  The formation of energetically favourable and metastable mineral phases
within the Fe-H2O system controls the long-term mobility of iron complexes as
well as other aqueous phase constituents in natural aquifers, soils and other
environmentally and industrially relevant systems. The fundamental mechanism
controlling the formation of these solid phases has remained enigmatic. Here,
we develop a general state-of-the-art partial equilibrium model and succeed in
predicting the rate of amorphous 2-line ferrihydrite precipitation, dissolution
and overall transformation to crystalline goethite at alkaline pH. All
mechanistic steps constituting the transformation mechanism accurately predict
the experimentally measured solid and aqueous phase composition over time,
involving only a single kinetic rate constant each. It is found that the
precipitation of goethite (i) occurs from solution and (ii) is limited by the
comparatively slow dissolution of the first forming amorphous phase 2-line
ferrihydrite. A generalised transformation mechanism applicable to near-neutral
and mildly acidic pH further illustrates that differences in the kinetics of
Fe(III) precipitation are controlled by the coordination environment of the
predominant Fe(III) hydrolysis product. Findings provide a framework for the
modelling of other iron(bearing) phases across a broad range of aqueous phase
compositions.
