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Hydrothermal synthesis of corrensite; a study of the transformation of saponite to corrensite
A coherent TEM- and XRD-description of mixed-layer illite/smectite
Oxygen isotope compositions of mixed-layer serpentine-chlorite and illite-smectite in the Tuscaloosa Formation (U. S. Gulf Coast); implications for pore fluids and mineralogic reactions
The chemical composition of serpentine/chlorite in the Tuscaloosa Formation, United States Gulf Coast; EDX vs. XRD determinations, implications for mineralogic reactions and the origin of anatase
The origin and diagenesis of grain-coating serpentine-chlorite in Tuscaloosa Formation sandstones, U.S. Gulf Coast
Petrology, chemistry, and clay mineralogy of a K-bentonite in the Proterozoic Belt Supergroup of western Montana
Rotationally disordered illite/smectite in Paleozoic K-bentonites
Illite from the Potsdam Sandstone of New York; a probable noncentrosymmetric mica structure
Front Matter
An Introduction to Computer Modeling of X-Ray Powder Diffraction Patterns of Clay Minerals: A Guided Tour of NEWMOD©
Abstract Discussion of the calculation of powder X-ray diffraction (XRD) profiles offers an excellent opportunity to introduce many of the concepts of XRD modeling which will be basic to understanding the succeeding chapters in this volume. This introductory chapter will discuss in detail the theory underlying the one-dimensional diffraction algorithm used by the NEWMOD® software series (Reynolds, 1985). Attention will also be given to some of the more specialized aspects of the NEWMOD® series especially the statistics used to describe interstratifications. The NEWMOD® series is familiar to many researchers in the clay field, and calculating XRD profiles with it is common in many investigations. However, the details of the calculations may not be familiar to all. In this paper, NEWMOD® variable names are given in all capitals, for example SIGSTAR or LOWN, and are included to help interested persons navigate through the source code.
Inverting the NEWMOD© X-Ray Diffraction Forward Model for Clay Minerals Using Genetic Algorithms
Abstract We have developed a computer program, MatchMod, which inverts the NEWMOD® forward modeling program of Reynolds (1985a) and provides a “match” for a given experimental X-ray diffraction (XRD) pattern. As used here, a forward model is a set of equations, based on first principles, which can predict the state of a system from the variables which control that system. An inverse model, on the other hand, is one which estimates the control variables from some state of the system. Reynolds' series of tools (NEWMOD®, CLAYS®, MIXER®; Reynolds, 1985 a, b, c) have the potential for producing a quantitative clay mineral analysis based on die 001 peaks of oriented clay mineral aggregates because they account for most instrumental factors, preferred orientation, sample length, a host of mineral compositional and structural variables as well as mass absorption coefficient and unit cell volume. However, in the practice of pattern-matching (inverse modeling) these tools are extremely time-consuming and tedious to apply, primarily because of the large number of variables involved in the trial-and-error procedure.
Three-Dimensional X-Ray Powder Diffraction from Disordered Illite: Simulation and Interpretation of the Diffraction Patterns
Abstract Plancon and Tchoubar (1977a) developed a computer algorithm for calculating three-dimensional powder diffraction patterns of phyllosilicates with various types and degrees of disorder, two of which are treated herein. Turbostratic disorder is caused by randomly distributed random magnitude translations and rotations of the 2:1 silicate layers. This condition breaks the crystal up into thin (along Z) coherent diffracting domains which at the limit of disorder produces an assemblage of crystallites composed of single silicate layers. Many smectites have such a structure and it gives rise to the well-known two-dimensional diffraction bands whose heads are defined by reflections of the type hkO (Brindley, 1980).
Studies of Clays and Clay Minerals Using X-Ray Powder Diffraction and the Rietveld Method
Abstract The Rietveld method was originally developed (Rietveld, 1967, 1969) to refine crystal structures using neutron powder diffraction data. Since then, the method has been increasingly used with X-ray powder diffraction data, and today it is safe to say that this is the most common application of the method. The method has been applied to numerous natural and synthetic materials, most of which do not usually form crystals large enough for study with single-crystal techniques. It is the ability to study the structures of materials for which sufficiently large single crystals do not exist that makes the method so powerful and popular. It would thus appear that die method is ideal for studying clays and clay minerals. In many cases this is true, but the assumptions implicit in the method and the disordered nature of many clay minerals can limit titsapplicability. This chapter will describe the Rietveld method, emphasizing the assumptions important for the study of disordered materials, and it will outline the potential applications of the method to these minerals. These applications include, in addition to the refinement of crystal structures, quantitative analysis of multicomponent mixtures, analysis of peak broadening, partial structure solution, and refinement of unit-cell parameters.
Abstract There are two equally valid ways to describe illite-rich crystals composed of illite and smectite. From the MacEwan crystallite viewpoint (Altaner et al., 1988), such mixed crystals are composed of interlayered illite and smectite layers, and the percentage of each (expandability, or, alternatively, percent illite layers) can be determined readily from X-ray diffraction (XRD) peak positions for glycol-solvated samples (Srodon, 1980, 1984; Moore and Reynolds, 1989). According to the interparticle diffraction concept (Nadeau et al., 1984), illite/smectite (I/S) is composed of discrete “fundamental” illite particles that sorb water on their basal X-Y surfaces. “Smectite layers” are water-rich interfaces that diffract coherently along Z with the adjacent illite particles, thereby producing XRD effects for mixed-layer crystals.
