Chapter 10 Mineral affinities and distribution of selenium and other trace elements in black shale and phosphorite of the phosphoria formation

Abstract

Mineral affinities and quantitative distributions of Se and other trace elements of envir- onmental concern in the Permian Phosphoria Formation were studied in samples from three variably weathered sections using microprobe, X-ray absorption spectroscopy (XAS), and sequential-extraction analyses. The results show a clear difference in the distribution of Cd, Cu, Ni, Se, V, and Zn between non-weathered and weathered samples. In unweathered sam- ples, sulfides (mainly pyrite and sphalerite) host the majority of Cd, Cu, Se, and Zn and a large proportion of the Ni and V Most of the non-sulfide fraction of these elements in unweathered samples is associated with organic matter and oxyhydroxides. A small fraction of Se is present in elemental form. Apatite is the primary host for U. Both apatite and organic matter may host a significant fraction of Mo. Of the elements investigated, only Cr, U, and V were found to have minimal association with organic matter in unweathered rocks. Acid-insoluble phases (assumed to be silicates and oxides) host the majority of Cr and a sig- nificant amount of V Molybdenum was the only element for which a significant fraction is easily leachable (assumed to be weakly adsorbed or in a very soluble phase). In weathered samples, acid-soluble oxyhydroxides are the primary hosts for all the aforementioned elements except Cr and U, which are associated with relatively stable phases in non-weathered samples. Organic-bound Ni, which represents a substantial part of total Ni in unweathered samples, apparently is more resistant to weathering than is organic-bound Mo, which appears to be readily lost during weathering. Despite the obvious shift in mineral residences of Se and associated trace elements upon weathering, weathered rocks may still contain high concentrations of these elements. As many of the elements of concern appear to be associated primarily with oxyhydroxides in weathered rocks, both dissolved and sorbed species released to the surface environment must be considered. Furthermore, trace elements in unweathered and minimally weathered rocks are hosted in a number of phases that have variable oxidation rates and a wide range of particle sizes. Some of these host phases occur as inclusions within phases more resist- ant to oxidative weathering, such as phosphate pellets. These factors indicate that the release of these elements to the environment will be a variable and long-term process.