Selenium in certain sedimentary rocks

The selenium contents of some sedimentary rocks were studied and tentative averages are presented. The distribution pattern of selenium in sedimentary rocks can be summarized as follows: The highest contents are found in clayey sediments (excluding possibly oxidates). Carbonaceous matter clearly has an increasing effect on the amount of selenium present. In slates the geochemistry of selenium partly coincides with that of uranium and vanadium. The organic carbonate sediments contain less selenium than would be expected, that found being is primarily bound by organisms. The organic matter and selenium are probably oxidized by the high calcium content, the selenium migrating out of the sedimentary rock with percolating waters. The rocks formed from sands and evaporites are nearly devoid of selenium