Clay minerals of four bentonites and two shales from Upper Cretaceous continental sedimentary rocks in western Alberta were studied in detail using X-ray diffraction and fluorescence methods. Study of the bentonites was oriented toward appraising their usefulness in correlation. The shales were investigated to help evaluate the influence of depositional environment on clay composition.

The bentonites range in composition from 19 to about 90 per cent mixed layer clay, 3 to 15 per cent kaolinite, no illite or chlorite and from none to almost 80 per cent montmorillonite. The shales lack the mixed layer clay, contain from about 25 to 50 per cent kaolinite, 40 to 75 per cent montmorillonite, small amounts of illite, and one sample contains some chlorite.

Fourier transforms of the mixed-layer clays show three component interstratifications which in three of the bentonites are within the compositional limits of 50 to 62 per cent illite, 14 to 35 per cent vermiculite and 15 to 30 per cent montmorillonite. The mixed-layer clay from the fourth bentonite contains only 20 per cent illite, 18 per cent vermiculite and 62 per cent montmorillonite which is partly present as a mechanical mixture. Calculations and combination transform peaks suggest a tendency toward a long range ordering of the components.

X-ray spectrochemical analyses checked against wet chemical analyses prove to be promising as a rapid, non-destructive method of determining the major rock forming elements, except Na+, in complex clay systems. At present accuracy of this method limits it to a semi-quantitative analysis. The major chemical components of the four bentonites are quantitatively very similar. The shales contain more Fe, Mn, Mg and Ti, and less K and A1 than the bentonites.

It is concluded that mineralogically the clays may be of use in correlating these bentonites, whereas the chemical compositions are too similar to be used in diffrentiating them. The shales are mineralogically and chemically different from the bentonites, but similar to one another. Assuming that the shales and bentonites were deposited in similar environments the environment of deposition only slightly modifies the composition of clay minerals.

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