Electrostatic Potential at the Basal (001) Surface of Talc and Pyrophyllite as Related to Tetrahedral Sheet Distortions

-Maps of the electrostatic potentials at the basal plane of talc and pyrophyllite, computed using a two-dimensional Ewald lattice-sum, reveal the effects caused by structural distortion of the phyllosilicate layer. Rotation and tilting of basal tetrahedra in phyllosilicates dramatically perturb the electrostatic potential near the (001) surface. A potential high exists at the center of each six-fold ring of the talc (001) surface. Concerted counter-rotations of basal tetrahedra by 10", as are typical in pyrophyllite, cause the potential lows above basal oxygens rotated into the ring to overlap, eliminating the ring-centered potential highs. Expansion of the vacant site in dioctahedral minerals tilts the basal tetrahedra by 4* and moves one-third of the basal oxygens about 0.2/~ toward the center of each phyllosilicate layer and away from the (001) surface, thereby producing corrugations of the basal surface. This shift dramatically reduces the contribution of these displaced basal oxygens to the (001) surface potential. Rotation and tilting of basal tetrahedra may influence the arrangement of interlayer water molecules on smectites and other swelling phyllosilicates by the effect that these distortions have on the (001) surface potential. Key Words--Basal oxygen sheet, Electrostatic potential, Pyrophyllite, Talc, Tetrahedral sheet distortion. I N T R O D U C T I O N Since Pauling (1930) first proposed the structure for phyllosilicate minerals, much has been learned about layer stacking, cation ordering, and various structural distortions. All contribute in some way to subtle differences in the properties of this important class of aluminosilicates. This present paper reports evidence that structural distortions in phyllosilicates dramatically alter the electrostatic potential at the phyllosilicate (001) surface and that the electrostatic potential at the surface of these minerals is determined primarily by those atoms in the first two atomic planes parallel to the surface, i.e., the silicon and basal-oxygen atoms. The tetrahedra in phyllosilicates form a net of threeconnected, 6-fold rings. The point symmetry of the rings in this net can be as high as Crv (6mm); however, a simple structural distortion exists in nearly all dioctahedral phyllosilicates. In this distortion, tetrahedra are counter-rotated about axes normal to the plane of the layer, lowering the point symmetry of the rings in the net to C3v (3m). The A1-O bond lengths in an unrotated octahedral sheet would be much longer than those in gibbsite. Inasmuch as Mg-O bond lengths in talc are close to those in brucite, no such rotational distortion is required to match the dimensions of the tetrahedral to the octahedral sheet. Such reasoning follows directly from the structural principles used by