Soft corals contain small spicules of calcium carbonate called "sclerites", which are biomineralized structures composed of an organic matrix and a mineral fraction. Since informations concerning the structure of sclerites, their crystallization properties and mineral composition are scarce, we have studied these parameters in the alcyonarian, Sinularia polydactyla. In the first step, we have determined the structure and shape of the sclerites. Then, we have identified the polymorphs of calcium carbonate both by X-ray diffraction (XRD) and Raman microprobe analysis. A mineral phase in the sclerites was identified as calcite, as revealed by the calcite (104) reflection. An electron probe microanalyzer was used to analyze the element on the sclerite. This analysis indicates that the sclerite has both Ca and Mg bearing calcite, in which Mg showed a relatively high concentration (5-10mol%). Analysis of proteinaceous components of calcified sclerites reveals the significance of protein composition to calcification. The function of matrix proteins during bio-calcification was studied by simulating the nucleation and growth of calcium carbonates in vitro. After precipitation of CaCO3 in the experimental design, the obtained crystals were characterized by scanning electron microscope (SEM) and XRD. The matrix proteins, which were purified from the calcified sclerites, can control the morphology of CaCO3 crystals and are potentially involved in the process of bio-calcification. (c) 2008 Elsevier B.V. All rights reserved.