I wasn’t very far into reading Bentonites: Characterization, Geology, Mineralogy, Analysis, Mining, Processing and Uses when I came across the following phrase: ‘The clay of 1000 applications’. This might have been a very apt subtitle for a book on bentonites given the wide variety of uses to which bentonites have been put, across many fields. An underlying theme of the book also emphasizes how variability in the characteristics and properties of bentonites – often from the same mine – and more specifically their dominant smectite mineral component may have a profound influence on the suitability of one bentonite compared to another for any particular use. The approach taken to investigate this variability is unique in using a wide-ranging collection of some 40 samples to perform an internally consistent testing and data-rich exploration of characteristics and properties, both in terms of the material bentonite and its key mineral component: smectite. The second part of the book takes a detailed tour of 17 key industrial applications of bentonites and the challenges that their variable properties may pose. There are seven chapters in total, preceded by a short introduction that sets out the scope and aims of the book. Foremost among these aims is to document industrial state-of-the-art perspectives and to consider the diversity of bentonites, which defines many of the research challenges that remain for future industrial and academic investigation. At the outset, a key point that is stressed is that many of the properties that affect bentonite applications cannot always be measured accurately, and others may yet be unknown. In terms of diversity, it also stressed that this knowledge must be shared by interdisciplinary teams that may be working on a common application but from different directions. The introduction also lists some key books and references into which the reader may wish to delve for additional background, particularly on methods. Among these are two classic texts that were published only in German, but which I suspect go a long way towards explaining why so many accomplished contributors to this book on bentonites have a close association with the DTTG (Deutsche Ton- und Tonmineralgruppe), an association of mainly German, Swiss and Austrian clay researchers. (As a tangent, I cannot resist the temptation to appeal to the DTTG to find a way to reprint these classic German texts in English, so that a much wider audience might benefit from their wisdom; it’s never too late!)

Chapter 1 opens with a nicely written, compact and precise outline of the crystal chemistry of the smectites. This is followed by a summary of the terminology and classifications that are applied to smectites and bentonites. Bentonite classification schemes are often based on the name of a locality or a prevalent characteristic, which, although not an ideal foundation for classification, is certain to persist, especially in industry. The remainder – and, indeed, the bulk – of Chapter 1 focuses on the genesis of bentonite deposits. It begins by proposing a new pragmatic and simple classification that divides bentonites into primary and secondary types, a scheme that sounds analogous to the well-known and widely utilized classification of kaolins, although this is not mentioned. This is a very well-illustrated part of the chapter with its focus mainly on primary bentonites, most of which formed from volcaniclastic deposits. Details of the roles of the original rock (protolith), temperature, fluids and hydrology are considered. Rarer primary bentonites, for which the protolith is not a volcanic rock, and primary chemically precipitated lacustrine bentonites, which are typically rich in Mg-smectites, are briefly considered. A final short section considers secondary bentonites, for which sedimentary transport and redistribution are key factors in the genesis of the bentonite, although clearly such types are a minority. In my view, the only obvious omission from this opening chapter is that it might have been instructive to include some mineralogical and geochemical information on the characteristics of the various processes and pathways by which volcanic glass alters to smectite, and perhaps also to outline some of the main common questions encountered surrounding bentonite origins.

Chapter 2 is the briefest of the seven chapters, at just five pages. It gives broad details of the origins of each of the 40 samples from some 18 different countries, including colour photographs of most and brief details of the overarching high-level, bulk-sample preparation. The motivation for assembling this collection has its origins in 2002, based on the already maturing schemes for the inclusion of bentonite as a component of the barrier systems in many concepts for the safe disposal and containment of high-level radioactive waste (HLRW). Given the diversity of parameters requiring investigation, the rationale was that a comparison of just a few key samples, as was often done previously, would be woefully inadequate, but the set of 40 samples here, including some well-known sources such as ‘MX80’, as well as two dominated by illite/smectite rather than smectite, should be sufficient to reveal ‘the secrets of bentonites’.

At 100 pages, Chapter 3 on sample characterization accounts for more than a third of the text, and it is undoubtedly the heart and core of this book. The range of properties that are measured is comprehensive, and, in addition to a presentation and analysis of the analytical data, each section begins with a short discussion of the salient points of each method, although some are covered in more detail than others. Topics covered include drying, grinding and disaggregation, preparation of purified and homo-ionic samples, particle-size fractionation, submicron particle-size analysis, chemical composition of bentonite, structural formulae, X-ray diffraction (XRD), intercalation for XRD analysis, infrared spectroscopy, thermal analysis, pH of bentonites, permanent charge, layer-charge distribution, variable charge, tetrahedral and octahedral charge, microstructure by scanning electron microscopy (SEM), smectite morphology, external surface area, internal and total surface area, porosity of bentonites, quantitative mineralogical composition, amorphous components and, lastly, iron in smectites and bentonites. The chapter is rich in tabulated data for the samples: 10 of the tables in this chapter detail parameters for all 40 samples, reflecting an extraordinary analytical effort. Although with some other characteristics, such as structural formulae, constraints such as persistent contamination of the fine fractions required to determine these characteristics compromised the data for most samples. Also, the lack of a table detailing the Brunauer–Emmett–Teller (BET) parameters for the samples seems like an oversight, especially as such data are used in figures in this (3.70) and the following chapter (4.17), so presumably most samples were measured. Most analysis of the data has been performed by correlation plots, although in some instances it is tricky to ascertain whether the plots include points for all 40 samples or are restricted to subsets of the data; an ‘N = X’ would have been useful. If you are prepared to carefully read through this long chapter, you will be rewarded with constant insights as to how various characteristics appear to relate to each other, but also, and perhaps more importantly, you will see where gaps in knowledge and understanding are still present, as well as where current methods still struggle to readily characterize the diversity of bentonites, such as variations in morphology and fine particle-size analysis, identifying where new methodological innovations are required.

