ESR Studies on the Paramagnetic Species Detected in Marine Seed Pearls

Abstract

ESR observations were carried out for the seed pearls obtained from Uwa Sea in Ehime prefecture. The seed pearls used for ESR observation were classified in several groups on the basis of their different appearance and, color. The satin white and the pale yellow seed pearls revealed ESR hyperfine structure typical to manganese(II) ion doped into the aragonite matrix in calification phase of biominerals. On the other hand, however, the pearls contaning the blackish materials gave ESR signal of an organic free radical assignable to that contained in melanin, as justified from the isotopic g-factor (g), line width(8) (g=2.0045 and δ=0.8mT), and the microwave saturation behaviour. In this case, the observed hyperfine structre of manganese(II) was found to be much similar to that of manganese (II) doped in calcite rather than that in aragonite matrix. With increasing the ESR intensity of organic free radical (spin/g), the ESR of manganese(II) showed a concomitant decrease in the apparent signal intensity. This suggests that the accumulation of melanin-like free radical in the prismatic pearl layer would be one of the important mechanism to explain the blackish color change occurring in the pearl during the cultivation.