In this paper, we present a structural model for the M 1 polymorph of tricalcium silicate Ca 3 SiO 5 from Powder X-Ray Diffraction (XRD) data, including weak intensity superstructure Bragg lines. As no single crystal is available, this structural model has been deduced using the structural relationships found between the previously known triclinic and monoclinic M 3 polymorphs. We find that the better starting set of atomic positions for Rietveld refinement is the triclinic set and not the monoclinic set. A key observation is that the monoclinic M 1 structure is closer to the low temperature triclinic T 3 structure (within the Golovastikov model) than to the higher temperature monoclinic M 3 structure. The unit cell and the set of atomic positions of the two best models, called 3〈 M 〉 Pc and 3〈 T 〉 Pn models, are provided. We prefer the 3〈 M 〉 Pc model for it better reproduces the weak characteristic Bragg lines of the superstructure.