Various chemical methods of freeing the clay minerals that occur in limestones and dolomites have been used without regard to the effect the chemicals might have on the properties and structures of these clay minerals. A procedure is here suggested that will accomplish the separation without significantly affecting the character of randomly interstratified illite-montmorillonite, chlorite, illite, or kaolinite, as shown by their X-ray diffraction diagrams. The limestone or dolomite is treated at room temperature with acetic acid (less than 0.3 molar) or hydrochloric acid (0.11 molar). The weaker the acid concentration the greater the possibility that the clay minerals will be unchanged. Some carbonate rocks must be present throughout the period of acid treatment.