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Calcium concentration and calcite supersaturation (X) needed for calcium carbonate nucleation and crystal growth in Pyramid Lake (PL) surface water were determined during August of 1997August of , 2000August of , and 2001. PL surface water has X values of 10-16. Notwithstanding high X, calcium carbonate growth did not occur on aragonite single crystals suspended PL surface water for several months. However, calcium solution addition to PL surface-water samples caused reproducible calcium carbonate mineral nucleation and crystal growth. Mean PL surface-water calcium concentration at nucleation was 2.33 mM (n = 10), a value about nine times higher than the ambient PL surface-water calcium concentration (0.26 mM); mean X at nucleation (109 with a standard deviation of 8) is about eight times the PL surface-water X. Calcium concentration and X regulated the calcium carbonate formation in PL nucleation experiments and surface water. Unfiltered samples nucleated at lower X than filtered samples. Calcium concentration and X at nucleation for experiments in the presence of added particles were within one standard deviation of the mean for all samples. Calcium carbonate formation rates followed a simple rate expression of the form, rate (mM/min) = A (X) ? B. The best fit rate equation ''Rate (D mM/D min) = -0.0026 X ? 0.0175 (r = 0.904, n = 10)'' was statistically significant at greater than the 0.01 confidence level and gives, after rearrangement, X at zero rate of 6.7. Nucleation in PL surface water and morphology of calcium carbonate particles formed in PL nucleation experiments and in PL surface-water samples suggest crystal growth inhibition by multiple substances present in PL surface water mediates PL calcium carbonate formation, but there is insufficient information to determine the chemical nature of all inhibitors.