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

► Calcium carbonate was precipitated in the presence of a pluronic triblock copolymer, F-68. ► The polymer is used in analytical grade form and after purification. ► Mesocrystals are produced with the non-purified polymer. ► Unmodified calcite rhombohedra precipitate in presence of purified polymer. ► Morphological effects of analytical grade F-68 are due to impurities in the polymer.