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Abstract

Because clay particles cannot be seen with a microscope, the initial petrographic-mineralogical approach, optical petrographic microscopic study, cannot be used. This fact has been a hindrance to the identification, description and systematic study of clay minerals. The foundations of the knowledge of clay structures were laid, and the basic mineral names given, in the late nineteenth and first half of the twentieth centuries. However, much imprecision characterized clay mineralogy. In favourable cases, such as samples with only one mineral type present, it was possible to give a general chemical formula and, with the aid of the initial stages of X-ray diffraction, establish the structural family of clay samples. However, clays are most often found in aggregates containing several different minerals, with millions of grains per gram of sample, and the methods of bulk chemistry or powder diffraction did not permit a sufficiently precise definition of the clay minerals, so that often a mixture of phases was present when one assumed a pure species.