Mg3Mn2(Si2O7)2 crystallizes in the monoclinic C2/c space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), one O(2), one O(4), and two equivalent O(7) atoms to form distorted MgO5 square pyramids that share  a cornercorner with one Mn(2)O6 octahedra,  a cornercorner with one Si(2)O4 tetrahedra, corners with three equivalent Si(1)O4 tetrahedra,  a cornercorner with one Mg(2)O4 trigonal pyramid,  an edgeedge with one Mn(2)O6 octahedra, and  an edgeedge with one Mg(1)O5 square pyramid. The corner-sharing octahedral tilt angles are 42°. In the second Mg site, Mg(2) is bonded to two equivalent O(1) and two equivalent O(3) atoms to form distorted MgO4 trigonal pyramids that share corners with two equivalent Mn(2)O6 octahedra, corners with two equivalent Mg(1)O5 square pyramids, and corners with four equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 59°. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded in a 6-coordinate geometry to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms. In the second Mn site, Mn(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form MnO6 octahedra that share corners with two equivalent Mg(1)O5 square pyramids, corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, corners with two equivalent Mg(2)O4 trigonal pyramids, and edges with two equivalent Mg(1)O5 square pyramids. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Mn(2)O6 octahedra, corners with three equivalent Mg(1)O5 square pyramids, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 49°. In the second Si site, Si(2) is bonded to one O(1), one O(3), one O(5), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one Mn(2)O6 octahedra,  a cornercorner with one Mg(1)O5 square pyramid, corners with two equivalent Si(1)O4 tetrahedra, and corners with two equivalent Mg(2)O4 trigonal pyramids. The corner-sharing octahedral tilt angles are 73°. There are seven inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Mg(2), one Mn(2), and one Si(2) atom to form distorted corner-sharing OMg2MnSi tetrahedra. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mn(1), and one Mn(2) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Mg(2), one Mn(1), and one Si(2) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Mg(1), one Mn(1), one Mn(2), and one Si(1) atom. In the fifth O site, O(5) is bonded in a bent 120 degrees geometry to one Si(1) and one Si(2) atom. In the sixth O site, O(6) is bonded in a bent 120 degrees geometry to one Si(1) and one Si(2) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one Si(1) atom.
Mg3Mn2(Si2O7)2 crystallizes in the monoclinic C2/c space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), one O(2), one O(4), and two equivalent O(7) atoms to form distorted MgO5 square pyramids that share  a cornercorner with one Mn(2)O6 octahedra,  a cornercorner with one Si(2)O4 tetrahedra, corners with three equivalent Si(1)O4 tetrahedra,  a cornercorner with one Mg(2)O4 trigonal pyramid,  an edgeedge with one Mn(2)O6 octahedra, and  an edgeedge with one Mg(1)O5 square pyramid. The corner-sharing octahedral tilt angles are 42°. The Mg(1)-O(1) bond length is 2.11 Å. The Mg(1)-O(2) bond length is 2.06 Å. The Mg(1)-O(4) bond length is 2.30 Å. There is one shorter (2.04 Å) and one longer (2.21 Å) Mg(1)-O(7) bond length. In the second Mg site, Mg(2) is bonded to two equivalent O(1) and two equivalent O(3) atoms to form distorted MgO4 trigonal pyramids that share corners with two equivalent Mn(2)O6 octahedra, corners with two equivalent Mg(1)O5 square pyramids, and corners with four equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 59°. Both Mg(2)-O(1) bond lengths are 2.06 Å. Both Mg(2)-O(3) bond lengths are 2.02 Å. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded in a 6-coordinate geometry to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms. Both Mn(1)-O(2) bond lengths are 1.91 Å. Both Mn(1)-O(3) bond lengths are 2.03 Å. Both Mn(1)-O(4) bond lengths are 2.39 Å. In the second Mn site, Mn(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form MnO6 octahedra that share corners with two equivalent Mg(1)O5 square pyramids, corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, corners with two equivalent Mg(2)O4 trigonal pyramids, and edges with two equivalent Mg(1)O5 square pyramids. Both Mn(2)-O(1) bond lengths are 2.39 Å. Both Mn(2)-O(2) bond lengths are 1.89 Å. Both Mn(2)-O(4) bond lengths are 2.00 Å. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Mn(2)O6 octahedra, corners with three equivalent Mg(1)O5 square pyramids, and corners with two equivalent Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 49°. The Si(1)-O(4) bond length is 1.64 Å. The Si(1)-O(5) bond length is 1.64 Å. The Si(1)-O(6) bond length is 1.64 Å. The Si(1)-O(7) bond length is 1.62 Å. In the second Si site, Si(2) is bonded to one O(1), one O(3), one O(5), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one Mn(2)O6 octahedra,  a cornercorner with one Mg(1)O5 square pyramid, corners with two equivalent Si(1)O4 tetrahedra, and corners with two equivalent Mg(2)O4 trigonal pyramids. The corner-sharing octahedral tilt angles are 73°. The Si(2)-O(1) bond length is 1.65 Å. The Si(2)-O(3) bond length is 1.63 Å. The Si(2)-O(5) bond length is 1.64 Å. The Si(2)-O(6) bond length is 1.66 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Mg(2), one Mn(2), and one Si(2) atom to form distorted corner-sharing OMg2MnSi tetrahedra. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mn(1), and one Mn(2) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Mg(2), one Mn(1), and one Si(2) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Mg(1), one Mn(1), one Mn(2), and one Si(1) atom. In the fifth O site, O(5) is bonded in a bent 120 degrees geometry to one Si(1) and one Si(2) atom. In the sixth O site, O(6) is bonded in a bent 120 degrees geometry to one Si(1) and one Si(2) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one Si(1) atom.