Polymorph Control of Calcium Carbonate Using Insoluble Layered Double Hydroxide

This paper demonstrates crystallization of calcium carbonate in the presence of insoluble Mg–Al layered double hydroxides (LDH). The metastable aragonite phase is preferentially stabilized over the thermodynamically more stable calcite phase in the presence of this insoluble inorganic matrix at ambient temperature. Such aragonite particles possess an unconventional “rosette-like” morphology. In the intermediate stages of crystallization, another metastable phase, vaterite, having a “flaky-floret-like” morphology, is initially formed, and an unusual transformation of ‘vaterite to aragonite’ is observed. The kinetics of such a transformation is systematically studied and captured through their crystal characterization. The presence of Mg-rich layers and the surface hydroxyl groups in LDH is postulated as one of the probable factors for such phase selectivity. The role of “soluble” Mg 2+ ions in affecting polymorph selectivity is ruled out based on the very low concentration of Mg 2+ ions leaching out of LDH. This is the first report of the use of Mg–Al LDH as an “insoluble inorganic material” to promote polymorph selectivity in calcium carbonate. This can also have potential significance in biomineralization.