MgMn3Cd2O8 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(3), one O(4), one O(5), one O(6), and one O(7) atom to form MgO6 octahedra that share corners with two equivalent Mn(3)O6 octahedra and  a faceface with one Mn(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 43-48°. There are three inequivalent Mn sites. In the first Mn site, Mn(1) is bonded in a 6-coordinate geometry to one O(3), one O(4), one O(5), one O(6), one O(7), and one O(8) atom. In the second Mn site, Mn(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(6), and one O(8) atom. In the third Mn site, Mn(3) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(7) atom to form MnO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 43-48°. There are two inequivalent Cd sites. In the first Cd site, Cd(1) is bonded in a 5-coordinate geometry to one O(2), one O(3), one O(5), one O(6), and one O(8) atom. In the second Cd site, Cd(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(4), one O(5), one O(7), and one O(8) atom. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Mg(1), one Mn(2), one Mn(3), and one Cd(2) atom. In the second O site, O(2) is bonded in a distorted see-saw-like geometry to one Mn(2), one Mn(3), one Cd(1), and one Cd(2) atom. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Mg(1), one Mn(1), one Mn(2), one Mn(3), and one Cd(1) atom. In the fourth O site, O(4) is bonded in a 5-coordinate geometry to one Mg(1), one Mn(1), one Mn(2), one Mn(3), and one Cd(2) atom. In the fifth O site, O(5) is bonded in a 5-coordinate geometry to one Mg(1), one Mn(1), one Mn(3), one Cd(1), and one Cd(2) atom. In the sixth O site, O(6) is bonded to one Mg(1), one Mn(1), one Mn(2), and one Cd(1) atom to form distorted corner-sharing OMgMn2Cd trigonal pyramids. In the seventh O site, O(7) is bonded in a distorted see-saw-like geometry to one Mg(1), one Mn(1), one Mn(3), and one Cd(2) atom. In the eighth O site, O(8) is bonded to one Mn(1), one Mn(2), one Cd(1), and one Cd(2) atom to form distorted corner-sharing OMn2Cd2 tetrahedra.
MgMn3Cd2O8 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(3), one O(4), one O(5), one O(6), and one O(7) atom to form MgO6 octahedra that share corners with two equivalent Mn(3)O6 octahedra and  a faceface with one Mn(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 43-48°. The Mg(1)-O(1) bond length is 2.15 Å. The Mg(1)-O(3) bond length is 2.09 Å. The Mg(1)-O(4) bond length is 2.27 Å. The Mg(1)-O(5) bond length is 2.12 Å. The Mg(1)-O(6) bond length is 2.09 Å. The Mg(1)-O(7) bond length is 2.11 Å. There are three inequivalent Mn sites. In the first Mn site, Mn(1) is bonded in a 6-coordinate geometry to one O(3), one O(4), one O(5), one O(6), one O(7), and one O(8) atom. The Mn(1)-O(3) bond length is 2.08 Å. The Mn(1)-O(4) bond length is 2.40 Å. The Mn(1)-O(5) bond length is 2.11 Å. The Mn(1)-O(6) bond length is 2.00 Å. The Mn(1)-O(7) bond length is 1.93 Å. The Mn(1)-O(8) bond length is 1.96 Å. In the second Mn site, Mn(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(6), and one O(8) atom. The Mn(2)-O(1) bond length is 2.05 Å. The Mn(2)-O(2) bond length is 2.24 Å. The Mn(2)-O(3) bond length is 2.55 Å. The Mn(2)-O(4) bond length is 2.02 Å. The Mn(2)-O(6) bond length is 1.98 Å. The Mn(2)-O(8) bond length is 1.95 Å. In the third Mn site, Mn(3) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(7) atom to form MnO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 43-48°. The Mn(3)-O(1) bond length is 1.98 Å. The Mn(3)-O(2) bond length is 1.91 Å. The Mn(3)-O(3) bond length is 1.97 Å. The Mn(3)-O(4) bond length is 1.97 Å. The Mn(3)-O(5) bond length is 1.98 Å. The Mn(3)-O(7) bond length is 1.98 Å. There are two inequivalent Cd sites. In the first Cd site, Cd(1) is bonded in a 5-coordinate geometry to one O(2), one O(3), one O(5), one O(6), and one O(8) atom. The Cd(1)-O(2) bond length is 2.22 Å. The Cd(1)-O(3) bond length is 2.51 Å. The Cd(1)-O(5) bond length is 2.23 Å. The Cd(1)-O(6) bond length is 2.23 Å. The Cd(1)-O(8) bond length is 2.33 Å. In the second Cd site, Cd(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(4), one O(5), one O(7), and one O(8) atom. The Cd(2)-O(1) bond length is 2.21 Å. The Cd(2)-O(2) bond length is 2.37 Å. The Cd(2)-O(4) bond length is 2.54 Å. The Cd(2)-O(5) bond length is 2.69 Å. The Cd(2)-O(7) bond length is 2.22 Å. The Cd(2)-O(8) bond length is 2.20 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one Mg(1), one Mn(2), one Mn(3), and one Cd(2) atom. In the second O site, O(2) is bonded in a distorted see-saw-like geometry to one Mn(2), one Mn(3), one Cd(1), and one Cd(2) atom. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Mg(1), one Mn(1), one Mn(2), one Mn(3), and one Cd(1) atom. In the fourth O site, O(4) is bonded in a 5-coordinate geometry to one Mg(1), one Mn(1), one Mn(2), one Mn(3), and one Cd(2) atom. In the fifth O site, O(5) is bonded in a 5-coordinate geometry to one Mg(1), one Mn(1), one Mn(3), one Cd(1), and one Cd(2) atom. In the sixth O site, O(6) is bonded to one Mg(1), one Mn(1), one Mn(2), and one Cd(1) atom to form distorted corner-sharing OMgMn2Cd trigonal pyramids. In the seventh O site, O(7) is bonded in a distorted see-saw-like geometry to one Mg(1), one Mn(1), one Mn(3), and one Cd(2) atom. In the eighth O site, O(8) is bonded to one Mn(1), one Mn(2), one Cd(1), and one Cd(2) atom to form distorted corner-sharing OMn2Cd2 tetrahedra.