|MadSci Network: Earth Sciences|
I beleive you are probably talking about bentoninte clays Bentonite is a type of clay that belongs to the groups of clays called smectites. These clays are known as 2:1 swelling clay This means that the clay swells when it is wet and shrinks when it is dry This makes bentonite very useful for lining ponds. It swells and becomes nearly watertight when wet. One company web page may offer some more info http://www.bhbentonite.com/
Try the links in the MadSci Library for more information on Earth Sciences.