MgMnO2 crystallizes in the triclinic P-1 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), one O(4), and two equivalent O(3) atoms to form distorted MgO4 trigonal pyramids that share corners with three equivalent Mg(2)O4 tetrahedra,  a cornercorner with one Mn(2)O5 trigonal bipyramid, corners with two equivalent Mg(1)O4 trigonal pyramids, corners with four equivalent Mn(1)O4 trigonal pyramids, and  an edgeedge with one Mn(2)O5 trigonal bipyramid. In the second Mg site, Mg(2) is bonded to one O(3), one O(4), and two equivalent O(2) atoms to form MgO4 tetrahedra that share corners with two equivalent Mg(2)O4 tetrahedra, corners with four equivalent Mn(2)O5 trigonal bipyramids,  a cornercorner with one Mn(1)O4 trigonal pyramid, corners with three equivalent Mg(1)O4 trigonal pyramids,  an edgeedge with one Mn(2)O5 trigonal bipyramid, and  an edgeedge with one Mn(1)O4 trigonal pyramid. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form distorted MnO4 trigonal pyramids that share  a cornercorner with one Mg(2)O4 tetrahedra, corners with four equivalent Mn(2)O5 trigonal bipyramids, corners with two equivalent Mn(1)O4 trigonal pyramids, corners with four equivalent Mg(1)O4 trigonal pyramids, and  an edgeedge with one Mg(2)O4 tetrahedra. In the second Mn site, Mn(2) is bonded to one O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted MnO5 trigonal bipyramids that share corners with four equivalent Mg(2)O4 tetrahedra,  a cornercorner with one Mg(1)O4 trigonal pyramid, corners with four equivalent Mn(1)O4 trigonal pyramids,  an edgeedge with one Mg(2)O4 tetrahedra, edges with two equivalent Mn(2)O5 trigonal bipyramids, and  an edgeedge with one Mg(1)O4 trigonal pyramid. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Mn(2), and two equivalent Mn(1) atoms to form distorted OMgMn3 trigonal pyramids that share corners with four equivalent O(2)Mg2Mn3 trigonal bipyramids, corners with two equivalent O(1)MgMn3 trigonal pyramids, and corners with four equivalent O(3)Mg3Mn trigonal pyramids. In the second O site, O(2) is bonded to two equivalent Mg(2), one Mn(1), and two equivalent Mn(2) atoms to form distorted OMg2Mn3 trigonal bipyramids that share  a cornercorner with one O(3)Mg3Mn trigonal pyramid, corners with four equivalent O(1)MgMn3 trigonal pyramids, edges with two equivalent O(2)Mg2Mn3 trigonal bipyramids, and  an edgeedge with one O(3)Mg3Mn trigonal pyramid. In the third O site, O(3) is bonded to one Mg(2), two equivalent Mg(1), and one Mn(1) atom to form distorted OMg3Mn trigonal pyramids that share  a cornercorner with one O(2)Mg2Mn3 trigonal bipyramid, corners with two equivalent O(3)Mg3Mn trigonal pyramids, corners with four equivalent O(1)MgMn3 trigonal pyramids, and  an edgeedge with one O(2)Mg2Mn3 trigonal bipyramid. In the fourth O site, O(4) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Mg(2), and two equivalent Mn(2) atoms.
MgMnO2 crystallizes in the triclinic P-1 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), one O(4), and two equivalent O(3) atoms to form distorted MgO4 trigonal pyramids that share corners with three equivalent Mg(2)O4 tetrahedra,  a cornercorner with one Mn(2)O5 trigonal bipyramid, corners with two equivalent Mg(1)O4 trigonal pyramids, corners with four equivalent Mn(1)O4 trigonal pyramids, and  an edgeedge with one Mn(2)O5 trigonal bipyramid. The Mg(1)-O(1) bond length is 2.05 Å. The Mg(1)-O(4) bond length is 1.98 Å. Both Mg(1)-O(3) bond lengths are 2.00 Å. In the second Mg site, Mg(2) is bonded to one O(3), one O(4), and two equivalent O(2) atoms to form MgO4 tetrahedra that share corners with two equivalent Mg(2)O4 tetrahedra, corners with four equivalent Mn(2)O5 trigonal bipyramids,  a cornercorner with one Mn(1)O4 trigonal pyramid, corners with three equivalent Mg(1)O4 trigonal pyramids,  an edgeedge with one Mn(2)O5 trigonal bipyramid, and  an edgeedge with one Mn(1)O4 trigonal pyramid. The Mg(2)-O(3) bond length is 1.98 Å. The Mg(2)-O(4) bond length is 1.92 Å. Both Mg(2)-O(2) bond lengths are 2.07 Å. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form distorted MnO4 trigonal pyramids that share  a cornercorner with one Mg(2)O4 tetrahedra, corners with four equivalent Mn(2)O5 trigonal bipyramids, corners with two equivalent Mn(1)O4 trigonal pyramids, corners with four equivalent Mg(1)O4 trigonal pyramids, and  an edgeedge with one Mg(2)O4 tetrahedra. The Mn(1)-O(2) bond length is 2.17 Å. The Mn(1)-O(3) bond length is 2.15 Å. There is one shorter (2.04 Å) and one longer (2.07 Å) Mn(1)-O(1) bond length. In the second Mn site, Mn(2) is bonded to one O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted MnO5 trigonal bipyramids that share corners with four equivalent Mg(2)O4 tetrahedra,  a cornercorner with one Mg(1)O4 trigonal pyramid, corners with four equivalent Mn(1)O4 trigonal pyramids,  an edgeedge with one Mg(2)O4 tetrahedra, edges with two equivalent Mn(2)O5 trigonal bipyramids, and  an edgeedge with one Mg(1)O4 trigonal pyramid. The Mn(2)-O(1) bond length is 2.16 Å. There is one shorter (2.26 Å) and one longer (2.34 Å) Mn(2)-O(2) bond length. Both Mn(2)-O(4) bond lengths are 2.14 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Mn(2), and two equivalent Mn(1) atoms to form distorted OMgMn3 trigonal pyramids that share corners with four equivalent O(2)Mg2Mn3 trigonal bipyramids, corners with two equivalent O(1)MgMn3 trigonal pyramids, and corners with four equivalent O(3)Mg3Mn trigonal pyramids. In the second O site, O(2) is bonded to two equivalent Mg(2), one Mn(1), and two equivalent Mn(2) atoms to form distorted OMg2Mn3 trigonal bipyramids that share  a cornercorner with one O(3)Mg3Mn trigonal pyramid, corners with four equivalent O(1)MgMn3 trigonal pyramids, edges with two equivalent O(2)Mg2Mn3 trigonal bipyramids, and  an edgeedge with one O(3)Mg3Mn trigonal pyramid. In the third O site, O(3) is bonded to one Mg(2), two equivalent Mg(1), and one Mn(1) atom to form distorted OMg3Mn trigonal pyramids that share  a cornercorner with one O(2)Mg2Mn3 trigonal bipyramid, corners with two equivalent O(3)Mg3Mn trigonal pyramids, corners with four equivalent O(1)MgMn3 trigonal pyramids, and  an edgeedge with one O(2)Mg2Mn3 trigonal bipyramid. In the fourth O site, O(4) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Mg(2), and two equivalent Mn(2) atoms.