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Home | Publications | EMU Notes in Mineralogy series | EMU volume 8, chapter 6

EMU Notes in Mineralogy - volume 8

Nanoscopic approaches in Earth and planetary sciences (F. Brenker and G. Jordan, eds)

Chapter 7: Reactivity of mineral surfaces at nano-scale: kinetics and mechanisms of growth and dissolution

Carlos M. Pina and Guntram Jordan

Introduction

Natural sciences have experienced important developments when the insights into both matter and its interaction with the environment advanced to the next scale in space and time – irrespective whether the direction was towards larger or smaller scales. A landmark advance took place at the end of the 19th century when the understanding of matter as composed of atoms eventually outranged the perception of matter as an energetic continuum. Although physical and mathematical discontinuities always pose serious problems, the atomistic perception of matter had the advantage of providing a valuable microscopic aside to thermodynamics, a new perspective to study the interaction between matter and radiation, and a better understanding of numerous properties of solids and liquids. Additionally, the concept of atoms has been proven to be a key to understand and quantify the kinetics of chemical reactions. The atomic age had begun.

However, the advances derived from the idea of atoms not only led to new insights into matter, they also made it necessary to develop new models for the reactions of a solid composed of atoms (i.e. composed discontinuously) with a liquid phase composed of the same or a different component (i.e. melt or solvent). Among the reactions that can occur between a solid and a liquid, growth and dissolution are ubiquitous and very important to understand. Growth can be described as the formation of a (crystalline) solid from a liquid phase whereas dissolution can be perceived as the decomposition of a solid and the transfer of its constituents into a liquid phase.

In the late 1920s, Kossel (1927) and Stranski (1928) independently of each other developed a model to describe the growth and dissolution of a discontinuously composed solid from/into a melt or solvent. The model considered the solid as composed of monotypic building units with an exclusively octahedral coordination. Thus, growth and dissolution were interpreted as the incorporation/removal of such building units into/from specific positions on the crystal surface, respectively. Ironically, the flaw in the model was that the solid has been perceived as flawless: the solid missed defects which later were understood to play a key role in the kinetics of the reaction between a crystal and a liquid phase under close-to-equilibrium conditions. In the mid-19th century Burton, Cabrera and Frank (1951) published the first comprehensive theory on ‘The growth of crystals and the equilibrium structures of their surfaces’. One of the main ideas in the so-called BCF-model was to consider the role of lattice defects and, particularly, screw dislocations in growth and dissolution.

Unfortunately, despite the long-standing models of atoms, ions and molecules as well as the models describing reactions between solids and liquids on an atomic or molecular scale, experimental data and proof of the theory were largely lacking, mainly because access to the atomic scale still was extremely limited. The available methods which gained insight into the molecular scale were either spatially integrating over many orders of magnitude of atoms (e.g. X-ray diffraction) or were operating in vacuum and, therefore, were unapt to provide access to the dynamic nature of solid-liquid interaction at an atomic or molecular scale. The most advanced studies in those days were probably interferometry investigations (Griffin, 1950; Verma, 1951) and scanning electron microscopy (SEM) investigations on decorated mono-layer steps, which at least proved the existence of spirals on crystal surfaces (e.g. Bethge, 1962a,b).

The situation changed in the last two decades of the 20th century with an important development. The in situ access to solid-liquid interfaces with molecular resolution became possible by applying atomic force microscopy (AFM). AFM is basically a derivative of the scanning tunnelling microscope (STM) which in its full capabilities had been introduced by Binnig et al. (1982). (The description of a new microscope, known as a topografiner, which technically had much in common with the STM later developed by Binnig and Rohrer, was published by Young et al. (1972). The topografiner can be considered as a clear precursor of all the so-called Scanning Probe Microscopes.) For the development of the STM, Binnig and Rohrer shared the 1986 Nobel prize in physics with Ruska, who was awarded the prize for his contributions to the development of electron microscopy. Both the scanning probe microscope family and the electron microscope family for the first time provided real experimental access to the molecular-scale: the nano-age had begun.

Since then, vigorous experimental activity has led to extensive knowledge of the kinetics of solid-liquid interface reactions. Parallel to these insights, rapid development of further methods with nano-scale resolution led to new perspectives on the processes taking place in the nanometre range and on physical and chemical properties of nanometre-sized objects. Three points became clear: (1) there are very few macro-scale analogies for many properties of nanometre-sized objects; (2) in order to control or just to understand macro-scale processes, it is essential to understand or even control the processes in the scale ranging from the size of a single atom or molecule to a few tens of nanometres; and (3) spatial integration of methods is generally unsuitable at providing the necessary nano-scale information.

Molecular-scale processes taking place at the interfaces of minerals and aqueous solution have huge implications on macro-scale processes. Macro-scale processes range from the size of single crystals or aggregates (local-scale), via the size of geological units such as deposits, catchments, aquifers, lakes (regional scale), to the global cycling of chemical compounds and elements (global scale). Local-scale processes are numerous in nature and technology. The synthesis of single crystals by crystal growth techniques as well as the formation of binders or ceramics are examples of local-scale technological processes. However, there are also undesirable local-scale processes to control such as the scale formation in boreholes of oil wells or in pipes of geothermal power plants. In nature, biomineralization is a local-scale process which has recently attracted broad attention. One reason for this attention is the unknown mechanisms of the generation of very sophisticated crystal morphologies and textures which often lead to remarkable materials with outstanding properties. Crystal morphology, texture and properties, however, are local-scale consequences of crystal-water interface processes which are taking place in the scale ranging from the size of atoms or molecules up to a few nanometres.

Nano-scale interactions between mineral surfaces and aqueous solutions also actively control regional-scale conditions and thus control e.g. the formation of hydrothermal deposits or the chemical composition of catchments, lakes and aquifers. On a larger scale, molecular-scale mineral-water interactions, for instance, even control the transfer rate of chemical elements between the hydrosphere and the lithosphere as part of the global cycling of elements. Thus, a detailed study at nano-scale of mineral-water interactions is essential for an understanding of the global cycles of chemical elements which significantly influence the present and as well as the ancient climate. Moreover, when natural environments (on a regional or planetary scale) are contaminated as a result of anthropogenic activities, the fate of the environment depends on the ability of mineral surfaces to immobilize the contaminants.

In general, mineral-water interactions at interfaces essentially comprise (1) alterations in physical and chemical properties of both the solid and the liquid side of the interface and (2) dynamic material fluxes between the adjacent phases. Material fluxes can occur in very small amounts, e.g. in the case of sorption of trace elements. However, the main processes associated with material flux are crystal growth and dissolution. Thus, in this chapter we will focus on these two reactions. Even though the number of studies on nano-scale aspects of growth and dissolution is immense, we will try to summarize a few of the most recent contributions to the ongoing research of growth and dissolution from a nano-scale perspective. In doing this, our main focus lies on information obtained by AFM. The high spatial resolution of AFM has been proven to be a very helpful tool in investigations on the nano-scale aspects of crystal growth and dissolution, regardless of whether the liquid phase is a multi-component aqueous solution, an aqueous solution containing organic or inorganic impurities, or merely pure water. The in situ capabilities of AFM have made the method a primary tool for providing access to the dynamics of interface processes. Thus, it is a logical consequence that we will place special emphasis on the nano-scale aspects of the dynamics of growth and dissolution.

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