A Computer Technique for Rapid Decomposition of X-Ray Diffraction Instrumental Aberrations from Mineral Line Profiles
Abstract The convolution of instrumental aberrations with the true mineral line profile is represented by P(29) = [(W 002A G) 002A S](20) + background (Balzar, 1992; Huang and Parrish, 1975). W is the wavelength distribution function, G is all other instrumental functions, S is the specimen profile that is caused by pure physical broadening, 002A represents the convolution operations, and the convolution operation is over the 29 scale. For instrumental effects, Alexander (1954) calculated a convolution total of six instrumental line profiles that are reproduced by Klug and Alexander (1974, page 292). They showed that there was excellent agreement between a theoretical profile and that of the 101 quartz profile. The five instrumental profiles that contribute to the observed line profile are: the source (represented by W above) which is approximated by a symmetrical Gaussian distribution; flat specimen surface which is an asymmetrically truncated function that affects the lower 29 side of a peak; axial divergence which is asymmetrical and also affects the lower 29 side of a peak; specimen transparency which is also asymmetrical and affects the lower 29 side of a peak; and the effects of the receiving slit and misalignment which are symmetrical distributions. These five instrumental aberrations are represented by G above. Inasmuch as flat specimen effects, axial divergence, and specimen transparency contribute to the asymmetric character of diffraction profiles, we attribute all asymmetry on the lower 29 side of the profiles used as examples in this chapter as due to instrumental aberrations. Such an assumption is
Abstract The advent of high-speed desk-top computers has made it possible to solve many crystallographic problems which heretofore required long hours of calculating. More importantly, many problems can be addressed that were simply inaccessible by any means thirty years ago. Some of the programs described in this volume complete more calculations in minutes than an individual could accomplish over a lifetime even if every second of that lifetime was spent with pencil and paper in hand. And the necessary computing tools are widely available at prices comparable to two or three of the best slide rules in the 1950's. We do routinely what could not have been imagined by scientists of a few generations ago. And this is just the beginning-we will do Rietveld analyses some day on laptop computers that will be disposed of when their batteries run down.
Abstract The mineral referred to as kaolinite/smectite has been identified in soils and paleosols from Holocene to Pennsylvanian in age (and perhaps older) and most fireclays, ball clays, and other poorly crystallized kaolinites. It is a mixed-layered mineral composed of an expandable 2:1 (9.7Å, collapsed) layer and a kaolinitic 1:1 (7Å) layer. Models of bonding between layers or units of this mineral can be calculated by using stacking sequences of three structural modules-2:l layers of parent material, 1:1 layers, and 2:1 layers hydrogen bonded to a 1:1 layer. This structural modeling suggests constraints on mechanisms of kaolinitization and types of interlayers that will expand, bond, form stable crystallites, or some combination of these. It also suggests possible ordering mechanisms and limits to particle size. Layer charge on the 2:1 parent material and on the 2:1 and 1:1 alteration products ranges from the maximum of that for illite to a minimum near zero. The wide range of possible layer charge on the 2:1 layer suggests that mixed-layered kaolinite/expandables ( K/E ) would be a better name than kaolinite/smectite. The mechanism of formation of K/E is poorly understood. Our evidence supports a mechanism in which tetrahedral sheets are stripped from 2:1 layers and bonding occurs between the hydroxyl sheet of the newly formed 1:1 layer and the adjacent 2:1 layer. Inherited octahedral and tetrahedral substitutions may result in atypical 7 Å layers. Growth of K/E may be terminated by encounters between crystallites with opposite c* directions of their 1:1 layers or by strains resulting from inherited substitutions within the 7 Å layer. Pedogenic K/E occurrences are correlated with iron substitution within kaolinitic layers of K/E. Where order can be determined, we have observed ~R1.5. R1 has been reported in the literature. There appear to be two continuous genetic sequences within the kaolin group: 1) a series from allophane through halloysite, and 2) a series from 2:1 parent materials through K/E . Transformations from halloysite or K/E to well-crystallized kaolinite probably require recrystallization and therefore the last step in both sequences is discontinuous. Conceptually and structurally, we can make several useful comparisons between smectite to illite or illite to smectite transformations and the 2:1 to K/E transformation. Detection of kaolinite/expandibles is readily made by XRD studies of < 2 µ m or finer fraction samples after air drying, ethylene glycol solvation, and a heating routine (300°C, 350°C, 400°C). K/E with a composition near kaolinite-a peak near 7Å--can be distinguished from halloysite and well-crystallized kaolinite by the rapid intercalation of the latter two phases by many agents. This intercalation shifts the kaolinite and halloysite peaks to the 10Å to 12Å area of the diffractogram and leaves only the peak for K/E in the area between 7Å and 10Å. The 17Å peak of expandable-rich K/E (001 kaol /001 exp ) with ethylene glycol solvation is extremely broad even at low percentages of 7Å interlayering. This peak broadening distinguishes K/E with a low proportion of kaolinitic layers from smectite and I/S peaks near 17Å. After heating to 300°C, elevated background intensity or a peak on XRD traces between the 7Å and 10Å positions is the most sensitive diagnostic method to detect K/E. Loss of the high-angle, K/E shoulder on the 10Å peak (assuming one or more discrete 2:1 phases are present) after heating to 350-400°C can be an equally sensitive method for detecting and quantifying K/E. The difficulty of detecting K/E, especially samples with low contents of kaolinitic interlayers, suggests that K/E is much more widespread than previously thought. The A and B zones of a soil profile typically contain the most K/E and the highest proportion of kaolinitic to expandable layers. The parent 2:1 clay minerals, climate, plant community, and degree of drainage determine whether K/E, halloysite, well-crystallized kaolinite, bauxite, or a combination of these phases forms. Determination of the types and amounts of kaolin minerals in soils may offer valuable insights into the nature of soil formation and associated processes and rate of soil formation below unconformities.