Interplay between Thermally Induced Aragonite–Calcite Transformation and Multistep Dehydration in a Seawater Spiral Shell (Euplica scripta)

While heating a seawater spiral shell (Euplica scripta), thermally induced aragonite–calcite (A–C) transformation occurred within the temperature region of multistep thermal dehydration. Here, the kinetic interplay between the A–C transformation and thermal dehydration was studied as a possible cause of the reduction in the A–C transformation temperatures. The kinetics of the A–C transformation was systematically investigated under isothermal conditions by powder X-ray diffractometry and under linear nonisothermal conditions by Fourier transform infrared spectroscopy. The thermal dehydration was characterized as a partially overlapping, three-step process by thermogravimetry–differential thermal analysis coupled with mass spectroscopy for the evolved gases. The A–C transformation occurred in the temperature range of the final part of the second dehydration step and the initial part of the third dehydration step. The kinetics of A–C transformation and thermal dehydration were characterized by contracting geometry-type models, in which the respective transformations were regulated by a constant linear advancement rate and diffusional removal of water vapor, respectively. Based on the kinetic results, the mutual interaction of those thermally induced processes is discussed as a possible cause of the reduction in the A–C transformation temperature.