Diffraction effects calculated for structural models of K-saturated montmorillonite containing different types of defects

Abstract

A B S T R A C T :The general approach to the problem of the real structure of smectites requires an analysis based on the relationship between structural characteristics and diffraction patterns. This paper, which considers only the models corresponding to dioctahedral smectites saturated by K-cations and collapsed, includes: 1. Successive consideration of all models which are crystallochemically possible. These models may differ in (i) the structure and chemical composition of layers and interlayer spaces; (ii) the azimuthal orientations, translations and the mode of alternation of the layers; (iii) independent parameters which describe quantitatively the models (e.g. abundance of each type of layer, probability parameters defining the succession of layers .... ). 2. Calculation, in all accessible domains of reciprocal space, of the distribution of intensities and profile variations, obtained by changing only one parameter at a time, that defines one type of structural feature (e.g. cation distribution in individual layers, stacking of the layers, nature of stacking faults, . . .). 3. A systematical analysis of the calculated diffraction patterns to establish the diffraction criteria which will help to interpret the experimental data explicitly.In dioctahedral smectites only two of the three possible octahedral sites in the half unit cell are occupied by cations. These smectites are classified into montmorillonites, beideUites and nontronites, according to the composition of tetrahedral and octahedral sheets and the magnitude of the negative charge on the layer. To describe the crystal structure of smectites requires knowledge of (i) the composition and structure of the layers and interlayer spaces and (ii) the orientation and succession of these structural elements. For natural samples determination of both (i) and (ii) is usually difficult due to the small size of the particles and poor structural ordering. Normally, diffraction patterns of smectites contain only two types of reflections: basal 00l reflections and practically unmodulated (hk) bands corresponding to a two-dimensional diffraction. Obviously such data are not enough for a detailed determination of crystallochemical and structural characteristics of smectites. Determination of the internal structure of the 2:1 layers can be aided by a better ordering of layer stacking. For example, Mamy & Gaultier (1976) pointed out that montmorillonite shows a better ordering of layer stacking after K-saturation and several wetting and drying cycles (WD cycles), the resulting X-ray patterns contain strong reflections which have been indexed in a monoclinic one-layer unit cell. Based on selected-area electron diffraction results these authors supposed that after WD treatment 9 1984 The Mineralogical Society

Illite from the Potsdam Sandstone of New York: A Probable Noncentrosymmetric Mica Structure

“…Drits et al (1984) is shown by Table 3. The experimental XRD pattern resembles the calculated cv-1M pattern over the 20 range 39 to 44 ~ (Figure 5), although the 040 peak on the experimental profile is too weak and resolution of the 222 and i33 reflections is poorly matched between calculated and observed patterns.…”

“…The first is the cell geometry. Drits et al (1984) maintained that ccos~/a > -0.383 indicates some occupancy of the M1 octahedral site, and that each of the three octahedral sites are 2/3 filled if c cos ~/a = -I/3. c cos ~/a is further increased if the M1 site is more than 2/3 filled, with the limit of -0.308 for a cv structure (one vacant M2 site).…”

“…The top trace is the experimental pattern, the center is the calculated cv-lM profile, and the bottom shows the calculated 3T pattern. Drits et al (1984).…”

“…The MI site is larger, requiring the upper tetrahedral sheet in the centrie 2:1 layer to shift relative to the lower one by more than the ideal value of a/3, leading to a typical monoclinic angle (/3) of about 101.3 ~ (Bailey, 1984). The cis-vacant structure in C2, however, produces an Copyright 9 1993, The Clay Minerals Society undershifl with/3 = 99.13 ~ Drits et al, 1984). An intermediate structure proposed by Drits et al has each of the three octahedral positions 2/3 filled to produce a near ideal B value of 100.3 ~ Gavrilov and Tsipursky (1988) reported such a mineral from Jurassic sediments of the central Caucasus, and Zvyagin et al (1985) described the hydrothermal occurrence of a dioctahedral 1M mica with a structure very close to the ideal cis-vaeant type.…”

“…The treated minerals produced the electron diffraction maxima necessary for a full structural determination, and they concluded that many montmorillonites contain significant proportions of cisvacant layers. Drits et al (1984) calculated three-dimensional XRD patterns of K-saturated montmorillonite that quantify the diagnostic 02l:lll reflections for the cis-vacant model and its possible variations. Their results are applicable to illite, and we have based the present study on the seminal work ofTsipursky, Drits, and their co-workers.…”