Humidity-induced aragonite formation from amorphous calcium carbonate (ACC) with addition of n-butylamine

Direct conversion from amorphous calcium carbonate (ACC) to aragonite has been extremely difficult compared to the other two polymorphs, calcite and vaterite. In the present study, aragonite formation with a high polymorph fraction (>97 %) was achieved from ACC immersed in n-butylamine under 90 % RH (relative humidity) at 30 °C for 2 h. It is noteworthy that aragonite with a high polymorphic fraction was obtained without the addition of Mg2+ ion, which is well known to promote aragonite formation. To understand the effects of hydrophobic and basic properties of n-butylamine, hexane, NH3 aq., and hexane + NH3 aq. were mixed with ACC and left for two weeks. Aragonite was formed only from the mixture of hexane and NH3 aq. Although the detailed mechanism requires further investigations, the obtained results suggest that both hydrophobic and basic properties are required for the crystallization of ACC to aragonite.