X-ray powder diffraction data for synthetic materials MgMnSiO4 and Mg0.6Mn1.4SiO4 are reported. Samples were prepared by firing mixtures of MgO, MnCO3, and SiO2 in prescribed molar ratios at 1523 K. Powder diffraction data were collected with a laboratory X-ray source (CuKα) for refinement of unit-cell parameters and synchrotron radiation (λ=1.1980 Å) for intensity measurement of individual reflections. Crystallographic data were MgMnSiO4, orthorhombic, Pnma (No. 62), a=10.4510(1), b=6.12446(5), c=4.80757(4) Å, V=307.717(4) Å3, Z=4, and Dx=3.697 g·cm−3, and Mg0.6Mn1.4SiO4, orthorhombic, Pnma (No. 62), a=10.5241(1), b=6.17903(6), c=4.83927(5) Å, V=314.692(5) Å3, Z=4, and Dx=3.873 g·cm−3.