The structural changes of sepiolite under thermal treatment, and their reversibility, have been examined by thermogravimetry. These changes have been related to the textural changes, determined by N-2 adsorption under similar conditions. The folding of this material starts at 390 degrees C, from this temperature the microporosity of the material disappears. This folding is reversible, becoming permanent at 570 degrees C. From 390 to 570 degrees C the surface area remains stable, corresponding to the external surface area of the mineral.