The U.S. Geological Survey has prepared an attapulgite and a bentonite as reference samples because of their economic importance. The attapulgite (ATT‐1) is from the Meigs‐Attapulgite‐Quincy fuller's earth district of Georgia and Florida, and the bentonite (CSB‐1) is from the Northern Black Hills bentonite district of Wyoming, Montana, and South Dakota. The chemical and mineralogical properties of these two samples are quite different. Provisional values calculated from the few analytical data and the X‐ray diffraction mineralogy are presented for those who wish to use the clays as referenc