Aragonite Crystal Growth and Solid-State Aragonite-Calcite Transformation: A Physico-Geometrical Relationship via Thermal Dehydration of Included Water

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.