Mn2As2O9 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(7), and one O(9) atom to form MnO6 octahedra that share corners with two equivalent As(2)O4 tetrahedra, corners with three equivalent As(1)O4 tetrahedra, and edges with two equivalent Mn(2)O6 octahedra. In the second Mn site, Mn(2) is bonded to one O(1), one O(3), one O(5), one O(6), one O(7), and one O(8) atom to form distorted MnO6 octahedra that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra, and edges with two equivalent Mn(1)O6 octahedra. There are two inequivalent As sites. In the first As site, As(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form AsO4 tetrahedra that share corners with two equivalent Mn(2)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra, and  a cornercorner with one As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-60°. In the second As site, As(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form AsO4 tetrahedra that share corners with two equivalent Mn(1)O6 octahedra, corners with three equivalent Mn(2)O6 octahedra, and  a cornercorner with one As(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-59°. There are nine inequivalent O sites. In the first O site, O(5) is bonded in a distorted trigonal planar geometry to one Mn(1), one Mn(2), and one As(2) atom. In the second O site, O(6) is bonded in a bent 120 degrees geometry to one Mn(2) and one As(2) atom. In the third O site, O(7) is bonded in a distorted trigonal planar geometry to one Mn(1), one Mn(2), and one As(2) atom. In the fourth O site, O(8) is bonded in a single-bond geometry to one Mn(2) atom. In the fifth O site, O(1) is bonded in a trigonal planar geometry to one Mn(1), one Mn(2), and one As(1) atom. In the sixth O site, O(9) is bonded in a single-bond geometry to one Mn(1) atom. In the seventh O site, O(2) is bonded in a bent 120 degrees geometry to one Mn(1) and one As(1) atom. In the eighth O site, O(3) is bonded in a trigonal planar geometry to one Mn(1), one Mn(2), and one As(1) atom. In the ninth O site, O(4) is bonded in a bent 120 degrees geometry to one As(1) and one As(2) atom.
Mn2As2O9 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(7), and one O(9) atom to form MnO6 octahedra that share corners with two equivalent As(2)O4 tetrahedra, corners with three equivalent As(1)O4 tetrahedra, and edges with two equivalent Mn(2)O6 octahedra. The Mn(1)-O(1) bond length is 2.16 Å. The Mn(1)-O(2) bond length is 2.03 Å. The Mn(1)-O(3) bond length is 2.17 Å. The Mn(1)-O(5) bond length is 2.18 Å. The Mn(1)-O(7) bond length is 2.05 Å. The Mn(1)-O(9) bond length is 1.74 Å. In the second Mn site, Mn(2) is bonded to one O(1), one O(3), one O(5), one O(6), one O(7), and one O(8) atom to form distorted MnO6 octahedra that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra, and edges with two equivalent Mn(1)O6 octahedra. The Mn(2)-O(1) bond length is 2.12 Å. The Mn(2)-O(3) bond length is 2.15 Å. The Mn(2)-O(5) bond length is 2.18 Å. The Mn(2)-O(6) bond length is 1.99 Å. The Mn(2)-O(7) bond length is 2.23 Å. The Mn(2)-O(8) bond length is 1.75 Å. There are two inequivalent As sites. In the first As site, As(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form AsO4 tetrahedra that share corners with two equivalent Mn(2)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra, and  a cornercorner with one As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-60°. The As(1)-O(1) bond length is 1.73 Å. The As(1)-O(2) bond length is 1.68 Å. The As(1)-O(3) bond length is 1.73 Å. The As(1)-O(4) bond length is 1.79 Å. In the second As site, As(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form AsO4 tetrahedra that share corners with two equivalent Mn(1)O6 octahedra, corners with three equivalent Mn(2)O6 octahedra, and  a cornercorner with one As(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-59°. The As(2)-O(4) bond length is 1.79 Å. The As(2)-O(5) bond length is 1.73 Å. The As(2)-O(6) bond length is 1.69 Å. The As(2)-O(7) bond length is 1.74 Å. There are nine inequivalent O sites. In the first O site, O(5) is bonded in a distorted trigonal planar geometry to one Mn(1), one Mn(2), and one As(2) atom. In the second O site, O(6) is bonded in a bent 120 degrees geometry to one Mn(2) and one As(2) atom. In the third O site, O(7) is bonded in a distorted trigonal planar geometry to one Mn(1), one Mn(2), and one As(2) atom. In the fourth O site, O(8) is bonded in a single-bond geometry to one Mn(2) atom. In the fifth O site, O(1) is bonded in a trigonal planar geometry to one Mn(1), one Mn(2), and one As(1) atom. In the sixth O site, O(9) is bonded in a single-bond geometry to one Mn(1) atom. In the seventh O site, O(2) is bonded in a bent 120 degrees geometry to one Mn(1) and one As(1) atom. In the eighth O site, O(3) is bonded in a trigonal planar geometry to one Mn(1), one Mn(2), and one As(1) atom. In the ninth O site, O(4) is bonded in a bent 120 degrees geometry to one As(1) and one As(2) atom.