Minerals belonging to the siderite-magnesite series have been studied using Fourier transform infrared spectroscopy. Pronounced shifts were observed in several of the carbonate vibrational bands. Shifts in the 1430-cm-1 and 880-cm-1 bands and a shift of about 60 cm-1 across the series for the 350-cm-1 band indicate that ftir can be used to distinguish members of this solid-solution series. Plots of mole fraction Fe + Mn against band frequency yielded straight lines for the Mg-rich minerals that deviated from linearity as the siderite end of the series was approached. The observed shifts reflect the differing mass and ionic radius of substituting cations and possible compression of the carbonate ion.