The geochemistry of cadmium in some sedimentary rocks

The cadmium contents of 11 shales, 11 sandstones, 7 limestones, 14 metamorphic rocks and 28 stream sediments, all from Pennsylvania, were determined by flameless atomic absorption spectrometry. On the basis of these cadmium values and those in the literature, cadmium is found to be enriched in dark shales and soils, depleted in red shales, sandstones and limestones, and about the same in stream sediments as in most igneous and metamorphic rocks and the crust. In stream sediments, cadmium correlates most significantly with zinc, followed by carbon, weight loss on ignition, cobalt, readily extractable lead and manganese. Examination of the correlation plots and of an Eh-pH diagram indicates that the primary cause of cadmium enrichment in the sedimentary environment is the adsorption and/or complexation of cadmium with organic matter followed by the accumulation of organic debris in a reducing depositional environment.