Clay minerals were initially defined on the basis of their crystal size. They were determined as the minerals whose particle diameters were less than 2 μm. This limit was imposed by the use of the petrographic microscope where the smallest particle which could be distinguished optically was of this size. Clays were essentially those minerals which could not be dealt with in a conventional nineteenth century manner. Chemical analyses were nevertheless made of fine grain size materials, most often with good results. However, the crystal structure and mineralogical family were only poorly understood. This was mainly due to the impurities present in clay aggregates, either as other phases or in multiphase assemblages. Slow progress was made in the early twentieth century, but the advent of reliable X-ray diffractometers allowed one to distinguish between the different mineral species found in the <2 μm grain size fraction. Today we know much more about clay mineral XRD (X-ray diffraction) properties; perhaps too much at times.
KeywordsClay Mineral Octahedral Site Clay Particle Sheet Structure Octahedral Layer
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