THERMAL ANNEALING OF RADICALS IN ARAGONITIC CACO3 AND CAHPO4.2H2O

High-temperature annealing of radicals in aragonitic CaCO3 and in CaHPO4.2H2O has been studied by electron spin resonance (ESR), thermometric analyses (DTA and TGA) and thermally stimulated gas release (TSGR). An isotropic ESR signal at g = 2.0007 is detected in coral and irradiated CaHPO4.2H2O. The signal in aragonitic coral is annealed out at 150-210-degrees-C, but the same signal in CaHPO4.2H2O fades out at 100-140-degrees-C. The signal annihilation is accompanied by water release from the specimens. The formation yield of the g = 2.0007 signal by irradiation is reduced by sample preheating. The experimental results indicate that isotropic CO2- radicals responsible for the g = 2.0007 signal are closely associated with the crystal water in aragonitic CaCO3 and in CaHPO4.2H2O. Some isotropic CO2- radicals may be associated with CO2- ions isolated by surrounding water molecules.