The Source Clays Program of The Clay Minerals Society was initiated in 1972 to distribute a set of reference clays, so that distributed clays could be identical for all recipients. Because most clays do not consist of a single phase, the immediate objective was not to produce a pure product consisting of one clay mineral, but to provide a uniform product. These materials were collected and processed carefully, and sufficient amounts were collected so that material was available for researchers for many years, Large numbers of researchers were thereby assured of working on identical material. Initial descriptions of these materials were presented in the Data Handbook (van Olphen and Fripiat, 1979). An updated version of this book was suggested several years ago because of the availability of new analytical techniques and to provide descriptions of material added to the reference set since 1979.

Clay minerals share a basic set of structural and chemical characteristics (e.g. they are largely aluminosilicates with layer structures) and yet each clay mineral has its own unique set of properties that determine how it will interact with other chemical species. The variation, in both chemistry and structure, among the clays leads to their applications in extremely diverse fields. Common and important industrial applications of clays are in the manufacture of paper, paint, plastics and rubber. One of their more recent and most economically important applications is in the petlitter industry where their adsorptive and deodorizing properties are used. Specialty uses include clay additive to chicken feed to boost nutritional uptake by the chicken, and in using clay as fillers and major ingredients in pharmaceuticals and cosmetics. Clays are used for their catalytic properties and for their ability to adsorb greases, fats and other organic materials. Those who exist with scarce resources frequently collect clays from local deposits and ingest them as a source of dietary minerals. It is difficult for a day to go by without using a product incorporating clay minerals, as we all use ceramics such as dinnerware and sanitaryware.

In any clay deposit, the nature of the mineral assemblage and the composition of individual clay minerals can change radically in a few em. Consequently, any given locality can contain many subtly different types of clay minerals. Results from different laboratories on ostensibly the same clay material may not always be comparable because the samples may indeed not contain an assembly of identical clay minerals. Such confusion slows the understanding of this important group of minerals. Several attempts were made to provide investigators with reasonably constant clay materials, the first being that of the American Petroleum Institute Project 49 (Kerr, 1949). The Clay Minerals Society Source Clays project proposed to provide investigators with gently homogenized clay materials, carefully collected and processed under the supervision of clay specialists. The collection would include metric ton amounts to ensure a long-lasting collection. The program began in 1972, with the introduction of the materials described in this paper. Later the program expanded to include the Special Clays. These samples are materials not amenable to homogenization, or they are available in very small amounts.

Chemical analysis is an essential step to establish the nature of minerals (Newman, 1987). The techniques used in rock and mineral analyses are generally valid for the analyses of clays. Additional information from other analytical techniques, which are mentioned here, is needed for accurate interpretation of the chemical analysis results of major elements (Gabis, 1979). In traditional chemical analyses, the aim is to obtain accurate analyses for all elements present in the sample, in such a way that the sum of elements expressed as oxides, including hydration and structural water, approaches the sample weight as closely as possible.

Inductively coupled plasma-mass spectrometry (ICP-MS) is ideally suited for the rapid and simultaneous analysis of multiple elements in geological materials, including soils, organic substances and most rock types (Eggins et al., 1997; Long erich et al., 1990; Jenner et al., 1990). The application to clay minerals is more recent (Jain et al., 1994). This technique is highly sensitive, and is capable of analyzing a wide range of isotopes covering the entire mass spectrum Consequently, ICP-MS is used to measure directly rare earth and platinum group elements at ppb levels without preconcentration.

The layer charge is perhaps the single most significant characteristic of 2: 1 layer phyllosilicates. Layer charge affects cation-retention capacity and adsorption of water, and various polar organic molecules. The effects of layer charge on the sorptive properties of organo- clays were illustrated by Lee et al. (1990). It is generally agreed that the classification of 2: 1 silicate clays, which is a continuing problem, may be resolved by taking into account the magnitude of the layer charge (Bailey et al., 1971; Malla and Douglas, 1987). Studies on structural chemistry also confirm the importance of the layer charge for the characterization of the 2:1 phyllosilicates (Newman and Brown, 1987).

The Clay Minerals Society maintains a repository of Source Clays to provide scientists and researchers with a readily available supply of consistent materials so that research conducted by different groups can be correlated to identical material. These Source Clays include kaolinite, smectite, chlorite, vermiculite, illite, palygorskite and other minerals. As most of the Source Clays are naturally occurring materials, they typically contain minor to significant amounts of other mineral impurities. When conducting research using these samples, it is important that their mineral content be well characterized. It is also often desirable to remove these impurities to produce pure clay samples. For example, when studying the biological effects of a particular clay mineral, it is imperative that all contaminants (e.g. crystalline silica minerals) be removed from the clay in question so that experimental results can be attributed to the clay alone. If purification is required, the clays must be purified in a manner that does not significantly alter the physical or chemical properties of the samples. This chapter describes the X-ray diffraction (XRD) characteristics of a suite of Source Clays of The Clay Minerals Society and demonstrates methods of purification based on Stokes' law of settling in suspension that can be used to purify the clays.

Infrared (IR) spectroscopy has a long and successful history as an analytical technique and is used extensively (McKelvy et al., 1996; Stuart, 1996). It is mainly a complementary method to X-ray diffraction (XRD) and other methods used to investigate clays and clay minerals. It is an economical, rapid and common technique because a spectrum can be obtained in a few minutes and the instruments are sufficiently inexpensive as to be available in many laboratories. An IR spectrum can serve as a fingerprint for mineral identification, but it can also give unique information about the mineral structure, including the family of minerals to which the specimen belongs and the degree of regularity within the structure, the nature of isomorphic substituents, the distinction of molecular water from constitutional hydroxyl, and the presence of both crystalline and non-crystalline impurities (Farmel, 1979).

Thermal analysis involves a dynamic phenomenological approach to the study of materials by observing the response of these materials to a change in temperature. This approach differs fundamentally from static methods of analysis, such as structural or chemical analyses, which rely on direct observations of a basic property of material (e.g. crystal structure or chemical composition) at a well-defined set of conditions (e.g. temperature, pressure, humidity). Clay minerals are highly susceptible to significant compositional changes in response to subtle changes in conditions. For example, changes in the fugacity of water affect the stability of interlayer H2O in a clay mineral (see below). Therefore, care must be taken that all experimental conditions are known with accuracy and precision

The cation exchange capacity (CEC) of fine-grained materials, and especially clay minerals, is a fundamental property of these materials, and can be determined routinely. A search of the recent literature illustrates the great interest of this property to researchers. For example, a search of the GeoRef database for references to “cation exchange capacity” for the years 1980 to 1999 yields 2559 citations.

The interface between the surfaces of clay minerals and other materials (aqueous solution, organic moieties, biomolecules, etc.) is of great importance to many geological, technological and biological processes. The examination of the structure and composition of mineral surfaces is advancing rapidly. However, the crystal structure of a mineral is only part of the information needed to understand the activity of the surface; we still do not fully understand the chemical and physical interactions at the surface or interface. One very powerful methodology for the study of surfaces and interfacial interactions is the determination of the thermodynamic properties of the surface. It became possible in the late 1980s to determine quantitatively the apolar and polar surface-tension components and parameters of liquids and solids (van Oss et al., 1988; van Oss, 1994), and the experimental techniques necessary for obtaining surface free-energy components of minerals, and particularly of colloid-sized minerals, are also a recent development (Costanzo eta!., 1990; van Oss et al., 1990, 1992; Giese et al., 1991).