Electron spin resonance studies of Mn2+ in freshwater snail shells: S-ingallsiana, P-ampullacea, P-canaliculata lamarck and fossilized snail shell

摘要

We have studied paramagnetic Mn2+ ions present in the shells of today's univalve freshwater snails, Sinotaia ingallsiana (FS), Pila ampullaceal (PA), Pomacea canaliculata lamarck (PCL) and the fossilized freshwater snail (FFS): Viviparus which are abundant in Thailand. The FS, PA and AG shells in our study were ground into fine powder. A set of seven samples was each then separately annealed for 2 hours in air atmosphere at 300degreesC, 400degreesC 450degreesC, 500degreesC, 550degreesC, 600degreesC and 900degreesC, respectively, while the FFS powder was characterized as received. The FS. PA and PCL shells mainly consist of aragonite and a fraction of calcite. The heat treatments higher than 450degreesC of the FS, PA and PCL powder samples resulted in an irreversible phase tran formation from aragonite to calcite. However, it is found that the FFS shell is mainly made of calcite, with a minor fraction of aragonite. The crystal structure of high temperature annealed FS, PA and PCL samples are quite similar to that of FFS, which indicates that the metamorphosis (aragonite calcite) in the FFS shell had occurred but, not yet completed. although they remained tinder the pressure and temperature of the Earth's Crust, over millions of years. Our detailed ESR spectral analyses of FS, PA, PCL and FFS show that Mn2+ ions enter Ca2+ sites during a biomineralization process. Typical simulated ESR. parameters of FS-500 of Mn2+ at a uniaxial site of calcite are g(x) = g(y) = 2.078+/-0.001, g(z) = 2.002+/-0.001, A(x) = A(y) = 87.50+/-1.00 G, A(z) = 89.00 +/- 1.00 G and D = 115+/-1 G, respectively. It is surprising to find that the ratio of Mn2+ concentration present, in FFS to those in FS, PA and PCL shells evaluated front ESR spectra is as much as 10:1. It is thus possible to gain some insight of manganese incorporation into the freshwater shells. during the biomineralization process.