Zinc concentrates often have a limit on the silica content to avoid problems in the subsequent metal extraction process. The mechanism(s) of silica contamination is, therefore, a concern. This investigation focuses on one possibility: formation of sphalerite-silica aggregates. To study the interaction between sphalerite and silica, a combination of zeta-potential and settling measurements (including color gradation in the suspension) was used in the presence and absence of calcium ions. In the absence of Ca2+, the following was observed: at about pH 2 and 8.5, sphalerite homoaggregated and some silica was dispersed; from about pH 3 to 7, the system heteroaggregated; and above pH 9.5, the suspension was dispersed. This behavior did not correlate with settling results on the minerals alone. In the presence of Ca2+, from about pH 2 to 7, the sphalerite homoaggregated and the silica was dispersed while above pH 7, heterdaggregation was observed. This behavior did correlate with that of the minerals alone. The zeta-potential results provided only a partial interpretation. An attractive force between sphalerite particles appears to exist at moderately alkaline pH. (C) 2001 Elsevier Science B.V. All rights reserved.