Mg2As2O7 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(3) and four equivalent O(1) atoms to form distorted MgO6 pentagonal pyramids that share corners with six equivalent As(1)O4 tetrahedra and edges with three equivalent Mg(1)O6 pentagonal pyramids. As(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form AsO4 tetrahedra that share corners with six equivalent Mg(1)O6 pentagonal pyramids and  a cornercorner with one As(1)O4 tetrahedra. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one As(1) atom. In the second O site, O(2) is bonded in a linear geometry to two equivalent As(1) atoms. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one As(1) atom.
Mg2As2O7 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(3) and four equivalent O(1) atoms to form distorted MgO6 pentagonal pyramids that share corners with six equivalent As(1)O4 tetrahedra and edges with three equivalent Mg(1)O6 pentagonal pyramids. Both Mg(1)-O(3) bond lengths are 2.09 Å. There are two shorter (2.05 Å) and two longer (2.20 Å) Mg(1)-O(1) bond lengths. As(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form AsO4 tetrahedra that share corners with six equivalent Mg(1)O6 pentagonal pyramids and  a cornercorner with one As(1)O4 tetrahedra. The As(1)-O(2) bond length is 1.71 Å. The As(1)-O(3) bond length is 1.69 Å. Both As(1)-O(1) bond lengths are 1.68 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one As(1) atom. In the second O site, O(2) is bonded in a linear geometry to two equivalent As(1) atoms. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one As(1) atom.