1. a soil separate consisting of particles < 0.002 mm (0.00008 in) in equivalent diameter —see soil separates 2. a textural class —see soil texture 3. clay mineralogy a naturally occurring material, composed primarily of fine-grained minerals, which is generally plastic at appropriate water contents and will harden when dried or fired —note 1. although clay usually contains phyllosilicates, it may contain other materials that impart plasticity and harden when dried or fired —note 2. associated phases in clay may include materials that do not impart plasticity and organic matter —see sand, silt This definition last updated 10/25/2008