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.

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ChairmanScientists involved with a clay, even in the smallest sample, immediately face problems inherent in dealing with an enormously large number of exceedingly small crystals. These crystals frequently vary among themselves and often represent only a metastable compromise with their environment. The nature of the crystals can change radically in various parts of the same geologic deposit. Furthermore, the process of separation from the raw sample can alter the clay minerals to a significant degree. Thus, results from different laboratories on ostensibly the same clay mineral cannot always be compared because that sameness may be illusory.Consequently, the Clay Minerals Society has established at the University of Missouri a Source Clay Minerals Repository, which will stock a metric ton each of several abundant natural clays and lesser quantities of more exotic clay minerals as such become available. The supply of each abundant mineral will be as homogenous as is possible within the constraints imposed by laboratory processing of a ton of material. The prime goal is to accumulate physical and chemical data on these samples. Initial characterization will be made in several laboratories and published in Clays and Clay Minerals. However, data generated by any investigator should be deposited in the repository. Thus over time, an extensive fund of information will emerge. This project, by its nature, is concerned with development of information. It should not be considered competitive with any other clay mineral distribution service.The samples have been selected at the deposit by qualified investigators. Because any beneficiation technique can cause changes in the clay minerals, treatment will be held to a bare minimum. It will involve only low temperature drying and crushing. As stated, homogenization will be as complete as practicable for such large samples.

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