Surprisingly, this temperature-dependent cyclic phase change confirms a range of physicochemical conditions where bassanite is thermodynamically more stable than gypsum. Additionally, it also shows that the formation of anhydrite, the CaSO 4 phase with the lowest solubility is kinetically hindered at T = 90 °C, . Importantly, the almost instantaneous gypsum-bassanite conversion occurs at a distinct temperature boundary. In a different variant of our scattering experiment, we precipitated gypsum at T = 35 °C and heated this slurry in 5 °C increments up to T = 90 °C, with each temperature being maintained for 900 s (15 min) (SI: Fig. ). The scattering data revealed that the rapid transition is viable only at a threshold temperature of T = 80 °C, which is significantly lower than the one predicted by the solubility calculations shown in Fig. (T = ~110 °C). Furthermore, in static experiments (i.e. without stirring), we obtained bassanite at T = 70 °C after 5 days (SI: Table ). Overall, these observations contradict the calculated solubility data shown in Fig. and, but they do confirm the previously reported stability behaviour of the calcium sulfate phases precipitating from brines solutions shown in Fig. . We further focused experimentally on a single cycle of gypsum-bassanite-gypsum conversion with extended reaction times for 35 °C < T < 90 °C (>25000 s with scattering and >70000 s with Raman spectroscopy, Fig. ). With these measurements, we intended to exclude a possibility that the occurrence of the cyclic transformations is driven by the residual seeds of the bassanite. Such seeds, hypothetically, might nucleate the hemihydrate phase, although it would be thermodynamically unfavourable . To fully exclude this scenario in Fig. , the experiments start with gypsum. Moreover, the equilibration periods, when only gypsum exists, are extended with respect to However, in Raman, the conversion of bassanite to gypsum was notably slower, with bassanite persisting for an extended period (~51000 s) after the solution commenced to cool down to T = 35 °C (T < 90 °C at ~24000 s, T = 35 °C at ~47000 s). Bassanite started to convert to gypsum at ~75000 s. The longer persistence of bassanite in the in situ Raman experiment compared to the in situ scattering experiment is most likely the result of the inherent differences in the hydrodynamic regimes between both setups . In the latter, the solution was vigorously stirred (1000 rpm) in the reactor vessel and continuously recirculated through a capillary using a peristaltic pump. In contrast, the Raman setup involved gentler stirring (400 rpm) of the solution in the reaction vessel.