The formation of oil-mineral aggregates (OMA) in water is initiated by a variety of physical and chemical factors that are not readily amenable to differentiation in the field or in the laboratory. This study illustrates a method that may be used to isolate the limiting parameter responsible for the formation and stabilization of OMA based on the microstructure and sedimentation characteristics of the aggregates. A flocculation index based on the sedimentation behavior of a sheared crude oil-mineral-water mixture was used to quantify the degree of interaction of oil and minerals in water. The degree of crude oil-mineral interaction was found to be dependent on the viscosity of the crude oil and the type of mineral present. Hydrophilic quartz and kaolinite interact more strongly with low-viscosity oils than with high-viscosity oils, whereas calcite (an oleophilic mineral) interacts strongly with crude oils irrespective of their viscosities. It was observed that the water chemistry must favor flocculation before the minerals can effectively stabilize oil-mineral flocs. In a fresh water analogue, quartz and kaolin interact more strongly with crude oils than montmorillonite but the reverse is true in seawater. Calcite flocculates crude oils in fresh or seawater more strongly than the hydrophilic minerals tested. In all the minerals tested, the degree of oil-mineral interaction increases and then plateaus as mixing energy is increased. Confocal laser images of the oil-mineral structures reveal two types of OMA. In low-viscosity oils with hydrophilic minerals, negatively buoyant flocs comprising minerals stabilizing oil droplets in a water-continuous phase are predominant. OMA with calcite show mineral-stabilized oil droplets and mineral-rich positively buoyant oil-continuous phase. Crown Copyright (C) 2003 Published by Elsevier Science Ltd. All rights reserved.