The components of the interfacial surface tension of talc and pyrophyllite were determined by measuring the rate of the capillary rise of a number of liquids through thin, sedimented deposits of the powdered minerals. The rate of capillary rise of a liquid in a powder is related to the contact angle between the liquid and the solid by the Washburn equation. The contact angles thus derived were used to determine the apolar (Lifshitzvan der Waals) component, 7 Lw, and the polar, electronacceptor and electron-donor parameters, 7e and 7 e respectively, of the Lewis acid/base component of the total interracial surface energy using the Young equation. The values of 7 Lw for talc and pyrophyllite (31.5 and 34.4 mJ/m 2) are slightly smaller than for smectite clay minerals (e.g., the value for hectorite is 39.9 mJ/mZ), the electron donor parameter values are roughly comparable for talc and pyrophyllite (7~= 2.4 and 1.7 mJ/m 2) as are the values of the electron acceptor parameter (7e=2.7 and 3.2 m J/m2). The well-known hydrophobicity of these two minerals is due to the remarkably small value (for silicate minerals) of 7 e (7 ~ is normally small or zero for silicates and many other oxides). The small values of both y e and 7e mean that the Lewis acid/base interactions between talc or pyrophyllite and highly polar water molecules are very weak. In contrast, low-charge smectites, the minerals most similar chemically and structurally to talc and pyrophyllite, have much greater values of 7e (>_ 30 mJ/m 2) and are hydrophyllic.