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STACKING FAULTS IN THE KAOLIN-GROUP MINERALS: DEFECT STRUCTURES OF KAOLINITE

Previous studies of the defect structure of kaolinite have examined samples having a restricted level of defects. This study examined nine kaolinite samples having a large diversity of defect contents, as indicated by Hinckley indexes ranging from 1.44 to 0.18. The samples were chosen so as to cover this range in as regular a manner as possible. The types and abundances of the defects were determined by examining the X-ray powder diffraction profiles for the 02,11 and 20,13 bands. The diffraction intensities were measured by counting for a fixed time in steps of 0.01°2θ. Analysis of these diffraction profiles indicated that (1) the major defect is the existence of a translation between adjacent layers, which is not the usual t→1 (approximately a/3), but is related to that translation by the pseudo-mirror plane coincident with the long diagonal of the unit cell; (2) the existence of a few C layers among the B layer stacking is a minor defect; (3) many of the samples could be accurately modeled only by assuming the existence of two kaolinite phases; (4) the existence of only a few C layers in some samples does not support the idea of a continuous series from kaolinite to dickite through disordered intermediates; and (5) the Hinckley indexes of several samples depend on the relative proportions of the two types of kaolinite in the mixture. p ]The nine kaolinite samples fall into three groups: those having a low to moderate abundance of defects (Hinckley index > 0.43) are mixtures of two types of kaolinite (one almost free of defects, the other richer in defects); those having low Hinckley indexes (0.43 to 0.18) are single phases with different proportions of defects; and those which contain a single type of kaolinite, unlike the others in the nature of the interlayer translations and the greater abundance of C layers. The agreement between calculated and observed X-ray diffraction profiles is excellent for all specimens, except one sample (from Charentes) for which the fit is acceptable but not perfect.RésuméLes précédentes études des défauts structuraux de la kaolinite ont porté sur des échantillonnages restreints. Dans cette étude, ont été examinés neuf échantillons de kaolinite présentant une large diversité dans l’abondance des défauts, et choisis de façon à couvrir, de manière aussi régulière que possible, le domaine de variation de l’indice d’Hinckley (de 1,44 à 0,18). Les types et abondances des défauts ont été déterminés à partir des bandes 02,11 et 20,13 d’enregistrements de diffraction X de poudres. Les intensités ont été enregistrées en pas à pas, avec un pas de 0,01°T, et un comptage à durée constante. L’analyse des profils de diffraction indique que: (1) le principal défaut est l’existence d’une translation entre feuillets adjacents qui n’est pas la translation habituelle (approximativement -a/3), mais celle qui s’en déduit par le pseudo miroir passant par la grande diagonale de la maille élémentaire; (2) l’existence de feuillets C, au sein des empilements majoritairement composés de feuillets B, est urn défault mineur; (3) la plupart des écantillons ne peuvent être correctement modélisés que si l’on suppose l’existence de échantillons ne conforte pas l’idée d’une série continue allant de la kaolinite à la dickite, via les intermédiaires que seraient les kaolinites désordonnées; et (5) l’indice d’Hinckley de plusieurs échantillons dépend de la proportion des deux types de kaolinites dans les mélanges.Les neuf échantillons de kaolinite se répartissent en trois groupes: ceux du premier groupe, qui ont une faible ou moyenne abondance de défauts (indice d’Hinckley >0,43), sont des mélanges de deux types de kaolinites (l’une presque sans défauts, l’autre riche en défauts); ceux du second groupe, qui ont un faible indice d’Hinckley (0,43 à 0,18), sont monophasés avec différentes proportions de défauts; le troisième groupe contient un seul echantillon, qui se distingue des autres par la nature de la translation entre feuillets et par la plus grande abondance en feuillets C. L’accord entre les intensités de diffraction expérimentales et calculées est exellent pour tous les énchantillons, excepté celui du troisième groupe, pour lequel l’accord est acceptable, mais non parfait

