A Mg-stabilized triclinic tricalcium silicate form of type Ca3−xMgxSiO5, T3, was synthesized. Rietveld analysis using synchrotron X-ray powder diffraction data suggested that unlike the T1 form, the T3 structure was unmodulated. This refinement illustrated that the only existing model for a triclinic form of tricalcium silicate (T1) can be used to describe the nonmodulated T3 form.