Abstract:Abstract--A procedure leading to the systematic derivation of all maximum-degree-of-order (MDO) polytypes in the three mica families, based on the order-disorder (OD) interpretation of mica structures, has been developed. There are 6 non-equivalent (8 non-congruent) homo-octahedral, 14 (22) mesooctahedral, and 36 (60) hetero-octahedral MDO mica polytypes. The ideal ditrigonalization oftetrahedral sheets leads to a splitting of the polytypes in any mica family into two subfamilies with octahedral and trigonal p… Show more
“…MDO polytypes 4-6 have also a common st.~perposition structure with the symbol E.Eu, E,U.U and symmetry H63/mcm. These results are in complete agreement with those obtained by Backhaus and Durovi~ (1984, Figures 5a and 5b) and hold, evidently, also for the meso-and hetero-octahedral families. It is interesting to note that Zvyagin et al (1979, p. 143) also mentioned the existence of two "groups" of mica polytypes, identical with our subfamilies.…”
Section: Compilation Of Classification Tablessupporting
Abstract--The maximum-degree-of-order (MDO) polytypes in all three mica families have been classified into two subfamilies and five homomorphous MDO groups according to their superposition structure and the YZ projection of their structure, respectively. This classification, which is closely related to the diffraction pattern of micas, can be derived directly from the fully descriptive polytype symbols and facilitates the calculation of the identification diagrams as well as the recognition of the relations of homomorphy between the three mica families. All mica structures refined so far by the least-squares method according to the present standards, are those of MDO polytypes.
“…MDO polytypes 4-6 have also a common st.~perposition structure with the symbol E.Eu, E,U.U and symmetry H63/mcm. These results are in complete agreement with those obtained by Backhaus and Durovi~ (1984, Figures 5a and 5b) and hold, evidently, also for the meso-and hetero-octahedral families. It is interesting to note that Zvyagin et al (1979, p. 143) also mentioned the existence of two "groups" of mica polytypes, identical with our subfamilies.…”
Section: Compilation Of Classification Tablessupporting
Abstract--The maximum-degree-of-order (MDO) polytypes in all three mica families have been classified into two subfamilies and five homomorphous MDO groups according to their superposition structure and the YZ projection of their structure, respectively. This classification, which is closely related to the diffraction pattern of micas, can be derived directly from the fully descriptive polytype symbols and facilitates the calculation of the identification diagrams as well as the recognition of the relations of homomorphy between the three mica families. All mica structures refined so far by the least-squares method according to the present standards, are those of MDO polytypes.
“…Simulation of the powder XRD pattern indicated that the peak intensity and width of reflections with h = k will be modified to be weaker and larger, respectively, by these stacking faults, and this tendency was actually observed in the experimental XRD patterns (Kogure et al, 2008b). Of course, a single image along one of ± X i is still valid for determining the stacking sequence in micas (and other phyllosilicates), assuming that the mutual rotation between the adjacent layers is restricted to 0°, +120° or –120° (‘subfamily A’) (Backhaus & Ďurovič, 1984), which is indeed the case for most micas. Fig.…”
This review demonstrates that high-resolution transmission electron microscopy (HRTEM) imaging of clay minerals or phyllosilicates with an incident electron beam along the major zone axes parallel to the constituting layers, in which the contrast corresponds to individual cation columns in the images obtained, is indispensable for elucidating the enigmatic structures of these minerals. Several kinds of variables for layer stacking, including polytypes, stacking disorder and the interstratification of various kinds of unit layers or interlayer materials, are common in phyllosilicates. Local and rigorous determination of such variables is possible only with HRTEM, although examination as to whether the results obtained by the HRTEM images from limited areas represent the whole specimen should be made using other techniques, such as X-ray diffraction. Analysis of these stacking features in clay minerals provides valuable insights into their origin and/or formation processes. Recent state-of-the-art techniques in electron microscopy, including incoherent imaging, superior resolutions of ~0.1 nm and low-dose imaging using new recording media, will also contribute significantly to our understanding of the true structures of clay minerals.
The present review deals with some selected topics pertaining to the theory of OD structures of layers founded by K. Dornberger-Schiff. An attention is paid mainly to the notion of VC (Vicinity Condition) structures and to the position of OD (Order-Disorder) structures within the VC structures. Explained is also the notion of MDO (Maximum Degree of Order) structures and indicated their derivation. The importance of a suitable axial setting for periodic OD structures is illustrated by an OD interpretation of the structure of the mineral kermesite. Based on X-ray studies of the mineral cronstedtite, the choice of suitable crystals for X-ray studies, reasons for the occurrence of 'ghost' peaks in Fourier maps and the refinement strategy for OD structures, is also discussed.
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