EXPERIMENTAL STUDY OF SMECTITE INTERACTION WITH METAL Fe AT LOW TEMPERATURE: 1. SMECTITE DESTABILIZATION

Interaction between metal Fe and a variety of natural and synthetic smectite samples with contrasting crystal chemistry was studied by scanning electron microscopy and X-ray diffraction from experiments conducted at 80°C. These experiments demonstrate an important reactivity contrast as a function of smectite crystal chemistry. An XRD method involving the use of an internal standard allowed quantification of the relative proportion of smectite destabilized as a function of initial pH conditions as well as of smectite structural parameters. In mildly acidic to neutral pH conditions, a significant proportion of metal Fe is corroded to form magnetite without smectite destabilization. Under basic pH conditions, smectite and metal Fe are partly destabilized to form magnetite and newly-formed 1:1 phyllosilicate phases (odinite and crondstedtite). More specifically, systematic destabilization of both metal Fe and smectite is observed for dioctahedral smectites while trioctahedral smectites are essentially unaffected under similar experimental conditions. In addition, smectite reactivity is enhanced with increasing Fe3+ content and with the presence of Na+ cations in smectite interlayers. A conceptual model for smectite destabilization is proposed. This model involves first the release of protons from smectite structure, MeFe3+OH groups being deprotonated preferentially and metal Fe acting as proton acceptor. Corrosion of metal Fe results from its interaction with these protons. The Fe2+ cations resulting from this corrosion process sorb on the edges of smectite particles to induce the reduction of structural Fe3+ and migrate into smectite interlayers to compensate for the increased layer-charge deficit. Interlayer Fe2+ cations subsequently migrate to the octahedral sheet of smectite because of the extremely large layer-charge deficit. At low temperature, this migration is favored by the reaction time and by the absence of protons within the di-trigonal cavity. Smectite destabilization results from the inability of the tetrahedral sheets to accommodate the larger dimensions of the newly formed trioctahedral domains resulting from the migration of Fe2+ cations.

STACKING FAULTS IN KAOLIN-GROUP MINERALS IN THE LIGHT OF REAL STRUCTURAL FEATURES

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.РезюмеСравнение 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.

STRUCTURAL TRANSFORMATION OF 2:1 DIOCTAHEDRAL LAYER SILICATES DURING DEHYDROXYLATION-REHYDROXYLATION REACTIONS

The structural transformation of dioctahedral 2:1 layer silicates (illite, montmorillonite, glauconite, and celadonite) during a dehydoxylation-rehydroxylation process has been studied by X-ray diffraction. thermal analysis, and infrared spectroscopy. The layers of the samples differ in the distribution of the octahedral cations over the cis- and trans-sites as determined by the analysis of the positions and intensities of the 11l, 02l reflections, and that of the relative displacements of adjacent layers along the a axis (c cos ß/a), as well as by dehydroxylation-temperature values. One illite, glauconite, and celadonite consist of trans-vacant (tv) layers; Wyoming montmorillonite is composed of cis-vacant (cv) layers, whereas in the other illite sample tv and cv layers are interstratified. The results obtained show that the rehydroxylated Al-rich minerals (montmorillonite, illites) consist of tv layers whatever the distribution of octahedral cations over cis- and trans-sites in the original structure. The reason for this is that in the dehydroxylated state, both tv and cv layers are transformed into the same layer structure where the former trans-sites are vacant.The dehydroxylation of glauconite and celadonite is accompanied by a migration of the octahedral cations from former cis-octahedra to empty trans-sites. The structural transformation of these minerals during rehydroxylation depends probably on their cation composition. The rehydroxylation of celadonite preserves the octahedral-cation distribution formed after dehydroxylation. Therefore, most 2:1 layers of celadonite that rehydroxylate (~75%) have cis-vacant octahedra and, only in a minor part of the layers, a reverse cation migration from former trans-sites to empty octahedra occurred. In contrast, for a glauconite sample with a high content in IVA1 and VIAl the rehydroxylation is accompanied by the reverse cation migration and most of the 2:1 layers are transformed into tv layers.

New modeling of X-ray diffraction by disordered lamellar structures, such as phyllosilicates

Abstract The “classical” modeling of powder X-ray diffraction (XRD) patterns of lamellar structures, such as phyllosilicates, assumes that the samples are composed of “crystals” having various thickness and well-defined translations between layers. This model is able to describe the high-angle domain of XRD patterns but sometimes fails in the low-angle region. The new model proposed here considers the samples to be composed of “particles” that have larger sizes than crystals and contain defects such as cracks, inner-porosity, bent layers, edge dislocations, etc. These defects induce variations in the d-spacings, introduced in the calculation by distributions of the d-spacings. For phyllosilicates, this model is consistent not only with XRD, but also with small-angle X-ray scattering (SAXS) data, transmission electron microscopy (TEM) results, and high-resolution transmission electron microscopy (HRTEM) observations.

