The celestite problem in gypsum Sr geochemistry: An evaluation of purifying methods of gypsiferous samples

Celestite (SrSO4) is a very common mineral in gypsiferous rocks. In order to evaluate the possible influence of celestite occurrence in gypsum strontium analysis (Sr content), different fine-grained lithofacies with a high content of enclosed minerals (ranging from 5% to 35% of the bulk rock) were studied and a series of experiments with artificial reagents were carried out. The results showed that even very low concentrations (< 0.5%) of celestite can release enough Sr to produce significantly altered results. commended methodology to prevent celestite contamination in Sr content analysis of fine-grained gypsum lithofacies involves three basic steps: 1) celestite identification, 2) celestite elimination, and 3) geochemical analyses. We found that back-scattered electron microscopy is the best method to identify celestite over a wide range of celestite concentrations. In the case of celestite occurrence, microprobe analysis provided reliable results as the obtained Sr values corresponded to the gypsum lattice. If a bulk sample analysis is performed, celestite must be eliminated or partially separated from the pure gypsiferous sample in order to reduce contamination. For this, different methods were modified and tested, including a heavy liquid (dibromomethane) separation procedure. However, because none of these methods were completely satisfactory, a new method based on a HCl 2N leaching was developed and tested. Nevertheless, although the celestite content diminished considerably, it was not totally eliminated. clusion, the presence of celestite must always be taken into account when gypsiferous samples are analyzed for their Sr content, or their 87Sr / 86Sr ratio, in order to avoid seriously biased results and non-reliable geochemical and paleoenvironmental interpretations. In celestite-rich samples it is convenient to separate the celestite before carrying out the analysis.