ABSTRACT
Geologists, mineralogists, chemists, and soil scientists all approach the terms clay and clay mineral somewhat differently. Historically the term clay has referred to the small inorganic particles in the <2-µm portion of a soil fraction without regard to composition or crystallinity, and clay minerals has referred to the specific phyllosilicates (the term for sheet silicate structures) that are the layered, hydrous, magnesium or aluminum silicates in such a fraction. In 1995 an AIPEA report (1) defined clay as a “material composed primarily of fine-grained minerals which is generally plastic at appropriate water contents and will harden when dried or fired” and clay minerals as “phyllosilicate minerals and minerals which impart plasticity to clay and which harden upon drying or firing.” For the purposes of this chapter the two terms will be used interchangeably, but the intent is always to refer to (either) clay mineral definition.