Dehydroxylation of Fe3+, Mg-rich dioctahedral micas: (I) structural transformation

Abstract Celadonite and glauconite samples heated at different temperatures were studied by X-ray and electron diffraction. For dioctahedral micas the in-plane component of the translation between layers (ccosβ/a), which is strongly dependent on the position of the vacant octahedral site, significantly decreases at temperatures greater than the temperature of maximum dehydroxylation. The simulation of XRD patterns from different structural models reveals the actual crystal structure of dehydroxylated samples as well as the dynamics of the structural transformations. In the nonheated state the samples consist of tv (trans-vacant) 2:1 layers. During dehydroxylation, cations migrate from cis- into trans-octahedra and have 5-fold coordination. In the averaged unit-cell the ‘residual’ anions formed after the dehydroxylation reaction occupy the former OH sites with probability equal to 0.5. The migration of octahedral cations is accompanied by the transformation of the C-centred layer unit-cells into primitive ones. In contrast to Fe, Al and Mg cations have a greater ability to migrate.

A SIMPLE TECHNIQUE FOR IDENTIFICATION OF ONE-DIMENSIONAL POWDER X-RAY DIFFRACTION PATTERNS FOR MIXED-LAYER ILLITE-SMECTITES AND OTHER INTERSTRATIFIED MINERALS

A very simple technique is proposed for a quantitative or semiquantitative interpretation of X-ray diffraction (XRD) patterns for two-component mixed-layer structures. It is suitable for a determination of the Reichweite (R) values and proportions of component layers from graphical simulations of basal peak positions for mixed-layer structures with definite layer types. This technique can be successfully used for illite-smectites, but the accuracy of the results obtained for other mixed-layer structures is somewhat lower. In addition to the graphical technique, simple linear relationships are proposed for the calculation of layer proportions. Such relationships can be easily obtained for any mixed-layer structure with any R and any thicknesses of interstratified layers.Structural parameters reported in the literature for mixed-layer illite-smectites, kaolinite-smectites, etc., were used to check the reliability of the method presented. It is concluded that the technique works well and produces parameters that are in agreement with those published.

Étude par Diffraction X des Modes d'Empilement de la Nacrite Hydratée et Deshydratée

X-ray diffraction based on the comparison of experimental and calculated powder profiles enabled the determination of the structural characteristics of hydrated and dehydrated Tunisian nacrite. Using the concept describing the structure of natural nacrite, the stacking mode of the layers in the hydrated and dehydrated nacrite has been determined. The hydrate is characterized by an 8.42 A basal distance; one water molecule per Si2Al2O5(OH)4 is intercalated in the interlamellar space, located above the vacant octahedral site of the layer at z = 6.5 A and inserted inside the ditrigonal cavity of the tetrahedral sheet of the upper layer. The dehydrated nacrite obtained by heating of the hydrate at 423 K has the same interlayer shift t = −0.35a as the natural nacrite. Coherence domain sizes along c^{\ast} and in the ab plane are the same as those in the hydrate but different from those of the natural mineral. After dehydration, 5% of the layers had an interlayer shift similar to that obtained from the hydrate.

Order-disorder in clay mineral structures

Abstract Some recent works dealing with the concept of order-disorder in clay minerals are considered, including those aspects of order-disorder which appeared in the Brindley & Brown (1980) monograph, i.e. disorder in the distribution of cations, disorder in layer stacking, orderdisorder in mixed-layer systems and finite crystal size as a lattice disorder. Heterogeneity of samples and polymorphous transformations are also considered as other types of disorder. Most of these works emphasize that accurate structural characterization can only be obtained if several techniques are combined (e.g. XRD and IR, EXAFS and Mössbauer spectroscopies, etc.). Another conclusion is that accurate structural determination provides the key to the genesis of clays.

Calculation of intensity distribution in the case of oblique texture electron diffraction

A new method is described to determine accurately the intensities of reflexions in the case of oblique texture electron diffraction. The method is based on a comparison between experimental values of intensities and those calculated with a formalism that takes into account the orientation function of the particles. It allows the problems that arise from frequent overlapping of reflexions to be surmounted. A concrete example of the application of this method is given the structure refinement of a mica with muscovite–phengite composition.