Monohydrocalcite (CaCO(3)·H(2)O: MHC) is similar in composition and synthetic conditions to hydrated amorphous calcium carbonate (ACC), which is focused recently as a key intermediate compound of biomineralization and biomimetic mineralization of calcium carbonate polymorphs. Detailed comparisons of the physicochemical property and reactivity of those hydrated calcium carbonates are required for obtaining fundamental information on the relevancy of those compounds in the mineralization processes. In the present study, kinetics of the thermal dehydration of spherical particles of crystalline MHC was investigated in view of physico-geometrical mechanism. The reaction process was traced systematically by means of thermogravimetry under three different modes of temperature program. A distinguished induction period for the thermal dehydration and cracking of the surface product layer on the way of the established reaction were identified as the characteristic events of the reaction. By interpreting the kinetic results in association with the morphological changes of the reactant particles during the course of reaction, it was revealed that nucleation and crystal growth of calcite regulate the overall kinetics of the thermal dehydration of MHC. In comparison with the thermal dehydration of hydrated ACC, which produces anhydrous ACC as the solid product, the kinetic characteristics of the thermal dehydration of MHC were discussed from the viewpoint of physico-geometry of the component processes.

Thermal decomposition of Ag2CO3 to Ag2O was investigated to identify the physicochemical events that occur during the reaction and to reveal the interactions that cause the complex kinetic behavior of the reaction. Based on comparative investigation of thermal decomposition behavior of six different commercially available Ag2CO3, a physicogeometrical reaction model of two partially overlapping reaction stages is proposed. The reaction stages involve the formation of a surface product layer and an internal reaction in the as-produced core–shell structure of the reacting particles. Immediately after the formation of the surface product layer, the structural phase transitions of Ag2CO3 to two different high-temperature phases occur. Under these conditions, the thermal decomposition behavior is controlled by the diffusional removal of CO2 through the surface product layer and/or the increase in internal partial pressure of CO2. The growth of Ag2O particles in the surface product layer produces possible channels for the diffusion of CO2. The relative rates of the formation of the diffusion channels in the surface product layer and the increase in the internal partial pressure determine whether the internal reaction advances at a steady rate or arrests until thermal decomposition of Ag2O of the surface product layer occurs at higher temperatures. The sample and reaction conditions influence the kinetic behavior of different component processes, resulting in the complex thermal decomposition behavior.

Monohydrocalcite (MHC) and hydrated amorphous calcium carbonate (ACC) indicate many similarities in chemical composition, precipitation condition, and thermal behavior, from which the comparable thermodynamic state of these hydrated calcium carbonates can be deduced. In a reaction system of a mixed aqueous solution of Ca2+ and Mg2+ ions with CO3 2– ion, the MHC single phase is precipitated in a very restricted condition of the molar fraction of Mg2+ from 15 to 25% and in the temperature region from 288 to 303 K, which is surrounded by the precipitation conditions of calcite, hydrated ACC, and hydromagnesite. Two MHC samples with different crystallinities and morphologies are obtained within the present precipitation condition of MHC single phase. Although these MHC samples indicate apparently different thermal behaviors, those behaviors of thermal dehydration and subsequent transformations of as-produced anhydrous calcium carbonate can be correlated to those reported for hydrated ACC. The similarities in the precipitation conditions and thermal behaviors of MHC and hydrated ACC indicate the close relevancy of these compounds in the biomineraliztion and biomimetic mineralization processes of calcium carbonate polymorphs.

A relationship between the physico–geometrical mechanisms of aragonite crystal growth and the thermally induced aragonite–calcite transformation was revealed by focusing on the morphological changes during these processes. Thermal dehydration of the included water during the aragonite–calcite transformation was investigated to characterize the relationship. The trapping of water molecules at the twin boundaries is expected from the aragonite crystal growth mechanism of the twinning of poorly crystalline needle-like crystals to form pseudohexagonal columnar crystals. Heating the aragonite gives the two-step thermal dehydration of the included water (total mass loss due to the dehydration is less than 2% of original sample mass), in which the second dehydration process with rapid water vapor release simultaneously occurs with the aragonite–calcite transformation. During the transformation, the morphology of the aragonite crystal dramatically changes to form dumbbell-like crystal with cauliflower-like structures at each end. The splitting of the aragonite crystal is initiated at both ends of the columnar crystals and propagates to the column center along the twin boundaries. The kinetic behavior of the thermal dehydration during the aragonite–calcite transformation describes the physico–geometrical mechanism of the aragonite–calcite transformation well, and this is closely related to the crystal morphology and the crystallographic characteristics of the synthetic aragonite.

Aiming to find rigorous understanding and novel features for their potential applications, the physico-geometrical kinetics of the thermal decomposition of sodium hydrogencarbonate (SHC) was investigated by focusing on the phenomenological events taking place on a single crystalline particle during the course of the reaction. The overall kinetics evaluated by systematic measurements of the kinetic rate data by thermogravimetry under carefully controlled conditions were interpreted in association with the morphological studies on the precursory reaction, mechanism of surface reaction, structure of the surface product layer, diffusion path of evolved gases, crystal growth of the solid product, and so on. The precursory reaction was identified as the decomposition of impurity, taking place at the boundary between the surface of the SHC crystal and the adhesive small SHC particles deposited on the surface. In flowing dry N(2), the thermal decomposition of SHC proceeds by two-dimensional shrinkage of the reaction interface controlled by chemical reaction with the apparent activation energy of about 100 kJ mol(-1), after rapid completion of the surface reaction and formation of porous surface product layer. Atmospheric CO(2) and water vapor influence differently on the overall kinetics of the thermal decomposition of SHC. Added gas phase of CO(2) slightly inhibits the overall rate because of the increasing contribution of the surface reaction. Under higher water vapor pressure, the physico-geometrical mechanism of the surface reaction changes drastically, indicating the preliminary reformation of reactant surface and the formation of needle crystals of solid product on the surface. The mechanistic change and extended contribution of the surface reaction result in the deceleration of the surface reaction and acceleration of the established reaction.