The Mg:Ca ratio at which the calcium carbonate mineral that has pseudohomogeneously precipitated from seawater changes from calcite to aragonite was experimentally determined as a function of temperature. Results indicate a dramatic change in the critical Mg:Ca ratio over a relatively small temperature range. For example, in normal seawater with a Mg:Ca of 5:1, the critical temperature is 6 ± 3 °C. At a temperature of 25 °C, however, Mg:Ca must be less than 1:4 ± 0.1 for calcite to form. These data indicate that paleoclimatic conditions at the time of carbonate deposition must be considered when attempting to make inferences about variation in other factors, such as paleo-P</em>CO2 (P</em> = pressure) and paleo-Mg:Ca ratios, that may control carbonate mineral formation by abiotic processes. However, the inverse relation between inferred global paleoclimatic conditions and the dominant type of CaCO3 polymorph being abiotically deposited (i.e., warm conditions and low-Mg calcite, versus cool conditions and aragonite) suggest that substantial changes must have taken place in ocean chemistry over Phanerozoic time.