  CaSO4 minerals (i.e. gypsum, anhydrite and bassanite) are widespread in
natural and industrial environments. During the last several years, a number of
studies have revealed that nucleation in the CaSO4-H2O system is non-classical,
where the formation of crystalline phases involves several steps. Based on
these recent insights we have formulated a tentative general model for calcium
sulfate precipitation from solution. This model involves primary species that
are formed through the assembly of multiple Ca2+ and SO42- ions into
nanoclusters. These nanoclusters assemble into poorly ordered (i.e. amorphous)
hydrated aggregates, which in turn undergo ordering into coherent crystalline
units. The thermodynamic (meta)stability of any of the three CaSO4 phases is
regulated by temperature, pressure and ionic strength with gypsum being the
stable form at low temperatures and low to medium ionic strengths, and
anhydrite the stable phase at high temperatures and lower temperature at high
salinities. Bassanite is metastable across the entire phase diagram but readily
forms as the primary phase at high ionic strengths across a wide range of
temperatures, and can persist up to several months. Although the
physicochemical conditions leading to bassanite formation in aqueous systems
are relatively well established, nanoscale insights into the nucleation
mechanisms and pathways are still lacking. To fill this gap, and to further
improve our general model for calcium sulfate precipitation, we have conducted
in situ scattering measurements at small- and wide-angles (SAXS/WAXS) and
complemented these with in situ Raman spectroscopic characterization. Based on
these experiments we show that the process of formation of bassanite from
aqueous solutions is very similar to the formation of gypsum: it involves the
aggregation of small primary species into larger disordered aggregates.
