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CMS Pioneer in Clay Science Lecture

Laurent J. MICHOT
PHENIX, Sorbonne Université, Paris (France)

Crystalline and osmotic swelling of clay minerals: Recent advances

The way in which water interacts with swelling clay minerals is a fascinating subject that has been investigated for almost a century. In fundamental terms, the processes that occur are rather complex, as they involve a subtle interplay between electrostatics, hydration forces and osmotic effects. Since the seminal paper by Norrish in 19541, two types of behavior are classically considered depending on the ratio between water amount and solid clay content, i.e. crystalline swelling and osmotic swelling. Studying phenomena in these two swelling regimes requires the use of various experimental techniques and modeling approaches. In this talk, we will try to give an overview of recent results obtained in both water activity ranges. In experimental terms, we will try to show how the use of large-scale facilities (synchrotron and neutron reactors) provides unique opportunities for such studies and how progresses in clay minerals synthesis represent a major asset for such investigations. We will in parallel try to illustrate how simulation techniques and theoretical approaches widen our understanding of the various phenomena involved in clay water interactions provided that a constant collation between simulation results and experimental data is maintained.
 
References
[1] Norrish, K. Crystalline Swelling of Montmorillonite: Manner of Swelling of Montmorillonite, Nature, 73, 256–257 (1954)

 

George Brown Lecture - Clay Minerals Group

Toshihiro KOGURE
Department of Earth and Planetary Science, The University of Tokyo, Tokyo (Japan)

Visualization of Clays at the Atomic Scale

Structures of clay minerals, most of which belong to the phyllosilicate group, are often enigmatic owing to the existence of polytypes/stacking disorder, mixed-layer structures, and other various inhomogeneity. Because most of such structural variations or disorder in phyllosilicates are of one-dimensional character, direct imaging of atomic columns using electron microscopy is most effective with the incident beam parallel to the layers. However, at least in the author’s recognition, structure analysis of clay minerals by imaging close to the atomic scale is not as common as expected in spite of its importance. There are several reasons for this but we should continue and encourage such challenges for the further progress of clay science. As demonstrated in recent decades by a number of clay mineralogists operating electron microscopes, high-resolution transmission electron microscopy (HRTEM) with a point resolution of ~ 2 Å has unraveled complicated structures of clay minerals. For example, we successfully explained mysterious features in powder XRD patterns and the mechanism of clay formation. Fortunately, the progress of electron microscopy does not cease and new technologies appears every year owing to the various demands from material sciences. Now we should make efforts to introduce these into clay science to solve a number of remaining problems in this field.
In the first-half of my lecture, I would like to introduce several examples of the accomplishments of HRTEM to present-day clay mineralogy. They include the complete determination of the stacking sequence in long-period polytypes or heavily disordered samples from two images of the same area along two independent directions. Also the elucidation of the origin of stacking disorder in beam-sensitive clays like kaolinite, halloysite, pyrophyllite, etc. from sufficiently high-quality structure images and the determination of a novel 2:1 layer by comparison between experimental and simulated images, etc. by using conventional TEM. Annular-dark-field (ADF) imaging by STEM has an advantage over phase contrast imaging by TEM, in that the contrast can be corresponded unambiguously to heavy atomic columns and heavy atoms can be easily distinguished in the structure. For instance, we can search for cesium ions incorporated in vermiculite by ion-exchange at the atomic scale.
In the second-half of my lecture, I would like to discuss promising, state-of-the-art techniques for solving remaining problems in clay science. Cancelation of spherical aberration by Cs-corrector has improved the point resolution of TEM or STEM close to or beyond 1 Å. This is very useful for the complete resolution of cation columns in the clay structures, although we are still concerned with radiation damage. Beside such high resolution, or by the sacrifice of resolution to some extent, we can obtain a wider gap for the specimen in the pole piece, by which we may make environment-controlled observations, for example, high-resolution imaging without dehydration of clay samples. New generations of recording media, represented by C-MOS or direct detection cameras, will definitely overcome the limitation of imaging of clay structures by radiation damage. On the other hand, although it is not relevant to structure imaging, improvement of X-ray detection efficiency by more than one order in recent energy dispersive X-ray spectroscopy (EDS) will open a new window for the analysis of fine clays.

 

The Marilyn and Sturges W. Bailey Distinguished Member Award

Dennis D. Eberl
US Geological Survey, Boulder, CO (USA)

