According to this calculation, in hypersaline brines (in our case 4.8 M, see SI: Materials and Methods), the transition point between gypsum and anhydrite is shifted to a temperature of T < 30 °C, but bassanite supposedly remains metastable for the entire temperature range. Thus, bassanite should not form from solution under these conditions, which is incorrect, as we show later. However, it is important to note that these curves are merely extrapolations from experimental data, where the solubility of CaSO 4 phases in high salinity solutions has been mainly determined experimentally for some temperature ranges. This becomes apparent when the solubility curves (Fig. ) are calculated using the latest version of PHREEQC 3 with an updated Pitzer database (SI: Methods, Eq. S1, Table ). Updated and improved data suggest that bassanite does become the stable phase at T > ~110 °C in 4.8 M brines. This would explain the experimental observation that bassanite forms at this temperature in high salinity experiments , but it contradicts the observation that anhydrite should eventually form at these conditions (Fig. and ref. ). These uncertainties regarding the phase diagram of CaSO 4 have hampered the search for a large-scale synthesis route for bassanite at a lower temperature, and thus lower environmental cost.