We compare the frictional strengths of 17 sheet structure mineral powders, measured under dry and water-saturated conditions, to identify the factors that cause many of them to be relatively weak. The dry coefficient of friction ¿ ranges upward from 0.2 for graphite, leveling off at 0.8 for margarite, clintonite, gibbsite, kaolinite, and lizardite. The values of ¿ (dry) correlate directly with calculated (001) interlayer bond strengths of the minerals. This correlation occurs because shear becomes localized along boundary and Riedel shears and the platy minerals in them rotate into alignment with the shear planes. For those gouges with ¿ (dry) < 0.8, shear occurs by breaking the interlayer bonds to form new cleavage surfaces. Where ¿ (dry) = 0.8, consistent with Byerlee's law, the interlayer bonds are sufficiently strong that other frictional processes dominate. The transition in dry friction mechanisms corresponds to calculated surface energies of 2--3 J/m2. Adding water causes ¿ to decrease for every mineral tested except graphite. If the minerals are separated into groups with similar crystal structures, ¿ (wet) increases with increasing interlayer bond strength within each group. This relationship also holds for the swelling clay montmorillonite, whose water-saturated strength is consistent with the strengths of nonswelling clays of similar crystal structure. Water in the saturated gouges forms thin, structured films between the plate surfaces. The polar water molecules are bonded to the plate surfaces in proportion to the mineral's surface energy, and ¿ (wet) reflects the stresses required to shear through the water films.