Magnesian calcites with compositions between 0 and 24 mole% MgCO 3 were synthesized at high temperatures and pressures in cold-seal pressure vessels and in a piston-cylinder apparatus. X-ray powder diffraction and atomic absorption analyses of these phases reveal a non-linear, but smooth, variation in volume and c/a with composition up to about 20 mole% MgCO 3 . Negative excess volumes exist below 20 mole% MgCO 3 , and positive excess volumes exist above 20 mole% MgCO 3 . Several samples of biogenic magnesian calcites in the same composition range exhibit chemical heterogeneities. Echinoid skeletal parts vary by up to 5 mole% MgCO 3 . Algae has neighboring domains differing by up to 10 mole% MgCO 3 . Difference in unit cell geometry with respect to synthetic phases were also observed. Cell volumes and c/a ratios for most biogenic specimens do not vary smoothly with composition and generally exceed those of synthetic phases. Minor element concentrations do not account for unit cell volume discrepancies. As a result, use of existing X-ray determinative curves based on synthetic phases can lead to errors of over 5 mole% MgCO 3 in the estimation of biogenic magnesian calcite compositions.--Modified journal abstract.