Memoir of an Illitist

A study of illite crystal thickness distributions (CTDs) has led to an understanding of crystal growth in general. Illite CTDs have three fundamental shapes: asymptotic, lognormal and an intermediate shape formed as the first type of CTD grows into the second type. The asymptotic CTD forms during simultaneous nucleation and crystal growth, whereas the lognormal CTD results from growth without simultaneous nucleation. In these studies, illite crystal thicknesses are measured by Fourier analysis of XRD peak shapes according to the Bertaut-Warren-Averbach method (MudMaster program) after the effects of swelling on XRD peak broadening have been removed by treatment with PVP-10 polymer.
The relative growth rate of illite crystal thicknesses (T, in nm) can be described mathematically by the Law of Proportionate Effect (LPE): Tj+1 = Tj + ejTj, where Tj is the initial crystal thickness, Tj+1 is the thickness after one subsequent growth cycle, and ej is a random number that is independent of T and that varies between 0 and 1. The LPE algorithm is iterated for a given number of times for 1001 crystals by the GALOPER program, and, upon collection of the resulting thicknesses into size classes, naturally occurring illite CTD shapes are duplicated. These shapes are related to two parameters, alpha, which is the mean of the natural logs of the thicknesses, and beta2, which is their variance. Reaction paths for illite formation are revealed in plots of these parameters on an alpha-beta2 diagram.
From a study of crystal size distributions (CSDs) in the literature, and from the results of our own experiments with K-alum and calcite crystallization, as well as from our measurements of the sizes of naturally occurring crystals for a variety of minerals, we discovered that, like illite, most minerals display asymptotic or lognormal size distributions. Therefore, mineral growth in nature can be described by the LPE. The rate of illite thickness growth is limited by the rate at which new material can be incorporated into the illite surface (surface limited proportionate growth). However, as minerals grow larger than clay size, the growth rate becomes limited by the rate at which nutrients can be transported to the mineral surface. During such supply limited proportionate growth, ej in the above equation essentially becomes a constant (it appears as a scaling factor), and the early formed shapes of the CSDs are preserved as crystals grow larger (i.e., beta2 remains constant as alpha increases). Such growth is described as “proportionate growth” because crystals tend to grow at a rate that is proportional to their size. In other words, in the same system, larger crystals increase in linear dimension faster than smaller crystals.
One other fundamental CSD shape, rarely found nature, was crystallized in our experiments: the reverse skew of the universal steady state shape predicted by LSW theory for Ostwald ripening. We have not found this shape for clays, but have produced it for calcites nucleated at high levels of supersaturation (omega>100). This unique shape has also been observed for some naturally occurring garnets. Extremely small crystals, unstable due to their large surface areas, nucleate in heterogeneous solutions at high supersaturation, and then dissolve to contribute growth material to larger crystals that are thermodynamically more stable. The Ostwald CSD shape, initially formed at very small crystal sizes, then is preserved by supply controlled proportionate growth as the crystals grow larger.
In summary, the shapes of CSDs for illite and other minerals reveal growth mechanisms and reaction pathways for crystallization.

 

Jackson Mid-Career Clay Scientist Award

Colleen HANSEL
Woods Hole Oceanographic Institution, Woods Hole, MA (USA)

How microbes break the cryptic manganese cycle to form manganese oxides

The distribution of manganese (Mn) oxide deposits is variable throughout the geologic record and in modern marine sediments. Despite a wide diversity of aerobic microorganisms that can precipitate Mn oxides, the mechanisms of formation remain poorly understood. Further, the physiological basis for microbial Mn(II) oxidation remains an enigma.
We have recently revealed that Mn oxide formation by some fungal and bacterial species is a consequence of Mn(II) oxidation by the reactive oxygen species (ROS) superoxide (O2.-). This superoxide production occurs extracellularly, via activity of transmembrane, outer membrane, or secreted proteins. These Mn oxides are highly reactive and induce further Mn oxide formation via autocatalytic oxidation of Mn(II) that leads to oxide ripening and ultimate loss in reactivity. While we have shown that extracellular superoxide production is widespread in bacteria, this production does not necessarily confer the ability to produce Mn oxides. Indeed, the back-reaction between the products, Mn(III) and hydrogen peroxide (H2O2), formed upon Mn(II) and superoxide reaction inhibits Mn oxide formation. These findings indicate that at least two processes – the generation of superoxide and consumption of hydrogen peroxide – are requisite for superoxide-mediated Mn oxide formation.
Nevertheless, under controlled abiotic conditions, Mn oxides formed via ROS cycling remain as nanocrystalline phases that do not aggregate or ripen with time. We find high concentrations of organic carbon (OC), including enzymes involved in the superoxide production process, associated with the Mn oxides points. This association along with oxide growth morphologies point to a role for organic templates in the precipitation and aggregation of Mn oxides. Together, we find that microbial mediated Mn oxide formation involves a complex network of biotic and abiotic reactions necessary for Mn(II) oxidation, Mn(III) stabilization, and Mn oxide precipitation and aggregation. Controls on each of these factors will therefore dictate the precipitation and distribution of Mn oxides within the environment. Within the ocean, targeting the controls on these three processes may allow for insight into the geographic variability often observed in Mn oxide and associated nutrient profiles along continental shelves and within coastal environments.

 

George W. Brindley Clay Science Lecture

Bruno LANSON
ISTerre, Université Grenoble Alpes , Grenoble (France)

HCrystal structure of defective lamellar minerals and their X-ray identification: Implications for reactivity

Layered minerals and materials are ubiquitous and characterized by the frequent occurrence of stacking defects that are key to their reactivity. These defects range from local defects such as isomorphous substitutions, layer vacancies, or atomic displacements, to random or well-defined stacking faults induced by non-periodic layer rotations, translations or twinning, and to mixed layering resulting from the coexistence within a given crystal of layers having different structure, thickness, or interlayer displacement. The occurrence of stacking faults in lamellar structures is favoured by the energetic similarity of the different stacking modes, owing to the weak interactions between adjacent layers. In addition, layered materials and minerals often exhibit minute crystal sizes that may be considered as an additional type of defect because of the induced disruption of the three-dimensional crystal periodicity.
To determine, control, or predict mineral / material reactivity a detailed structural characterization of layered structures, including structure defects, is thus essential, and X-ray diffraction (XRD) has been the preferred method of investigation for this purpose. However, as a result of their non-periodicity or their reduced periodicity, the diffraction maxima recorded from these compounds do not strictly obey Bragg’s law, thus hampering the use of conventional diffraction approaches. Profiles and intensities of diffraction maxima are affected by the nature, content, and distribution of structure defects, however, thus allowing the determination of these parameters with diffraction techniques and specific algorithms have been developed.
From the results of a few examples, this presentation will outline how our understanding of defective structures and mixed layers has improved over the last decade or so and describe some of the new perspectives opened by this improvement, with special emphasis on the induced reactivity.

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