Clay Minerals (1996) 31,225-232 STABIL ITY OF SEP IOL ITE IN NEUTRAL AND ALKAL INE MEDIA AT ROOM TEMPERATURE

studied in 0.04 N NaOH (pH = 12.6), a saturated solution of Ca(OH)2 (pH = 12.6) and deionized/ distilled water (pH = 7). The chemical stability was evaluated by determining the concentrations of Mg, Si and Ca in the solid residues during the kinetic test. The structural stability was studied by determining the changes in specific surface area and 'crystallite ' size. The specific surface was determined by the BET method and the 'crystallite ' size by the profile of the X-ray diffraction line. The main conclusion was that sepiolite is a material with high chemical stability in neutral and alkaline media, with no significant structural changes. However, in alkaline media, greater dissolution of silica was observed than in neutral media, the silica being derived from the most external tetrahedra attached to the OH- groups. Sepiolite is a hydrated Mg silicate, with the formula, according to Brauner & Preisinger (1956): Si12MgsO3o(OH)a(H20)4.SH20. Its structure is composed of sheets of silica tetrahedra joined by O ions to a central octahedral plane of Mg ions. The silica tetrahedra re inverted every six units