In order to interpret the formation mechanism of calcium carbonate polymorphs, we propose and construct a new 'precipitation diagram', which has two variables: the driving force for nucleation and temperature. The precipitation experiments were carried out by mixing calcium chloride and sodium carbonate aqueous solutions. As a result, a calcite-vaterite co-precipitation zone, a vaterite precipitation zone, a vaterite-aragonite co-precipitation zone and an aragonite precipitation zone can be defined. Theoretical considerations suggest that the steady state nucleation theory can explain well the appearance of these four zones, and the first-order importance of the temperature dependency of surface free energy in the nucleation of aragonite. Furthermore, the addition of an impurity will likely result in the change of these energies, and this precipitation diagram gives a new basis for interpreting the nature of the polymorphs precipitated in both inorganic and biological environments.
Abstract--Spherical kaolinite has been synthesized for the first time from noncrystalline aluminosilicate material in hydrothermal experiments conducted between 150 ~ and 250~ and under autogenous vapor pressure. Sphernles, whose mean diameters depended on growth conditions (0.1-0.6 urn), were formed surrounding the noncrystalline aluminosilicate in all products of 150 ~ and 200*(2 runs and coexisted with platy or lath-shaped kaolinite in the products of 250~ runs. The estimated percentages of spherules in the products increased from about 1% in the 150~ days product to about 74% in 200~ days product, and decreased from about 21% in 250~ days product to 0% in 250~ days product. Lattice images by high-resolution electron microscol~e indicated that the spherules consisted of nearly concentric stackings of layers with a unit spacing of 7A, which were sectored by radiating boundaries. The mean chemical composition of the spherules (A1203/SiO 2 = 0.58) analyzed by the analytical electron microscope is similar to that of kaolinite (Al~O3/SiOz = 0.5). Even in the case of the product abundant in spherule (200~ days), X-ray powder diffraction patterns of the wetted products, e.g., of the 200~ day run, showed the 7.14-~ (001) reflection of kaolinite. The 020 reflection was broad, indicating the existence of abundant (001) layer displacements. The b axis (8.94 ,~) were within the kaolinite range (8.93-8.94 ~,). No infrared absorption peaks were observed at 3550 cm -~ which would correspond to halloysite. The differential thermal analysis slope ratios of the endothermic peak at about 550~ (1.4-2.3) were in the kaolinite range (0.78-2.39).
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