An X-ray diffraction (XRD) methodology has been developed for characterizing clays in unextracted oil sands. Application of the new technique to five estuarine and five marine ores directly identified three clay mineral properties that may impact bitumen recovery: (1) The specific surface area of illite was significantly greater for four oil sand ores identified as problematic in batch extraction unit tests. (2) The correlation of illite/kaolinite XRD peak area ratios with bitumen recovery produced a processability classification similar to that proposed in earlier work. (3) Significant amounts of chlorite, as measured by XRD, were observed only in marine oil sands: this may provide a means to distinguish marine from estuarine ores. A combination of XRD analysis on separated clays and laser diffraction determination of clay contents provided a quantitative estimate for the illite and kaolinite contents of the oil sands. Also, the contribution from ultrathin illite and kaolinite for each oil sand (i.e., the mass fractions of illite and kaolinite with crystallite thicknesses of 1-3 composite layers) was determined. This methodology thus provides a direct method for the determination of the ultrafines content in unextracted oil sands and obviates the necessity for the time-consuming wet chemistry technique for separation of this component. For the 10 oil sands analyzed here, ultrathin crystallites occurred almost entirely in the illite clay fraction. The amount of ultrathin illite was critical and closely matched the ultrafines concentration required to cause sludging (gelation) in the primary separation vessel, with concomitant loss of bitumen recovery during extraction.