Rheological properties of concentrated suspensions (∼70 wt%) of relatively coarse particles of crushed quartz (∼11 μm) in solutions of various metal chlorides were studied. The apparent viscosity of these settling suspensions, as measured with a “settling viscometer,” was compared with the zeta potential of the particles, the degree of aggregation, the sediment volume, and the degree of hydration of the metal cation. As the salt concentration increased and the suspensions coagulated, their viscosity increased. However, suspensions that were qualitatively completely aggregated showed large variations in apparent viscosity at100 s−1,depending on the particular metal cation of the salt solution. The viscosities increased in the orderLi