. . . . "nd therefor softer), as on the Gulf or Atlantic Coastal Plains, we refer to it as clay.As defined by the Wikipedia entry, Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and other cations.There are five groups (and other subgroups) of clay minerals, including some 14 minerals (one of which is not always con" . .