Clay contamination of aggregates used for road construction in Texas is an increasingly acute problem as high-quality aggregate sources in the state are depleted.  Concentrations of expandable layer silicate minerals - such as smectites - affect the shrink-swell properties of concrete, bituminous materials, and chip seals and may ultimately contribute to failure of roads and structures.  The Texas Department of Transportation (TxDOT) is sponsoring a study of the types and quantities of clay minerals present in benign and problematic aggregates from Texas and neighboring states.  Mineralogy and engineering test data for each aggregate are being correlated to establish more reliable field tests.  In this study, 11 aggregates from Texas and Arkansas are treated for carbonates and separated into sand, silt, coarse clay, and fine clay fractions.  X-Ray Diffraction patterns of coarse and fine clay fractions on oriented slides are recorded, and bulk samples undergo treatment in a spray drier for random particle orientation.  FTIR-ATR studies of coarse and fine clays and CEC tests of bulk clay fractions are included as well.  Rietveld refinements of bulk aggregates randomly-oriented through spray dryer treatment and NEWMOD analysis of fine and coarse clay fractions are this study's primary quantification tools.  Preliminary results indicate that total mass of clay-sized particles is not a viable predictor of engineering test failure.  Rather, concentration of expandable layer silicates such as smectites is expected to be a better indicator of aggregate suitability for road construction. Quantification data will be used by TxDOT to better foresee problems in aggregates and mitigate the negative effects of expandable layer silicates.