Hydration of anhydrite with substitution of strontium (Ca,Sr)SO4 - model experiments

Hydration of anhydrite with substitution of strontium (Ca,Sr)SO₄ - model experiments

Martyna NAWRACAJ¹, Julia RÓŻAŃSKA¹, Kacper STASZEL¹, Bartosz PUZIO¹, Aleksandra PUŁAWSKA¹, Maciej MANECKI¹

¹Department of Mineralogy, Petrography and Geochemistry, AGH University of Kraków, al. Mickiewicza 30,    30-059 Kraków, Poland

Infiltration of fresh water into the clay-anhydrite layers of the salt deposit (Bochnia Salt Mine, UNESCO World Heritage Site in southern Poland) results in the hydration of anhydrite (CaSO₄) to gypsum (CaSO₄·2H₂O) (Pitera and Cyran, 2008). This process is particularly complex and unusual because the parent anhydrite is partially substituted with Sr (0.1-0.2%, Pulawska et al., 2021), and the release of strontium during this transformation remains unclear.

To investigate this phenomenon, laboratory model experiments were performed. Synthetic analogs of Sr-substituted anhydrite with varying Sr content (0.1%, 1%, as well as  2%) were prepared, along with pure anhydrite and celestine (SrSO₄). All five syntheses were conducted for 3 hr at 120°C (Kamarou et al., 2021) and resulted in formation of Sr-doped anhydrite. A maximum Sr substitution in anhydrite was established at 1–2 wt.%. Synthetic sulfates were hydrated for 70 days in a controlled environment, using 500 mL of redistilled water with 2.5 g of solid material (1:10 solution-to-solid ratio). The solids were analyzed using powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). The phase transformations began as early as 21 days in both pure and 0.1% Sr-substituted anhydrite, forming bassanite (CaSO₄·0.5H₂O). Later on, the hemihydrate sulfate transformed into gypsum. Pure celestine did not undergo any phase transformation during the hydration process.

Model hydration experiments have successfully mirrored the natural phenomenon occurring in the Bochnia Salt Mine, including the release of strontium into solution. These findings leave the room for further research so as to understand the fate and influence of strontium on minerals in salt deposits.

References

• Kamarou, M., Korob, N., Hil, A., Moskovskikh, D., Romanovski, V. (2021). Low-energy technology for producing anhydrite in the CaCO₃–H₂SO₄–H₂O system derived from industrial wastes. Journal of Chemical Technology & Biotechnology, Vol 96, issues 7, p. 2065-2071
• Pitera, H., Cyran, K. (2008) Altered anhydrite from Bochnia Salt Mine (Poland). Geologia, Vol 34, issue 1, p. 5–17 (in Polish)
• Puławska, A., Manecki, M., Flasza, M., (2021). Mineralogical and Chemical Tracing of Dust Variation in an Underground Historic Salt Mine. Mineralas, 11, 686