Impurities in pluronic triblock copolymers can induce the formation of calcite mesocrystals

An interesting recent report by Zhou et al. (Chemical Geology, 2010, 279, 63–74) describes the formation of calcite mesocrystals with very well-defined, prismatic morphologies in the presence of F-68, a pluronic triblock copolymer. That this can be achieved with a polymer which is a linear, nonionic polyoxyethylene–polyoxypropylene runs counter to the current understanding that face-selective adsorption of a polymer on the precursor mineral building blocks is required for mesocrystal formation. In this paper we contrast the effect of analytical grade F-68 and the same polymer after purification by dialysis, and show that a strong morphological effect on calcite precipitation is only achieved with the analytical grade polymer. We therefore suggest that low molecular weight impurities present within analytical grade F-68 may have been responsible for the formation of CaCO3 mesocrystals rather than the F-68 macromolecules themselves.