, sudoite, etc. Especially direct observation of the stacking disorder in kaolin group minerals was an epoch-making work in the history of this topic started more than a half century ago.
Besides, several new techniques and ideas to characterize the structure (stacking sequence, crystal orientation, morphology, etc.) of fine-grained materials using Kikuchi patterns were developed and demonstrated. Less
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