Mg3Th crystallizes in the hexagonal P6_3/mmc space group. Mg(1) is bonded to eight equivalent Mg(1) and four equivalent Th(1) atoms to form distorted MgTh4Mg8 cuboctahedra that share corners with four equivalent Th(1)Mg12 cuboctahedra, corners with fourteen equivalent Mg(1)Th4Mg8 cuboctahedra, edges with six equivalent Th(1)Mg12 cuboctahedra, edges with twelve equivalent Mg(1)Th4Mg8 cuboctahedra, faces with four equivalent Th(1)Mg12 cuboctahedra, and faces with sixteen equivalent Mg(1)Th4Mg8 cuboctahedra. Th(1) is bonded to twelve equivalent Mg(1) atoms to form ThMg12 cuboctahedra that share corners with six equivalent Th(1)Mg12 cuboctahedra, corners with twelve equivalent Mg(1)Th4Mg8 cuboctahedra, edges with eighteen equivalent Mg(1)Th4Mg8 cuboctahedra, faces with eight equivalent Th(1)Mg12 cuboctahedra, and faces with twelve equivalent Mg(1)Th4Mg8 cuboctahedra.
Mg3Th crystallizes in the hexagonal P6_3/mmc space group. Mg(1) is bonded to eight equivalent Mg(1) and four equivalent Th(1) atoms to form distorted MgTh4Mg8 cuboctahedra that share corners with four equivalent Th(1)Mg12 cuboctahedra, corners with fourteen equivalent Mg(1)Th4Mg8 cuboctahedra, edges with six equivalent Th(1)Mg12 cuboctahedra, edges with twelve equivalent Mg(1)Th4Mg8 cuboctahedra, faces with four equivalent Th(1)Mg12 cuboctahedra, and faces with sixteen equivalent Mg(1)Th4Mg8 cuboctahedra. There are a spread of Mg(1)-Mg(1) bond distances ranging from 3.22-3.50 Å. There are two shorter (3.30 Å) and two longer (3.38 Å) Mg(1)-Th(1) bond lengths. Th(1) is bonded to twelve equivalent Mg(1) atoms to form ThMg12 cuboctahedra that share corners with six equivalent Th(1)Mg12 cuboctahedra, corners with twelve equivalent Mg(1)Th4Mg8 cuboctahedra, edges with eighteen equivalent Mg(1)Th4Mg8 cuboctahedra, faces with eight equivalent Th(1)Mg12 cuboctahedra, and faces with twelve equivalent Mg(1)Th4Mg8 cuboctahedra.