The effect of cationic substitution on the mid-infrared absorption spectra of synthetic, and natural inorganic and biogenic MgCO3-CaCO3 solid solutions (Mg calcites; space group R(3) over barc$; 0-23 mol% MgCO3) is investigated, and the results are compared to disordered proto-dolomite (Ca0.5Mg0.5CO3; space group R(3) over barc$), ordered dolomite (Ca0.5Mg0.5CO3; space group ), huntite (Ca0.25Mg0.75CO3; space group R32), and magnesite (MgCO3; space group R(3) over barc$). The measurements show that significant frequency shifts of the vibrational bands of the carbonate ion group occur as a function of chemical composition. The Mg2+ content of all Mg calcites correlates linearly with the position of the sharp nu 4 mode according to : nu 4 (cm(-1)) = 39.40X(Mg) + 712.20 (r = 0.97).