Growth process and crystallographic properties of ammonia-induced vaterite
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1272-PP03-06
Growth process and crystallographic properties of ammonia-induced vaterite Qiaona Hu1, Jiaming Zhang1, and Udo Becker1 1 Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, USA ABSTRACT Metastable vaterite crystals were produced by increasing the pH of Ca2+- and CO32+-containing solutions through diffusing ammonia gas. The SEM and TEM studies indicate that this ammonia-induced vaterite is polycrystalline with a 6-fold symmetry of the crystal aggregate. The morphology and crystallographic properties of this assemblage change during crystallization. One hour after nucleation starts, vaterite grains display a spherical structure composed of nano-particles (5-10 nm) with random crystallographic orientations. After that, horizontal layers begin to develop at the edge of the sphere and gradually tilt toward the center as they grow vertically, which results in a three-dimensional morphology with a dent in the center. The vaterite grains mature fully 16 hours after nucleation. TEM analysis indicates the grown vaterite grain (50-60 µm) consists of numerous hexagonal pieces of single crystals (1-2 µm) of similar crystallographic orientations. High-resolution TEM demonstrates that these single crystals grow along (001) with {110} hexagonal boundaries. INTRODUCTION Calcium carbonate (CaCO3) occurs widely on the Earth’s surface and represents the largest geochemical reservoir for carbon.1, 2 Vaterite, a less thermodynamically stable polymorph of CaCO3, has been of great interest because its crystallization is strongly associated with biogenic activities and itself an important precursor in several carbonate-forming processes. Due to its large surface area and high surface energy, vaterite is also largely applied as dispersant in industries, such as paper-making, oil-field drilling fluids, and regenerative medicine.3, 4 Therefore, understanding the vaterite growth process is important to both the materials industry and to the research of biomineralization mechanisms. Previous studies have intensively reported on vaterite formation induced by organic additives or precipitating it from salt solution that were highly supersaturated with respect to CaCO3.5-10 However, only few studies have focused on the ability of inorganic additives promoting vaterite growth.11 The growth process and the crystallographic properties of vaterite crystals have also been poorly documented. In this project, the growth process of vaterite and the function of ammonia as an inorganic additive in vaterite formation have been investigated. EXPERIMENTAL PROCEDURE Vaterite was produced by increasing the pH of Ca2+ and-CO32- (0.002 M) containing solutions by diffusing ammonia gas into solution and, thus, supersaturating the solution with respect to calcium carbonate at ambient conditions. The original pH was adjusted to 3.4 by adding 10% HCl. It was found that the polymorphic composition of the CaCO3 changes with the diffusion rate of ammonia. In this study, the NH3 diffusion rate was adjusted to increase the pH of the
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