Mössbauer measurements have been made on single crystals of metamagnetic FeCO3 (siderite) at temperatures from 2.5°K to above the Néel temperature (TN=38.4°±0.2°K) and in static magnetic fields of 0-120 kG applied parallel to the spin-ordering axis. Computer analysis of the spectra assuming a stochastic model for the atomic-spin relaxation is used to determine the order parameter η(T) and the atomic-spin relaxation frequency ΩS(T). The temperature dependence of the order parameter is consistent with that expected of a highly anisotropic, Ising-type magnetic system. The relaxation frequency is small compared to the nuclear precession frequency below T=0.7 TN and increases rapidly with temperature. Other parameters obtained are: the saturated hyperfine magnetic field Hsat≡Heff(0)=184±2 kG, quadrupole splitting e2qQ/2=2.06±0.03 mm/sec, and isomer shift I.S.=1.36±0.03 mm/sec relative to iron metal. There was no change in the latter parameters over the range of temperatures and applied fields used in this study. In addition, the sign of the effective magnetic field at the 57Fe nucleus was determined to be positive. © 1969 The American Institute of Physics.