The fourth of the chapters turns its focus to ‘properties and performance’, including sections on cation-exchange capacity (CEC), rheological properties, critical coagulation concentrations, water uptake, free swelling, swelling pressure and specific electrical resistivity. The data on CEC are also included as a table for all 40 samples, but this is the only table in this chapter, and variations in properties are primarily explored graphically. Very little is said directly about ‘performance’, so it did feel like it was simply left to the reader to assume that the properties covered, all of which relate to how bentonites may ‘behave’, are the key ones for assessing their ‘performance’ in most applications; a bit more of the performance context might have been useful. In many instances it was also clear that some of these key properties may suffer from the ‘observer effect’ in part because of the complex preparations that are required to measure them, such as where surface properties may be unavoidably affected by pretreatments; again, more scope for innovative method development perhaps.

Chapter 5 signals the start of a move away from the previous data-rich chapters, and it begins with some well-illustrated sections on the exploration and mining of bentonite. First, we have some details on production figures for tonnages up to 2018, which seem a little outdated for a book with a 2024 publication data, but the latest values are easy to access online, so this not a serious issue. It appears that there is as much diversity in mining as the bentonites themselves, but this mining is dictated by factors such as size and geography along with local traditions in terms of the level of technologies used. However, the chapter soon progresses to an analysis of data in terms of quality management of the mine and in the laboratory. The latter covers various pragmatic methods widely used in industry to assess key quality indications, including bentonite composition, water uptake and swelling, along with various rheological parameters.

Processing is the subject of Chapter 6, and, following a brief historical account of the early developments in this field in Germany, the chapter covers alkali activation, acid activation, organic activation, pillared clays, various means of drying, purification and blending. This is another richly illustrated chapter, with 17 high-quality colour figures, although Figure 6.7(E), showing XRD traces of progressive decomposition with acid attack, has an error in the x-axis scale. In general, I only noted half a dozen or so minor typos in the book, and the quality of editing and production is very good throughout.

At 51 pages, Chapter 7 – the final chapter of the book – is the second largest of them all. It covers 17 different applications of bentonite, broadly divided into those that use bentonite in a dry form and those that use it in a wet form. The chapter begins with a general discussion of bentonite as an adsorber, which is the central function in many dry applications. Three of four of the main applications of bentonite (by tonnage) are included in the dry-form applications, namely cat litter, foundry bentonites and pelletizing of iron ore. Other dry-form applications include animal feeds, landfill sealing, ceramic bodies and disposal of HLRW. Several pages are devoted to each topic, detailing the functions of bentonite in the various applications. Anyone in need of a good synopsis of the issues being considered in the use of bentonites in HLRW concepts will profit from this compact and well-structured account of this application, which is usefully divided into 10 subtopic areas of the issues that might arise and the research efforts aiming to understand them. The so-called wet applications that are covered include clarification of beverages, civil engineering for sealing- and barrier-wall applications (where bentonite is often used in conjunction with cement), drilling fluids, paper production, healing clays, bleaching earths for the refining of fats and oils, organoclays in paints, catalysts and uses in nanocomposites. Many of the wet applications are still niche applications in many respects, so the main details of these application areas are well summarized for most, with the possible exception of nanocomposites, a rapidly growing area of research application, for which the summary is already bit dated, although, to be fair, this is one application that is likely to remain niche in terms of the global tonnages of bentonite that industry will consume.

Overall, this is a very welcome addition to the literature on bentonites. All of the chapters are well written and presented, although the style and layouts vary somewhat, with only the first four chapters making use of summary callouts. If you are working with bentonites in industry, you will find this book an invaluable reference; if you are engaged in research on bentonites or smectites, you will find it a rather useful pointer to where current knowledge is still lacking or incomplete. Stephan Kaufhold as editor has done a great job of getting this book over the activation energy barrier (which I know from speaking with Stephan over the past several years has required much perseverance), and as the author or a coauthor of all seven chapters, he has clearly been the catalyst for fusing the academic and industrial perspectives together to more than a simple sum of these sometime disparate endeavours. I know of no other ‘clay’ book that does this, and I wholeheartedly recommend it as a very useful addition to your bookshelf. If you buy it only to explore the data, you will have found a bargain for sure.