What is clay？
Clay refers to minerals whose particle size is below 2μm. It does not refer to specific mineral or clay mineral. Generally, common clay on the ground surface include not only clay minerals and quartz, but also feldspar minerals, przibramite and minerals of clay-particle size produced in surrounding parent-rock environment. Therefore, clay is not a mineralogical term.
For more information, please go to the “Taiwan Geoscience Portal” website.