Scanning Probe Microscopy of Clay Minerals

Edited by Kathryn L. Nagy and Alex E. Blum


A set of surface-sensitive analytical techniques, collectively called scanning probe microscopy (SPM), has been developed and applied to a wide variety of materials. With SPM one can image nearly any surface in vacuum, in air, or in solution, and often can actually observe surfaces during reaction. Scanning tunneling microscopy (STM) and scanning force microscopy (SFM) have been used successfully in the geosciences to characterize mineral surface structure and topography, surface reactivity, and the rates and mechanisms of mineral-water reactions. SPM techniques are most easily applied to materials with nearly flat surfaces, and minerals with good cleavage, particularly clays, which are excellent prospects for investigation. However, there have been relatively few applications of SPM to geologic materials, particularly in comparison to applications in physics, chemistry, material science, or even biological sciences. The purpose of this volume is to introduce the theory and operation of SPM to clay mineralogists, summarize previous work using STM and SFM in mineralogy, and outline the advantages and limitations of SPM for future research applications.

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    Rocks and minerals communicate chemically with their surroundings via reactions at surfaces. The surface of the Earth contains tens of trillions of square kilometers of mineral surface area that act as key chemical agents in geochemical processes at and near the Earth’s surface. Mineral surfaces influence the regional and global cycling of elements through partitioning, dissolution-precipitation, and catalytic reactions that occur in ground-, sea-, surface-, and atmospheric waters. For example, chemical weathering of silicate minerals, via surface-controlled dissolution reactions, is thought to be one mechanism for draw-down of atmospheric CO2 (Berner et al., 1983). Mountain-building episodes have been implicated in global climate control (Molnar and England, 1990; Raymo and Ruddiman, 1992; Berner, 1994). Global climate may be in part controlled by the rate of production of mineral surface area available for weathering. The study of reactions at mineral surfaces under a range of conditions is thus important for understanding processes from the local to the global scale. Understanding the behavior and influence of surfaces in natural processes is necessary in order to model natural systems at all scales and to predict their responses to perturbations. The structure and reactivity of mineral surfaces have therefore become an important and growing research subject.

    Because much or most of the surface area in geologic systems is associated with the clay minerals and clay-sized particles, these particles are often important in geochemical processes that depend on the catalytic, adsorbent, or other reactive behavior of surfaces. These minerals, and models/proxies for them and their surfaces

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    The development of the atomic force microscope (AFM) has opened exciting new possibilities for mineral studies. For the first time we can see the individual atoms, and the relationships between atoms, that make up mineral surfaces. It is also possible to use the AFM with a fluid cell to follow the progress of reactions between minerals and solutions as they take place in real time. In some cases the surface atoms can be deliberately removed with the AFM so that the atoms of the internal parts of structures can be seen. No other instrument provides the opportunity for such direct observation at such high, atomic-scale, resolution. The AFM was invented less than 10 years ago (Binnig et al., 1986) but has already become an important instrument in mineral studies, as this Workshop volume illustrates. The AFM has been used to study many problems in many fields other than mineralogy. Many of these uses have not required the high resolution the instrument is capable of, but it is the atomic scale resolution in studies of minerals that is one of the instrument’s more intriguing aspects and the subject of this chapter.

    The key to the AFM is the interaction between the atoms of the scanning tip and the atoms of the surface being studied. Ideally a single atom of the tip is attracted or repulsed by successive atoms of the surface being studied. However, this is a dynamic environment and there can be accidental or deliberate wear of the tip and

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    Mineral-water interactions are of fundamental importance in controlling the behavior of many natural and engineered earth systems. The processes of weathering and soil development, secondary precipitation and dissolution associated with diagenesis, oil field reservoir behavior, swelling properties of clay liners, and sorption of organic and inorganic contaminants are each influenced by mineral chemistry and physical properties. Until recently, investigations of processes occurring at mineral-water interfaces have necessarily relied upon experimental studies which measured changes in bulk solution chemical composition and/or made comparisons of initial mineral reactants with final reaction products. These methods give our current understanding of mineral solution interactions and behavior while also showing the tremendous complexity and heterogeneity of reacting mineral surfaces. Even from this knowledge base, our understanding of reaction processes continues to be severely limited at every scale from the time-dependent dynamics of macroscopic growth processes to the detailed kinetics and mechanisms of mineral-water reactions at the molecular level. With the recent invention of scanning force microscopy (SFM) and Fluid Cell attachments, it is now possible to span length scales from topographic to molecular and directly observe many different kinds of interfacial processes as they occur in aqueous solutions.

    The purpose of this chapter is to introduce the use of the Fluid Cell in SFM as a promising technique that complements traditional surface analysis and bulk geochemical methods in studies of mineral-water interactions. Our focus on the Fluid Cell necessarily limits us from considering other scanning probe techniques such as tapping mode, non-contact mode, magnetic or

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    Scanning force microscopy (SFM) is a new technique that can be used for the precise measurement of vertical and lateral dimensions of individual clay particles. SFM has several advantages over other techniques used for the observation of clay morphologies, such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Most notably, SFM can directly and quantitatively measure individual clay particle thicknesses, and reveal details of clay particle surface steps and other microstructures with an accuracy approaching ±0.15nm. Quantitative, high-resolution measurements of clay particle shapes and thicknesses with SFM provide direct evidence for the structure, chemistry and genetic origin of clays.

    The use of SFM is now common in the chemistry, physics and material science literature, but there have been few studies that apply SFM to clay mineralogy. Hartman et al. (1990) demonstrated molecular scale imaging of illites, and this is among the earliest published SFM studies. Lindgreen et al (1991) showed that SFM and scanning tunnelling microscopy (STM) could be used to measure illite particle thicknesses. However, SFM has not yet been widely applied to practical problems in clay minerals research, even though clay minerals have properties conducive to SFM study. Clay particles fall in a size range easily resolved with SFM, and clays have a large lateral extent relative to their heights. The sheet-like morphology reduces some of the severe limitations of SFM, such as limits on the resolution of rough topographies imposed by the tip geometry.

    This chapter concentrates on SFM imaging of clay particles at resolutions

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    Present address: University of Colorado, Dept. of Geological Sciences, Campus Box 399, Boulder, CO 80309-0399, USA.

    In the previous chapters we have seen that scanning force microscopy (SFM) is a powerful tool for examining physical and chemical properties of clay mineral surfaces. Eggleston (this volume) and Wicks et al. (this volume) have discussed the many applications of SFM to the determination of mineral surface structures and adsorption properties. Dove and Chermak (this volume) presented a summary of the utility of SFM for imaging mineral surfaces in solutions. Blum (this volume) showed how to acquire data on the morphologies of platy illites and thoroughly discussed the advantages and disadvantages of using SFM to measure particle morphologies. In this chapter, we go one step beyond the acquisition of morphological data on natural clay minerals and show how to use such data in conjunction with other geological information to interpret and quantify clay growth. We demonstrate how to use SFM to acquire data on morphologies of fibrous illite from sandstones and how to use the observed trends in morphology to extract reaction rates for the growth of fibrous illite. Such data complement the types of information that can be obtained using the methods described in this volume and provide a necessary link between the laboratory studies and nature.

    Morphological data in general can provide clues as to the type of reaction that last affected a mineral surface. For example, the size and density of etch pits on a mineral surface have been proposed as measures of a mineral’s dissolution rate (Lasaga, 1983; Lasaga and Blum, 1986; Brantley et al.)

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