MgTm(FeO2)2 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to two equivalent O(4) and three equivalent O(1) atoms to form distorted MgO5 trigonal bipyramids that share corners with four equivalent Tm(1)O6 octahedra, corners with four equivalent Mg(1)O5 trigonal bipyramids,  an edgeedge with one Tm(1)O6 octahedra, and edges with two equivalent Mg(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 14-80°. Tm(1) is bonded to three equivalent O(3) and three equivalent O(4) atoms to form TmO6 octahedra that share corners with three equivalent Fe(1)O4 tetrahedra, corners with four equivalent Mg(1)O5 trigonal bipyramids, edges with six equivalent Tm(1)O6 octahedra, and  an edgeedge with one Mg(1)O5 trigonal bipyramid. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(3) and three equivalent O(2) atoms to form FeO4 tetrahedra that share corners with three equivalent Tm(1)O6 octahedra and corners with six equivalent Fe(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 61-64°. In the second Fe site, Fe(2) is bonded in a 3-coordinate geometry to one O(2) and two equivalent O(1) atoms. There are four inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Mg(1) and two equivalent Fe(2) atoms to form distorted OMg3Fe2 trigonal bipyramids that share corners with four equivalent O(4)Tm3Mg2 square pyramids, corners with two equivalent O(2)Fe4 tetrahedra, corners with four equivalent O(1)Mg3Fe2 trigonal bipyramids,  an edgeedge with one O(4)Tm3Mg2 square pyramid, and edges with two equivalent O(1)Mg3Fe2 trigonal bipyramids. In the second O site, O(2) is bonded to one Fe(2) and three equivalent Fe(1) atoms to form OFe4 tetrahedra that share corners with three equivalent O(3)Tm3Fe tetrahedra, corners with six equivalent O(2)Fe4 tetrahedra, and corners with two equivalent O(1)Mg3Fe2 trigonal bipyramids. In the third O site, O(3) is bonded to three equivalent Tm(1) and one Fe(1) atom to form OTm3Fe tetrahedra that share corners with three equivalent O(4)Tm3Mg2 square pyramids, corners with three equivalent O(2)Fe4 tetrahedra, corners with six equivalent O(3)Tm3Fe tetrahedra, and edges with three equivalent O(4)Tm3Mg2 square pyramids. In the fourth O site, O(4) is bonded to two equivalent Mg(1) and three equivalent Tm(1) atoms to form OTm3Mg2 square pyramids that share corners with four equivalent O(4)Tm3Mg2 square pyramids, corners with three equivalent O(3)Tm3Fe tetrahedra, corners with four equivalent O(1)Mg3Fe2 trigonal bipyramids, edges with two equivalent O(4)Tm3Mg2 square pyramids, edges with three equivalent O(3)Tm3Fe tetrahedra, and  an edgeedge with one O(1)Mg3Fe2 trigonal bipyramid.
MgTm(FeO2)2 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to two equivalent O(4) and three equivalent O(1) atoms to form distorted MgO5 trigonal bipyramids that share corners with four equivalent Tm(1)O6 octahedra, corners with four equivalent Mg(1)O5 trigonal bipyramids,  an edgeedge with one Tm(1)O6 octahedra, and edges with two equivalent Mg(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 14-80°. Both Mg(1)-O(4) bond lengths are 2.28 Å. There is one shorter (2.05 Å) and two longer (2.15 Å) Mg(1)-O(1) bond lengths. Tm(1) is bonded to three equivalent O(3) and three equivalent O(4) atoms to form TmO6 octahedra that share corners with three equivalent Fe(1)O4 tetrahedra, corners with four equivalent Mg(1)O5 trigonal bipyramids, edges with six equivalent Tm(1)O6 octahedra, and  an edgeedge with one Mg(1)O5 trigonal bipyramid. There are two shorter (2.17 Å) and one longer (2.21 Å) Tm(1)-O(3) bond length. There are two shorter (2.29 Å) and one longer (2.37 Å) Tm(1)-O(4) bond length. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(3) and three equivalent O(2) atoms to form FeO4 tetrahedra that share corners with three equivalent Tm(1)O6 octahedra and corners with six equivalent Fe(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 61-64°. The Fe(1)-O(3) bond length is 2.14 Å. There is one shorter (1.95 Å) and two longer (2.01 Å) Fe(1)-O(2) bond lengths. In the second Fe site, Fe(2) is bonded in a 3-coordinate geometry to one O(2) and two equivalent O(1) atoms. The Fe(2)-O(2) bond length is 1.99 Å. Both Fe(2)-O(1) bond lengths are 2.30 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Mg(1) and two equivalent Fe(2) atoms to form distorted OMg3Fe2 trigonal bipyramids that share corners with four equivalent O(4)Tm3Mg2 square pyramids, corners with two equivalent O(2)Fe4 tetrahedra, corners with four equivalent O(1)Mg3Fe2 trigonal bipyramids,  an edgeedge with one O(4)Tm3Mg2 square pyramid, and edges with two equivalent O(1)Mg3Fe2 trigonal bipyramids. In the second O site, O(2) is bonded to one Fe(2) and three equivalent Fe(1) atoms to form OFe4 tetrahedra that share corners with three equivalent O(3)Tm3Fe tetrahedra, corners with six equivalent O(2)Fe4 tetrahedra, and corners with two equivalent O(1)Mg3Fe2 trigonal bipyramids. In the third O site, O(3) is bonded to three equivalent Tm(1) and one Fe(1) atom to form OTm3Fe tetrahedra that share corners with three equivalent O(4)Tm3Mg2 square pyramids, corners with three equivalent O(2)Fe4 tetrahedra, corners with six equivalent O(3)Tm3Fe tetrahedra, and edges with three equivalent O(4)Tm3Mg2 square pyramids. In the fourth O site, O(4) is bonded to two equivalent Mg(1) and three equivalent Tm(1) atoms to form OTm3Mg2 square pyramids that share corners with four equivalent O(4)Tm3Mg2 square pyramids, corners with three equivalent O(3)Tm3Fe tetrahedra, corners with four equivalent O(1)Mg3Fe2 trigonal bipyramids, edges with two equivalent O(4)Tm3Mg2 square pyramids, edges with three equivalent O(3)Tm3Fe tetrahedra, and  an edgeedge with one O(1)Mg3Fe2 trigonal bipyramid.