On the Transition Temperature to Calcite and Cell Lengths for Various Biogenic Aragonites

Abstract

In order to understand the mineralogical difference between biogenic aragonites and their geological or synthetic ones, the transition temperature from aragonite to calcite by heating and the cell lengths of a number of biogenic arago- nites have been measured using conventional and high-temperature XRD, as well as those of abiotic ones. Among 21 specimens, most biogenic aragonites showed a transition temperature 60–100 °C lower than that for abiotic ones. However, the shells of land snails showed almost similar transition temperatures. The temperature range from the beginning to the completion of the transition was also varied among the biogenic aragonites. On the other hand, the axial ratios (a/b and c/b) of arago- nites in marine molluscan species were considerably larger than those of abiotic ones. However, aragonites in freshwater molluscan species and land snails showed axial ratios similar to abiotic ones. X-ray microanalysis suggested that the origin of such abnormal cell lengths was sodium incorporated in the aragonite crystals, not due to lattice distortion induced by the intracrystalline organic molecules proposed in previous researches