The cyclic transformations of gypsum-to-bassanite in high-salinity brines offer an efficient route to produce hemihydrate. To demonstrate the practicality of this method, we also processed natural gypsum into bassanite. We used 0.2 M CaSO 4 equivalent solutions/slurries, containing 34.4 g/L of dispersed natural gypsum powder (see SI: Materials and Methods). We reproduced the processes at high salinity with 4.8 M NaCl brines. A detailed plot of the Raman spectra in the first 60000 s after raising the temperature to T = 90 °C shows, as expected, the evolution of the transformation of gypsum to bassanite and back to gypsum (SI: Fig. ). After raising the temperature, gypsum completely transformed into bassanite within 1500 s and that the hemihydrate remained stable for >30000 s after the temperature was lowered to T = 35 °C, until gypsum eventually reprecipitated.