BaMn2FeSi2O9 crystallizes in the monoclinic C2/m space group. Ba(1) is bonded in a 10-coordinate geometry to two equivalent O(2), four equivalent O(4), and four equivalent O(5) atoms. There are three inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(6) atoms to form MnO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Fe(1)O5 trigonal bipyramids, edges with two equivalent Mn(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, and edges with two equivalent Mn(3)O6 octahedra. In the second Mn site, Mn(2) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form MnO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, edges with two equivalent Mn(3)O6 octahedra, and edges with four equivalent Mn(1)O6 octahedra. In the third Mn site, Mn(3) is bonded to two equivalent O(6) and four equivalent O(3) atoms to form MnO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, corners with two equivalent Fe(1)O5 trigonal bipyramids, edges with two equivalent Mn(2)O6 octahedra, and edges with four equivalent Mn(1)O6 octahedra. Fe(1) is bonded to one O(6), two equivalent O(4), and two equivalent O(5) atoms to form distorted FeO5 trigonal bipyramids that share  a cornercorner with one Mn(3)O6 octahedra, corners with two equivalent Mn(1)O6 octahedra, and corners with four equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 62-64°. Si(1) is bonded to one O(2), one O(3), one O(4), and one O(5) atom to form SiO4 tetrahedra that share  a cornercorner with one Mn(1)O6 octahedra,  a cornercorner with one Mn(2)O6 octahedra,  a cornercorner with one Mn(3)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Fe(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 56-62°. There are six inequivalent O sites. In the first O site, O(4) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Fe(1), and one Si(1) atom. In the second O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Fe(1), and one Si(1) atom. In the third O site, O(6) is bonded to one Mn(3), two equivalent Mn(1), and one Fe(1) atom to form OMn3Fe tetrahedra that share  a cornercorner with one O(6)Mn3Fe tetrahedra, corners with four equivalent O(3)Mn3Si trigonal pyramids,  an edgeedge with one O(6)Mn3Fe tetrahedra, and edges with two equivalent O(3)Mn3Si trigonal pyramids. In the fourth O site, O(1) is bonded in a trigonal non-coplanar geometry to one Mn(2) and two equivalent Mn(1) atoms. In the fifth O site, O(2) is bonded in a bent 150 degrees geometry to two equivalent Ba(1) and two equivalent Si(1) atoms. In the sixth O site, O(3) is bonded to one Mn(1), one Mn(2), one Mn(3), and one Si(1) atom to form distorted OMn3Si trigonal pyramids that share corners with two equivalent O(6)Mn3Fe tetrahedra, corners with five equivalent O(3)Mn3Si trigonal pyramids,  an edgeedge with one O(6)Mn3Fe tetrahedra, and  an edgeedge with one O(3)Mn3Si trigonal pyramid.
BaMn2FeSi2O9 crystallizes in the monoclinic C2/m space group. Ba(1) is bonded in a 10-coordinate geometry to two equivalent O(2), four equivalent O(4), and four equivalent O(5) atoms. There is one shorter (2.97 Å) and one longer (3.02 Å) Ba(1)-O(2) bond length. There are two shorter (2.76 Å) and two longer (3.01 Å) Ba(1)-O(4) bond lengths. There are two shorter (2.77 Å) and two longer (2.92 Å) Ba(1)-O(5) bond lengths. There are three inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(6) atoms to form MnO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Fe(1)O5 trigonal bipyramids, edges with two equivalent Mn(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, and edges with two equivalent Mn(3)O6 octahedra. Both Mn(1)-O(1) bond lengths are 2.13 Å. Both Mn(1)-O(3) bond lengths are 2.23 Å. Both Mn(1)-O(6) bond lengths are 2.26 Å. In the second Mn site, Mn(2) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form MnO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, edges with two equivalent Mn(3)O6 octahedra, and edges with four equivalent Mn(1)O6 octahedra. Both Mn(2)-O(1) bond lengths are 1.92 Å. All Mn(2)-O(3) bond lengths are 2.20 Å. In the third Mn site, Mn(3) is bonded to two equivalent O(6) and four equivalent O(3) atoms to form MnO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, corners with two equivalent Fe(1)O5 trigonal bipyramids, edges with two equivalent Mn(2)O6 octahedra, and edges with four equivalent Mn(1)O6 octahedra. Both Mn(3)-O(6) bond lengths are 2.14 Å. All Mn(3)-O(3) bond lengths are 2.29 Å. Fe(1) is bonded to one O(6), two equivalent O(4), and two equivalent O(5) atoms to form distorted FeO5 trigonal bipyramids that share  a cornercorner with one Mn(3)O6 octahedra, corners with two equivalent Mn(1)O6 octahedra, and corners with four equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 62-64°. The Fe(1)-O(6) bond length is 1.89 Å. Both Fe(1)-O(4) bond lengths are 2.00 Å. Both Fe(1)-O(5) bond lengths are 2.00 Å. Si(1) is bonded to one O(2), one O(3), one O(4), and one O(5) atom to form SiO4 tetrahedra that share  a cornercorner with one Mn(1)O6 octahedra,  a cornercorner with one Mn(2)O6 octahedra,  a cornercorner with one Mn(3)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Fe(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 56-62°. The Si(1)-O(2) bond length is 1.66 Å. The Si(1)-O(3) bond length is 1.64 Å. The Si(1)-O(4) bond length is 1.63 Å. The Si(1)-O(5) bond length is 1.63 Å. There are six inequivalent O sites. In the first O site, O(4) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Fe(1), and one Si(1) atom. In the second O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Fe(1), and one Si(1) atom. In the third O site, O(6) is bonded to one Mn(3), two equivalent Mn(1), and one Fe(1) atom to form OMn3Fe tetrahedra that share  a cornercorner with one O(6)Mn3Fe tetrahedra, corners with four equivalent O(3)Mn3Si trigonal pyramids,  an edgeedge with one O(6)Mn3Fe tetrahedra, and edges with two equivalent O(3)Mn3Si trigonal pyramids. In the fourth O site, O(1) is bonded in a trigonal non-coplanar geometry to one Mn(2) and two equivalent Mn(1) atoms. In the fifth O site, O(2) is bonded in a bent 150 degrees geometry to two equivalent Ba(1) and two equivalent Si(1) atoms. In the sixth O site, O(3) is bonded to one Mn(1), one Mn(2), one Mn(3), and one Si(1) atom to form distorted OMn3Si trigonal pyramids that share corners with two equivalent O(6)Mn3Fe tetrahedra, corners with five equivalent O(3)Mn3Si trigonal pyramids,  an edgeedge with one O(6)Mn3Fe tetrahedra, and  an edgeedge with one O(3)Mn3Si trigonal pyramid.