Abstract: Studies have been made of the alteration of pure albite single crystals and powders. No structural modification of the feldspar itself has been detected. No evidence has been obtained for any preferential orientation of an alteration product in relation to the initial feldspar. Under hydrothermal conditions at 280° C and 430° C, albite flakes and powders have been subjected to attack by 0.1 N HCl for periods ranging from a few hours to 52 days. The flakes changed mainly to boehmite, the powders to a variety of products, including a well-defined kaolinite. This difference of behavior is interpreted in terms of Correns' ideas on the weathering of silicates.