Bassanite is considered to be metastable in solution with respect to gypsum and anhydrite 1 (Fig. ). This explains why bassanite deposits do not persist on Earth, except in extreme arid areas, such as the desert of Atacama or volcanic deposits 1 . However, experimental observations indicate that the persistence of bassanite is dramatically increased in solution with high ionic strength , the fact of which has been used to produce bassanite . Yet, when looking at the solubility of the different CaSO 4 phases in a concentrated NaCl solution the stability region of bassanite is not expected (and sometimes even not observed ). The problem mainly lies in the quality of the thermodynamic data. In Fig. we show solubility curves of the three calcium sulfate phases calculated using the widely used geochemical modelling software PHREEQC 2 and the included Pitzer database (see also SI: Materials and Methods).