The colloidal stability of quartz suspension was determined over a wide range of pH in aqueous copper nitrate where the state of Cu(II) is changed from mainly aqua ions and monohydroxyl complexes in the acid and neutral pH to polynuclear hydroxo complexes and colloidal precipitated copper hydroxide at higher pH. Two regions of instability were observed and in both cases the particles were shown to have low electrophoretic mobility. In the neutral pH region, the uptake of Cu(II) was sufficient to reduce the mobility of the particles to zero, while in the high-pH region evidence suggested coagulation between precipitated Cu(OH)(2) and the quartz particles. It was shown that in all cases the coagulation was reversible and that the uptake of Cu(II) was dependent on the uncharged surface hydroxyl density. Studies of the coagulation kinetics showed that extended time scales were involved (several minutes in the neutral pH region to tens of minutes at high pH).