Ca3SnSi2O9 was prepared by solid state reaction of CaCO3, SnO2 and SiO2. The crystal structure [a = 732.83(7), b = 1007.43(8), c = 1043.88(9)pm, beta = 91.064(2)degrees, monoclinic, P2(1)/c, Z = 4, R(p) = 2.48, R(wp) = 3.27%] has been determined with the Rietveld method from neutron powder diffraction data. The structure has isolated pairs of edge sharing Sn(IV)O-6 octahedra. These Sn2O10 groups show an efficient luminescence with a very large Stokes shift.