4. Clay Mineralogy

When discussing clays, it is important to understand what they are, their origins, their chemical and physical properties, and their crystal structures. Clay minerals, a subgroup of the phyllosilicates, are a major and important type of mineral in the Earth’s crust. Phyllosilicates exist over a crystal-size range from traditional clay-size range to very large crystals. However, in terms of their impact on everyday life, small grain-sized geomaterials are of most importance. Chemical weathering of primary minerals is one of the processes of principal interest when fine-grained crystalline hydrous aluminosilicates are considered. There are a variety of detailed approaches for the orderly classification of clay minerals, which is outside the scope of this work; for simplicity, the two principal layer types and three clay mineral families—kaolin, smectite, and chlorite—are discussed here. Importance of pillared and lateritic (a source of rare-earth elements) clays is also highlighted. While there is no comprehensive nomenclature for mixed-layer clays, statistical treatment of the binary mixed systems can be simplified by assuming three standard sequences: random, ordered, and segregated. The primary analytical tool used to sort out all these structural variations is powder X-ray diffraction. Though the structure of clay will inform many of the physical properties and allows for differentiation of the various clay minerals, substitution into the individual clay layers can alter the base physical properties and provides variation within the clay families. For more precision in clay mineral identification, additional analytical information is required (as further discussed in this book).