Combined geochemical and textural analyses of halite: First results of non-destructive µXRF measurements

The mineralogical-geochemical composition and texture of salt rocks plays an important role for the site selection and construction of a repository for heat-generating, highly active radioactive waste. As the bromide content in halite depends on the degree of evaporation and subsequent processes, Br is frequently analyzed to estimate the genetic history of the rocks (Braitsch 1971). Typically, geochemical analytical methods are applied on powder samples (e.g. ICP-OES, XRF), and textural analyses (e.g. using EBSD) require extensive sample preparation. In this study, first results of diapiric Upper Permian rock salt samples are presented using non-destructive µXRF on polished rock samples.

The µXRF Bruker M4 Tornado Plus (Nikonow & Rammlmair 2016) was used to map and quantify element distributions in rock salt. For calibration, in a first step, a certified reference material (CGL), consisting of mostly halite (NaCl) with minor content of anhydrite (CaSO₄), and sylvite (KCl), was pressed into pellets of 2 g and 1 cm diameter. For representativity, three spot measurements and a mapping of the center (1 cm²) were chemically quantified. The µXRF measurements correlate with the certified values yielding an R² of 0.995. In a second step, pressed pellets with a range of defined concentrations of Br in halite and Rb in sylvite were prepared to estimate the concentration ranges measurable by µXRF. For Br, the concentrations range from 1 to 0.005 wt.% Br in halite, and for Rb the concentrations range from 0.4 to 0.005 wt.% Rb in sylvite. Both data sets show a good correlation with R² of 0.99 (n=21 for Br and n=17 for Rb). Therefore, µXRF seems suitable for quantification of Rb and Br in salt rocks.

Furthermore, naturally deformed halite samples were analyzed simultaneously for their geochemically and textural properties, which were previously analyzed using “conventional” methods (ICP-OES and EBSD, respectively; Mertineit et al. 2023). The bromide content in halite is ca. 200 µg/g and thus comparable to the known values. The textural results show misorientations of few degrees within single halite grains and pronounced misorientations at halite grain boundaries, indicating bending of the crystals, but no pronounced texture of the bulk rock.

Although the results are in good agreement with published data, further test should follow, especially on the textural analyses including the misorientation angle resolution and the indexing of the halite crystal axis. However, the application of µXRF on salt rocks offers a fast, non-destructive method providing reliable combined geochemical and textural information.

References

Braitsch 1971. Springer-Verlag, https://doi.org/10.1007/978-3-642-65083-3

Mertineit et al. 2023. Tectonophysics 847, https://doi.org/10.1016/j.tecto.2023.229703

Nikonow & Rammlmair. 2016. Spectrochim Acta B 125, https://doi.org/10.1016/j.sab.2016.09.018