Traditionally, Athabasca oil sands have been classified by bitumen and fines (<44 mum) contents. However, these markers do not always identify ores with poor processing characteristics. Consequently, there is a need to discover other characteristics to define problem ores. Here we describe a separation scheme for oil sands solids fractions based on their physical and surface properties. Clay material (<3 mum) and its ultra-fine (<0.3 mum) component are of particular interest. Settling tests on ultra-fine clay suspensions confirm that this fraction is the major cause of sludging during primary bitumen separation. Waste units, or barren oil sands, are shown to be a major source of this component.