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Clays from the Dakota

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The basic properties of these clays are chemical composition, mineral composition, and particle size and shape. It is assumed that if the exact percentage and chemical composition of each of the minerals present were known, the chemical composition of the whole clay could be calculated. The real problem involved is that of identifying the clay minerals with sufficient exactness. Fairly accurate estimates of the percentage of each of the clay mineral groups present can be made. Kaolinite, which has a definite chemical composition, can be identified with relative ease, but the montmorillonite and illite groups include many members, each of which has a different chemical composition. Exact identification of members is difficult and often impossible. The correlation obtained in this study between (1) mineral identification and estimated percentages and (2) chemical composition, is due in part to the fact that the major constituents, kaolinite and quartz, have a definite chemical composition. It is also probable that the assumption of an "average" composition for each of the clay mineral groups was justified. The use of x-ray diffraction, the electron microscope, the petrographic microscope, and differential thermal analysis constitutes a concerted attack on the problem of mineral identification. A lack of agreement from one method to the other indicates error in a determination. The chemical analyses represent a positive check on the other methods, although it is one that is difficult to apply.

Particle shape and size determinations will correlate with those for specific surface inasmuch as they are different aspects of the same physical properties. Particle size and shape (or specific surface) is in turn directly related to the characteristic size and shape of the minerals found in the sample.

The basic or fundamental data should be considered as one class of information about the clay material, and the ceramic data another class. The relation between these two classes of data is of considerable importance. The basic data explain the behavior of the clay materials under the conditions of ceramic tests, and can be used as a guide to control ceramic properties. Sometimes the determination of one or more of the basic properties can be used as a positive check on the quality of clays mined for ceramic use, or as a means of finding a new clay deposit.

Very little use has been made of the fact that easily obtained ceramic data can be used as a method for the rough determination of basic properties. In many cases an estimate of the relative proportion of the minerals present in a clay material can be made from a careful study of ceramic data. The close correlation between drying shrinkages, shrinkage water, and specific surface has been shown. In materials having approximately the same clay mineral assemblage the ion-exchange capacity of the materials is indicated by the same properties. There is also a clear relation between the fluxes present in the clays and firing shrinkage, absorptions, and porosity. This factor is largely a reflection of the kind and proportion of clay minerals present but it is modified by the quantity and particle size of the quartz that is usually found in clay materials.

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Kansas Geological Survey, Geology
Placed on web June 22, 2007; originally published Dec. 1954.
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