An extension of the seawater secular-variation model that successfully predicted the observed timing of Phanerozoic MgSO4 vs. KCl marine evaporites and aragonite seas vs. calcite seas has been applied to the prediction of the secular variations in the major ion chemistry of seawater and aragonite seas vs. calcite seas during the Precambrian. Testing of the predictions was based on those Precambrian seafloor carbonate precipitates that have been interpreted by others to have formed originally as aragonite. Of 16 examples of Precambrian seafloor aragonite, 14 fall within the 6 periods of aragonite seas predicted for the Late Archean–Proterozoic by the model, 1 falls on the transition between an aragonite sea and calcite sea, and 1 falls in a period of calcite seas. This strong correlation supports the following predictions of the model: (1) Precambrian seawater was a saline NaCl water with Ca > HCO3 since at least the Late Archean. (2) The major ion compositions of Precambrian seawater chemistry and their secular variations are in the same ranges as those of Phanerozoic seawater. (3) The Mg/Ca mole ratio in seawater has controlled the types of CaCO3 polymorphs that have precipitated from Earth's oceans throughout the Phanerozoic and most, if not all, of the Precambrian.