Chemical synthesis of CaCO_3 minerals at low temperatures and implication of mechanism of polymorphic transition

摘要

Polymorphs of calcium carbonate were synthesized in a temperature range of 0 to 90 deg C by the slow decomposition of Ca~(2+)-Mg~(2+)-HCO_3-Cl~- -H_2O or Ca~(2+)-Mg~(2+)-Na~+-HCO_3~--Cl~- -H_2O solution and the overgrowth with SrCO_3, BaCO_3 or PbCO_3 as seed crystals. The results show that an increase in temperature favours the formations of metastable aragonite and unstable vaterite, and that an increase in Mg~(2+) concentration or a decrease in Ca(HCO_3)_2 solution concentration promotes the formation of aragonite. The processes and mineralogical mechanisms of polymorphic transition from aragonite to calcite and from vaterite to calcite were investigated by means of XRD and SEM techniques. The results indicate that the rates of polymorphic transition from aragonite to calcite and from vaterite to calcite in aqueous solution are very fast, only within the time order of a day or far shorter, and that their mechanism of polymorphic transition is related to dissolution-reprecipitation process.