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The host and impurity excitations in FeCO, containing 3% Mn2+ are studied in the range from 12.5 to 500 cm-1. The host excitations are observed both in the antiferromagnetic and the paramagnetic temperature range and are well described for a wide temperature range with a single ion Hamiltonian, including the interactions between all sublevels coming from the ground term 6D of the Fe2+ ion: the temperature dependent trigonal field, the spin-orbit interaction, and the Ising molecular field. For the first time in a n Ising antiferromagnet a pair mode (host excitation plus impurity excitation) characterized by the absence of magnon-magnon interaction is observed.The g-factors of the observed lines are measured. A sharp drop of the g-factors near T X cannot be explained by an existent theory of the g-factor renormalization.Ferrous carbonate has attracted the attention of scientists for over two decades, to a great extent owing to its extraordinary physical (magnetic in particular) properties. Without reviewing the niimerous stuaies of FeCO, as it. was done in [ 11, we shall remind of the essential data available.Ferrous carbonate has a calcite-type rhombohedra1 structure (Fig. I ) The crystal field operator of 0, symmetry is diagonal in the basis (1) and splits the ground term into two sublevels, 6T2, and sEg, with energy level separation of lODp (Dp = lo3 ern-l), the triply degenerate sublevel is the lowest. Allowing for the trigonal