X-ray diffraction has been widely used in analyzing Ca-Mg carbonates. Compositions of biogenic and inorganic (Ca,Mg)CO3 crystals are often calculated by comparing their d104 values with published empirical curves. However, previous studies suggested that these curves do not apply to very high-Mg calcite and disordered dolomite. Based on synthesized high-Mg calcite and disordered dolomite, a new empirical curve between values of magnesian calcite d104 and MgCO3 content in the calcite-disordered dolomite solid-solution series is constructed. This new curve is consistent with the significant cell parameter changes accompanying the Mg-Ca cation disorder in dolomite, and it can help the characterization of the MgCO3 content of both natural and synthetic magnesian calcite and disordered dolomite, especially for the mineral mixtures that are not suitable for other analysis methods.