Of Caco <sub> 3 </ Sub> In Vitro Simulation Of The Crystal Growth Process And Mechanism Study

The nacre of molluscan shell, which is composed of inorganic aragonite tablets and a few percent of organic polymers filling between the tablets, is a wonderful natural composite material. It shows complex forms, and fascinating mechanical and optical properties. The extraordinarily excellent properties are resulted from the unique layered microstructure, the crystallographic orientation of aragonite tablets and the exquisite control of crystallization over several length scales in biomineralization processes. All of these inspire the research of shell nacre. However, the mechanism of nacre formation is still poorly understood because of lack of sufficient experiment evidences on the nucleation and growth of CaCO₃ in biomineralization process of nacre.In the present study, we conducted SEM and X-ray diffraction experiments to study the microstructure of nacre of Hyriopsis cumingii Lea. The results showed that the nacre is built from a "Brick-Wall" of plate-like aragonite crystals and the aragonite tablets are aligned along (002) direction. Secondly, we conducted an in vitro experiment to simulate the growth of aragonite on nacre surface via the vapor diffusion method in the absence of any other additional additives. By varying the time, substrates, temperature of deposition and CaCl₂ solution concentration, the aragonitic crystals formed on the nacre surface were investigated with SEM, Raman and XRD. The results showed that tower-like stacks of layered aragonite tablets were deposited along the edge of the natural aragonite tablets of nacre, and these layered aragonite tablets obtained after deposition for 24h were transformed from amorphous particles formed at the early stage. In control experiment, the deposit on glass slices is composed of calcite and vaterite. The mechanisms of nucleation, growth, morphology and orientation of the CaCO₃ crystals deposited in vitro on nacre surface were discussed in this paper. Because the environment of the calcium carbonate formation in this experiment is close to the biomineralization process of nacre, we hope the information revealed in the present study will help understand the growth mechanisms of the nacre in shell.