Two compounds with the chemical composition Ca_2.32Mn_0.68Si_2.00O₇ were synthesized from a molten carbonating silicate with the melilite composition. The synthetic samples were subsequently characterized by X-ray powder diffraction, IR absorption and EPMA, and they proved to be the anhydrous phases isomorphous with kilchoanite Ca₃Si₂O₇ and åkermanite Ca₂MgSi₂O₇ respectively, and to be polymorphous with each other. Their crystallochemical properties suggest that Mn-åkermanite is a high-temperature structure type of Mn-kilchoanite. Substitution of Mn²⁺ with an intermediate ionic radius between those of Mg²⁺ and Ca²⁺ cations establishes the polymorphic relationship between åkermanite Ca₂MgSi₂O₇ and kilchoanite Ca₃Si₂O₇.