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The geochemical and biological study of corals
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Nucleation of aragonite in aqueous solutions: implication for coral biomineralization
Monday 7th @ 1510-1530, Conference Room 4
Zeing Long, State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing 100083, China
Al Katz, Department of Physics, CCNY, CUNY, New York, NY 10583, USA
Chen Zhu, Key Laboratory of Surficial Geochemistry, Nanjing University, Nanjing 210093, P.R. China
Liang Zhao, Key Laboratory of Surficial Geochemistry, Nanjing University, Nanjing 210093, P.R. China
Zhengrong Wang* , Department of Earth and Atmospheric Sciences, CCNY, CUNY, New York, NY 10583, USA
Presenter Email: zwang1@ccny.cuny.edu |
| Although marine organisms can secrete both calcite (e.g. mollusks) and aragonite (e.g. corals) to make their skeletons, only metastable aragonite naturally precipitated from supersaturated modern seawater inorganically. Previous studies suggest that Mg/Ca is one of the most important factor to determine the ca-carbonate minerals phases in seawater. In this study, we systematically investigate how solution chemistry (including Mg/Ca, salinity, temperature and other impurities) affects the nucleation dynamics.
Experiments were conducted to nucleate calcite, aragonite, vaterite and amorphous Ca-carbonates. In each experiment, about 3.0 ml of solution was mixed in a 3.5 ml semi-micro polystyrene cuvette from CaCl2/MgCl2, NaHCO3, and NaOH with known concentrations. Solutions were oversaturated relative to calcite/aragonite. Experiments were carried out over a range of saturation states, (Ca + Mg)/CO32- and Mg/Ca activity ratios between 15 and 40 oC, and the turbidity and pH of the solution were continuously monitored using a fiber optic spectrometers coupled with a CCD detector (measured at 700 nm wavelength), and a pH micro-probe, respectively. Induction time in each experiment was deduced from the turbidity variation with time. The saturation state of the solution and ion speciation were calculated using PHREEQC program and measured pH values. Using these information, the surface energy of precipitates can be extracted from the correlation between induction time and saturation states. Over the course of the experiments, precipitates were observed under SEM and mineral phases were determined by XRD method.
Our experimental results show analyses based on turbidity results provide a more accurate estimate of the induction-time compared with those based on pH data. Our precipitation experiments with similar metal (Ca or Mg)/CO32- activity ratios are consistent with classical nucleation theory (CNT), while the surface energy derived from CNT varies with temperature, mineral phases, Ca/CO32-, Mg/Ca, and some other impurities in solution.
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