Mutual Relationship between Solid-State Aragonite-Calcite Transformation and Thermal Dehydration of Included Water in Coral Aragonite

A specimen sample of coral skeleton (Pavona, Okinawa, Japan) was selected to study the mechanistic relationship between aragonite calcite transformation and the thermal dehydration of the included water in biogenic aragonite. The sample exhibited three mass loss steps attributed to the thermal dehydration of the included water (2.8% of the sample mass) before the thermal decomposition of CaCO3. The aragonite calcite transformation simultaneously occurred during the second dehydration step of the included water in a temperature region lower by approximately 100 K than in synthetic and geological aragonites. During the thermal dehydration of the included water and the aragonite calcite transformation, the root structure of polygonal plate stacking in the biogenic aragonite was preserved. A possible mechanistic relationship between the thermal dehydration of the included water and the aragonite calcite transformation is explained by the three successive dehydration steps: (a) the diffusional removal of the included water from the interstitial spaces between the polygonal aragonite plates, (b) the dehydration step accompanying the aragonite-calcite transformation in the polygonal aragonite plate, and (c) the diffusional removal of residual water accompanying the growth of as-produced calcite crystals.