The effects of pre-existing mineral phases on the nucleation and growth of calcium carbonates from solution are relatively poorly understood, despite the widespread co-occurrence of carbonate minerals with clays and other silicates in rocks, soils, and sediments. Previous studies suggested that sheet silicates template calcite nucleation. Moreover, the presence of certain clay minerals appeared to enhance Mg²⁺ incorporation, resulting in Mg-bearing calcite or protodolomite. Here, we present the results of titration experiments with an environmentally relevant experimental setup, designed to study the roles of swelling clay minerals in the formation of Mg-bearing CaCO₃ phases. We added both Mg-free and Mg-rich calcian solutions to carbonate buffers both in the presence and absence of smectite, and monitored the evolution of the solutions with pH and Ca ion selective electrodes, in order to identify nucleation and phase transition events. Initial products of the titration experiments were aged in their mother solutions for a few months. Both freshly formed and aged materials were studied using a variety of transmission electron microscopy (TEM) techniques. From Mg-free, homogeneous solutions vaterite was the first phase to precipitate. The addition of smectite triggered nucleation at lower supersaturation and generated calcite rather than vaterite. In Mg-rich solutions, aragonite was the first phase to precipitate both without and with clay minerals, and precipitation occurred at similar saturation levels in both samples. In the presence of clays, however, the aragonite nanocrystals were attached to smectite flakes. After the Mg-bearing systems were aged for several months, peculiar assemblages of protodolomite and low-magnesian calcite formed in association with smectite, whereas in the clay-free systems aragonite persisted. These observations suggest that if smectite is present in an environment where carbonates precipitate, the clay mineral has important and complex roles in the formation of Mg-bearing calcium carbonate phases. In addition to enhancing the nucleation of the first carbonate solid, smectite also triggers the formation of calcite-type structures, both at nucleation and in dissolution/reprecipitation reactions during aging.