Structure Of Freshwater Vaterite Pearl And Its Biomineralization Mechanism

Vaterite is an unstable polymorph of calcium carbonate, which is rare as biomineral in nature. Vaterite tablets/layers are found in Chinese freshwater cultured pearls. Due to the sparsity of natural vaterite, biomineralization about vaterite is deficient.The existence of vaterite results in the lackluster characteristic and low quality of pearls. Vaterite pearls were chose as research object and experimental material in this study. Compared with aragonite nacre, we studied crystallographic orientation, hiberarchy, protein component of vaterite nacre, and in vitro calcium carbonate growth under the control of different proteins and amino acids.Vaterite existed in lackluster pearls has good thermodynamic stability, this property is similar to that of aragonite in common pearls. Based on the analysis of micro-elements, organic content and structural color, we consider that the high content of organic matrix and the irregular layer structure result in the lackluster characteristic of vaterite nacre.Vaterite nacre has a similar hiberarchy to that of nacre with the'brick and mortar'structure. The individual vaterite tablet is batten-like with a mean size of 8μm×2μm×0.4μm. Most vaterite tablets have preferred orientation with [010] direction perpendicular to the tablet surface. In the interior of tablets, we found the twin stacking faults structure using high-resolution transmission electron microscopy (HRTEM).We observed in real time the growth process of aragonite on the fresh surface of nacre in supersaturated CaCO3 solution. The process is as follows: the deposition of ACC, the iso-oriented growth of nanostacks, and the nacre-like tablets forming. These hexagonal tablets with 3–4μm in diameter have a strong texture and the c-axis is perpendicular to the tablet surface. These nacre-like tablets were aligned closely to form a layer that is similar to a real nacre layer.The proteins extracted from aragonite and vaterite pearls are divided into acid insoluble protein (AIM), acid soluble protein (ASM) and water soluble protein (WSM). We found that there are some structural differences among the proteins of two kind pearls. And ASM of vaterite pearls can induce vaterite crystals.Vaterite pearl is considered as a typical biomineral system, and the research route of this study is'property—inorganic structure—protein component—in vitro study', which provides a good supplement for vaterite biomineralization.