Natural siderite is a valuable iron mineral composed of ferrous carbonate (FeCO3), which is commonly found in hydrothermal veins and contains no sulfur or phosphorus. In this paper, micro-sized FeCO3 crystallites are synthesized via a facile hydrothermal route, and almost all of them possess a rhombohedral shape similar to that of natural products. When applied as an anode material for lithium ion batteries, the synthetic siderite can deliver an initial specific discharge capacity of ∼1587 mAh g−1 with a coulombic efficiency of 68% at 200 mA g−1, remaining a reversible value of 1018 mAh g−1 over 120 cycles. Even at a high current density of 1000 mA g−1, after 120 cycles the residual specific capacity (812 mAh g−1) is still higher than the theoretical capacity of FeCO3 (463 mAh g−1). Moreover, a novel reversible conversion mechanism accounts for the excellent electrochemical performances of rhombohedral FeCO3 to a great extent, implying the potential applicability of synthetic siderite as lithium ion battery anodes.