RAPID D ISSOLUTION OF ALLOPHANE AND KAOL IN ITE-HALLOYSITE AFTER DEHYDRATION by

Differential dissolution in NaOH solution was investigated as a means of purification of clay. I t was discovered that a large quantity of allophane and free alumina and silica were dissolved from clay preparations in 0.5 N NaOH solution by boiling for as little as 2.5 rain, provided the ratio of clay weight to solution volume was kept less than 100 mg to 100 ml. Reprecipitation of dissolved silica occurred if a more prolonged digestion or higher sample to solution ratio was employed. Also, prolonged igestion in the NaOH solution (for 80 rain) dissolved as much as 50 percent of kaolinite (Merck) and 25 percent of Wyoming montmorillonite, but only a small quantity of these crystal-line minerals was dissolved uring the 2.5 rain digestion period. Subsequent dithionite-citrate-bicarbonate tr atment removed the released iron. Marked improvement ofx-ray diffraction patterns of the clays resulted. This rapid and selective dissolution of allo-phane and free oxides greatly improves the quantitative analysis of crystalline minerals by conventional methods. After dehydroxylation at 500 ~ kaolinite and halloysite became amorphous and also dissolved by the same differential dissolution procedure. Heat-stable (aluminous) montmorillonite and chlorite were only slightly dissolved by this treatment. The selective removal of the i: 1 layer aluminosilieates greatly improves the quantitative analysis of the remaining crystalline components of clays by conventional methods