  Understanding the gypsum (CaSO4.2H2O) formation pathway from aqueous solution
has been the subject of intensive research in the past years. This interest
stems from the fact that gypsum appears to fall into a broader category of
crystalline materials whose formation does not follow classical nucleation and
growth theories. The pathways involve transitory precursor cluster species, yet
the actual structural properties of such clusters are not very well understood.
Here, we show how in situ high-energy X-ray diffraction experiments and
molecular dynamics (MD) simulations can be combined to derive the structure of
small CaSO4 clusters, which are precursors of crystalline gypsum. We fitted
several plausible structures to the derived pair distribution functions and
explored their dynamic properties using unbiased MD simulations based on both
rigid ion and polarizable force fields. Determination of the structure and
(meta)stability of the primary species is important from both a fundamental and
applied perspective; for example, this will allow for an improved design of
additives for greater control of the nucleation pathway.
