Thermodynamics and Kinetics of Water-Rock Interaction

Book review by William M. White, Cornell University, Ithaca NY

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Thermodynamics and Kinetics of Water-Rock Interaction, edited by Eric Oekers and Jacques Schott, is the latest volume in the very valuable series, Reviews in Mineralogy and Geochemistry, published by the Mineralogical Society of America and the Geochemical Society. This 35 year-old series began with an exclusive focus on mineralogy but expanded to geochemistry a few years later with the 1981 publication of volume 8 of the series: Kinetics of Geochemical Processes. That volume is one the most well-thumbed book on my bookshelf; I have relied on it heavily in teaching geochemistry. It had a heavy focus water-rock interaction, so I was expecting this new volume to be, in part, an update and extension of that volume, as well as some the classic, but now aging, texts, such as Principles and Applications of Aquatic Chemistry by Morel and Hering (1993). It succeeds in this respect, but it is also more. The book is based on a short course held prior to the 2009 Goldschmidt meeting in Davos, but according to the preface its real origins date to summer courses for graduate students held over the previous decade. From this, one might reasonably assume that the target audience is graduate students. While some chapters do provide brief introductions to basic thermodynamics and kinetics, the book is written at a high level and is appropriate mainly for advanced students and those already familiar with the field. As is generally the case with multi-authored treatises, readers will likely be interested only in reading selected chapters rather than reading it cover to cover.

The first chapter, Thermodynamic Databases for Water Rock Interaction, provides a brief but useful overview of thermodynamic concepts and variables and background as to how fundamental thermodynamic data have been determined. But it mainly serves as a warning to those who rely on nominally sophisticated thermodynamic computer models such as PHREEQC. This is because the fundamental thermodynamic data, upon which these models depend, would seem to be in sorry shape. The authors point out the estimates of the equilibrium constant for fluoro-apatite dissolution range over 10 orders of magnitude. Table 2 of that chapter provides a selection of thermodynamic databases (datasets might be a better word - these are not organized, searchable databases in the modern sense), the most recent of which is from 1998 and the most extensive remains that of Helgeson et al. (1978). The authors' certainly succeed in making their point that 'it is best to consider the current state of thermodynamic databases as a work in progress rather than a completed task.' One can only hope that the chapter will lead to a community effort to address the situation.

The next few chapters provide an overview of the current understanding of modeling water-rock interaction in detail. I found them all useful and they enhanced my understanding of the subject and will impact my teaching. There is some duplication here, but that may be just as well as some authors' approaches will resonate with some readers, while other approaches will resonate with others. I found Manual Prieto's chapter on Thermodynamics of Solid-Solution Aqueous Solution Systems and David Sherman's chapter on Surface Complexation Modeling: Mineral-Fluid Equilibria at the Molecular Scale to be particularly useful. I also found the chapter by Schott, Pokrosky, and Oelkers entitled The Link Between Mineral Dissolution/Precipitation Kinetics and Solution Chemistry particularly useful because it did an excellent job of linking theory with experimental observation.

The longest chapter, over 100 pages, is entitled Organics in Water-Rock Interaction, but it may have been the chapter from which I took away the least. Organic compounds are, of course, ubiquitous in natural waters and the number of different organic compounds potentially present in water is overwhelming - the chapter contains a 5-page table just listing this compounds and their role in water-rock interaction. So it is certainly a broad topic. Consequently, the uninitiated, such as me, will be looking for simplifying and unifying concepts but won't find them in this chapter. That may be, of course, because there are none.

The chapter entitled Mineral Precipitation Kinetics begins with a useful review of the fundamental kinetic considerations in nucleation and crystal growth, and appendices usefully extend these concepts from the simply spherical case to more realistic geometries. Much of the chapter, however, describes the NANOKIN code developed by the authors and it applications. What is missing from this is any attempt to compare predictions of the code to observational data. One is thus left wondering how useful this code actually is.

The last few chapters take an integrative approach applying thermodynamics to the outcrop, watershed, and global scales. The first of these is a chapter entitled Towards an Integrated Model of Weathering, Climate, and Biospheric Processes. The chapter succinctly reviews efforts of the authors over the last few years to advance global models relating water-rock interaction to climate. This is of course, an important area of inquiry and one that attracted wide attention since the BLAG (Berner, Lasaga and Garrels) model was introduced 25 years ago. One comes away with the impression that the science has indeed advanced greatly since BLAG, for reasons that include a greater willingness to tackle the complexity of the issue, greater computer power, and greater understanding of underlying thermodynamics and kinetics. I also liked the 3 subsequent chapters because they tied theory to observation, demonstrated both present capabilities and limitations.

In describing the background history of the book in the preface, the editors relate that they 'received numerous demands from our students requesting a book to help them follow the subject as rapidly lost among the equations, symbols, and conventions, and standard states.' In this respect, the editors could have served students better. To begin with, one would hope that in the 21st century geochemistry would finally get around to using SI units exclusively. The first chapter, however, insists on using calories rather than joules as the unit of energy and one subsequent chapter uses both (!). There is little that is more daunting in geochemistry than the great number of parameters involved and this becomes all the more confusing when different symbols are used for the same parameters. Some of this may be unavoidable as convention has evolved so that some symbols represent more than one common parameter. For example, Q is commonly used to represent heat, mineral surface charge, the reaction quotient, and diffusive or advective flux. Readers would have been better served, however, had the editors made some effort to mitigate this problem and enforce a uniform system of symbols throughout the book. For example, ρ and I are used in the conventional way in several chapters to represent density and ionic strength, respectively, but are used, unconventionally, to represent radius and the ratio of reaction quotient to equilibrium constant, respectively, in the Mineral Precipitation Kinetics chapter. That ratio is elsewhere represented by Ω; why not use Ω throughout? Brantley and White go so far as to warn the reader in the introduction to their chapter on Approaches to Modeling Weathered Regolith that 'symbol definitions differ' throughout the chapter! At the very least, perhaps a list of symbol definitions at the start of each chapter would have helped.

Notwithstanding these complaints, the volume represents an up-to-date overview of water-rock interaction by the leaders in this field. It will be a useful addition to the bookshelves of those of us who teach geochemistry and to graduate students and others with an interest in this topic.

Helgeson, H. C., J. Delaney, H. W. Nesbitt and D. K. Bird (1978) Summary and critique of the thermodynamic properties of rock-forming minerals, Am. J. Sci., 278A:1-229.

Morel, F. M. M. and J. G. Hering (1993) Principles and Applications of Aquatic Chemistry, John Wiley and Sons, New York, 588pp.

Lasaga, A. C. and R. J. Kirkpatrick, eds. (1981), Kinetics of Geochemical Processes, Reviews in Mineralogy and Geochemistry, vol. 8, Mineral. Soc. Am., Washington 398pp.