Effect of Organic - Mediated Water Restructuring in Modifying Mineral Precipitation Rates Nita Sahai, Donald Mkhonto, and Yang Yang Department of Geoscience, University of Wisconsin, Madison, WI 53706 We have used Molecular Dynamics (MD) simulations to study the role of organic acid, succinate, on modifying calcite crystal morphology by affecting growth rates in specific crystallographic directions, and have identified that the effect is under kinetic, and not thermodynamic, control. Biocalcification by plankton is related to the processes involved in hydrocarbon formation, migration and storage. a Figure 3. Reaction energies (Gads) of succinate, Na+ and water adsorption on steps in different directions on calcite (104). Higher Gads implies less succinate adsorption, hence, less blocking of step movement. Faster moving steps appear earlier in the sequence. Increasing Gads sequence corresponds to step opening sequence of AFM experiment. Thus, morphology evolution is under kinetic control. Figure 2. Surface energies () of succinate, Na+ and water adsorption on steps in different directions on calcite (104). Lower  implies more stable step, hence, slower moving. Increasing step stability sequence does not correspond to step opening sequence of AFM experiment. Thus, morphology evolution not under thermodynamic control. Figure 1. AFM images of calcite (104) face with evolving etch pit morphology (a) in the presence of increasing concentrations of succinate (b) (from Teng et al., 2008, JACS). Morphology evlves because of preferential opening of steps in specific directions.