Mg4Ta2O9 is Ilmenite-like structured and crystallizes in the trigonal P-3c1 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to six equivalent O(2) atoms to form MgO6 octahedra that share corners with three equivalent Mg(2)O6 octahedra, corners with six equivalent Ta(1)O6 octahedra, edges with three equivalent Mg(1)O6 octahedra, and  a faceface with one Mg(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 47-59°. In the second Mg site, Mg(2) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted MgO6 octahedra that share corners with three equivalent Mg(1)O6 octahedra, corners with three equivalent Mg(2)O6 octahedra, corners with three equivalent Ta(1)O6 octahedra, edges with three equivalent Ta(1)O6 octahedra, and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-65°. Ta(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted TaO6 octahedra that share corners with three equivalent Mg(2)O6 octahedra, corners with six equivalent Mg(1)O6 octahedra, edges with three equivalent Mg(2)O6 octahedra, and  a faceface with one Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 47-59°. There are two inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Mg(2) and two equivalent Ta(1) atoms to form distorted OMg2Ta2 trigonal pyramids that share corners with four equivalent O(1)Mg2Ta2 trigonal pyramids, corners with eight equivalent O(2)Mg3Ta trigonal pyramids, edges with two equivalent O(1)Mg2Ta2 trigonal pyramids, and edges with two equivalent O(2)Mg3Ta trigonal pyramids. In the second O site, O(2) is bonded to one Mg(2), two equivalent Mg(1), and one Ta(1) atom to form distorted OMg3Ta trigonal pyramids that share corners with four equivalent O(1)Mg2Ta2 trigonal pyramids, corners with eight equivalent O(2)Mg3Ta trigonal pyramids,  an edgeedge with one O(1)Mg2Ta2 trigonal pyramid, and edges with three equivalent O(2)Mg3Ta trigonal pyramids.
Mg4Ta2O9 is Ilmenite-like structured and crystallizes in the trigonal P-3c1 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to six equivalent O(2) atoms to form MgO6 octahedra that share corners with three equivalent Mg(2)O6 octahedra, corners with six equivalent Ta(1)O6 octahedra, edges with three equivalent Mg(1)O6 octahedra, and  a faceface with one Mg(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 47-59°. There are three shorter (2.07 Å) and three longer (2.14 Å) Mg(1)-O(2) bond lengths. In the second Mg site, Mg(2) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted MgO6 octahedra that share corners with three equivalent Mg(1)O6 octahedra, corners with three equivalent Mg(2)O6 octahedra, corners with three equivalent Ta(1)O6 octahedra, edges with three equivalent Ta(1)O6 octahedra, and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-65°. All Mg(2)-O(1) bond lengths are 2.02 Å. All Mg(2)-O(2) bond lengths are 2.19 Å. Ta(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted TaO6 octahedra that share corners with three equivalent Mg(2)O6 octahedra, corners with six equivalent Mg(1)O6 octahedra, edges with three equivalent Mg(2)O6 octahedra, and  a faceface with one Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 47-59°. All Ta(1)-O(1) bond lengths are 2.12 Å. All Ta(1)-O(2) bond lengths are 1.90 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Mg(2) and two equivalent Ta(1) atoms to form distorted OMg2Ta2 trigonal pyramids that share corners with four equivalent O(1)Mg2Ta2 trigonal pyramids, corners with eight equivalent O(2)Mg3Ta trigonal pyramids, edges with two equivalent O(1)Mg2Ta2 trigonal pyramids, and edges with two equivalent O(2)Mg3Ta trigonal pyramids. In the second O site, O(2) is bonded to one Mg(2), two equivalent Mg(1), and one Ta(1) atom to form distorted OMg3Ta trigonal pyramids that share corners with four equivalent O(1)Mg2Ta2 trigonal pyramids, corners with eight equivalent O(2)Mg3Ta trigonal pyramids,  an edgeedge with one O(1)Mg2Ta2 trigonal pyramid, and edges with three equivalent O(2)Mg3Ta trigonal pyramids.