Pyrophyllite and talc were shown to have acid sites on their surface, and the strength and amount of the acidity were much higher for pyrophyllite than talc. The strength and amount of the acidity of both minerals were not affected by the exchangeable cation species. Water molecules previously adsorbed on the surface of the minerals lowered both the strength and amount of the acid sites, although the degrees of the decrease were smaller than the case of montmorillonite. When the water was introduced after the adsorption of indicators, the water molecules did not affect both the strength and amount of the acidity.
From above observations, it is suggested that the acidity of pyrophyllite and talc are not attributed to any known origin of the acidity for layer silicates. We concluded that the acidity of pyrophyllite and talc are the Lewis type, and derived from their nature as electron acceptor due to their ‘neutral’ layer compositions. The difference in the acidity between pyrophyllite and talc may come from the difference in their octahedral cation species.