Phase transitions of natural corals monitored by ESR spectroscopy

The main purpose of this work is to present a systematic study of structure of marine exoskeletons, Acropora coral and its structural transformation upon heat treatments. The coralline sample was ground and characterized as powder throughout this work. Structural identifications of all samples have been confirmed using X-ray diffraction and IR spectroscopy. It was clearly found that the fresh specimen is made of aragonite, a common phase of the mineral CaCO3. Thermal analyses, DSC and TGA were used to monitor structural and thermal decompositions and an irreversible solid-state phase transition from aragonite to calcite of the marine carbonate. Next, the coral powder was annealed at specific temperatures over the range 350–900 °C, and the effects of heat treatment on the structure of coralline samples were carefully studied by Rietveld refinement method. In addition, we have examined Mn2+ paramagnetic ions and free radicals present in the coral and changes of those upon heating by using ESR spectroscopy. The local environments of Mn2+ ions were verified from the calculated ESR spectra using appropriate spin Hamiltonian parameters, i.e. gyromagnetic tensor g, zero-field splitting D and hyperfine tensor A. This work reported structures and compositions as well as physical, chemical and thermal properties of the coralline material upon heat treatments qualitatively and quantitatively.