BECAUSE of the widespread occurrence of carbonates in marine sediments there has been a continuing interest in the mineralogy of carbonate phases that can be chemically precipitated from seawater1–9. The discovery of metastable carbonate minerals (aragonite and high-magnesium calcites) of non-skeletal origin occurring as cementing material in submarine sedimentary environments10–12 has further stimulated interest in mechanisms that might explain these occurrences. Aragonites have been formed from seawater in a variety of experiments approximating natural conditions7,8,13. Attempts at the synthesis of high-magnesium calcites in such conditions have met with little success. We only know of three published papers in which high-magnesium calcites have been precipitated from seawater2,5,9,. In only one of these was supporting X-ray diffraction evidence presented to document the mineralogy and in all cases the phases were produced by the addition of Na2CO3 or NaHCO3.