  Crystal precipitation from aqueous solution occurs through multiple pathways.
Besides the classical ion-by-ion addition, non-classical crystallization
mechanisms, such as multi-ion polymer and nano-particle attachment, could be of
great significance under certain circumstances. These non-classical
crystallization processes have been observed with advanced microscopy, yet
detailed quantification of their contribution in mineral precipitation remains
challenging. Building from paired Ca and Sr isotope observations, we develop a
new theoretical framework to quantify the relative contribution of classical
and non-classical crystallization pathways on the precipitation of the calcium
carbonate mineral calcite, one of the most common precipitates in nature. We
demonstrate that the classical (ion-by-ion) crystallization pathway alone is
insufficient to account for the observed isotope behaviors and, thus, the
entire calcite precipitation process. We present a new kinetic surface reaction
model to incorporate the non-classical crystallization pathway. This new model,
for the first time, enables the detailed characterization of the roles of
classical and non-classical crystallization mechanisms in calcite
precipitation. The results suggest that the relative contribution of
non-classical crystallization pathways increases with saturation state and can,
under high supersaturation levels, be comparable to or greater than
precipitation driven by the classical crystallization pathway. The presented
theoretical framework readily explains observed trace element partitioning and
isotope fractionation behaviors during calcite precipitation and can be further
expanded onto other mineral systems to gain insights into crystal growth
mechanisms.
