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Stacking Faults in Kaolin-Group Minerals in the Light of Real Structural Features

Published online by Cambridge University Press:  02 April 2024

A. S. Bookin
Affiliation:
Geological Institute, Academy of Sciences of the U.S.S.R., Pyzevsky 7, Moscow 109017, U.S.S.R.
V. A. Drits
Affiliation:
Geological Institute, Academy of Sciences of the U.S.S.R., Pyzevsky 7, Moscow 109017, U.S.S.R.
A. Plançon
Affiliation:
Centre de Recherche sur les Solides à Organisation Cristalline Imparfaite and Université d'Orléans, 45100 Orléans, France
C. Tchoubar
Affiliation:
Université d'Orléans, Laboratoire de Cristallographie, ERA#841, 45100 Orléans, France

Abstract

A comparison of the structural characteristics of the kaolin-group minerals, mainly kaolinite and dickite, shows that they differ in both the two-dimensional periodicity in the 1:1 layers and the rotation angles of the polyhedra. Distortions in a real 1:1 layer, compared with an idealized layer, do not allow such stacking faults as ± 120° layer rotations and vacancy displacements, because the second layer is incommensurable with the first. The 1:1 layer structure and the fact that the unit cell is symmetrical with respect to the plane passing through the long diagonal of the unit cell suggest the possibility of defects resulting from the two stacking sequences for the same layers. For a regular alternation of translations, a halloysite-like structure should be the end-member of such a series of defect kaolinite types.

The formation of layers having vacant octahedral C-sites is another possible type of fault. Because of the minor dilference between γ and 90°, dickite-like layers should exist. A regular alternation of B and C layers yields dickite as the end-member structure. In materials containing few defects, stacking faults of both types lead to similar X-ray powder diffraction patterns. Thus, the nature of the stacking faults is difficult to determine experimentally. In materials containing many defects, however, the two models lead to different calculated diffraction patterns. Therefore, only a study of defect-rich types of kaolinite can determine which types of defects exist in natural kaolinite samples.

Сравнение структурных характеристик минералов каолиновой группы, в основном као- линита и диккита, показало, что они различаются как в отношении двумерной периодичности их 1: 1 слоев, так и углами разворота полиэдров. Искожения реальных 1:1 слоев по сравнению с идеали- зированными не позволяют реализоваться таким дефектам упаковки, как вращние на ± 120° и смена положения вакансии, поскольку второй слой сказался бы несоразмерным с первым. Строение 1:1 слоя и элементарной ячейки, будучи симметричны относительно плоскости, проходящей через длин- ную диагональ элементарной ячейки, предопределяют возможность возникновения дефектов упа- ковки, вызванных двумя способами наложения однотипных смежных слоев, в случае их регулярного чередования конечным членом такого ряда дефектных каолинитов была бы структура галлуазитового типа.

Возникновение слоев с вакантной с позицией представляется другим допустимым типом ошибок. Вследствие малого отклонения угла гамма от 90° могли бы встречаться диккито-подобные встройки, давая при упорядоченном чередовании слоев в и с диккит как конечный член ряда, в образцах с низкми содержанием дефектов оба типа ошибок приводят к близкому профилю рентгеновской ди- фракции и природу ошибок экспериментально установить сложно, в образцах с высокой концентра- цией дефектов две модели ведут к различным дифракционным картинам, поэтому изучение сильно дефетных каолинитов может ответить на вопрос, какой тип дефектов встречается в природный образ- цах.

Type
Research Article
Copyright
Copyright © 1989, The Clay Minerals Society

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