Mg3(AsO4)2 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form MgO6 octahedra that share corners with six equivalent As(1)O4 tetrahedra, corners with two equivalent Mg(2)O5 trigonal bipyramids, and edges with two equivalent Mg(2)O5 trigonal bipyramids. In the second Mg site, Mg(2) is bonded to one O(1), one O(3), one O(4), and two equivalent O(2) atoms to form MgO5 trigonal bipyramids that share  a cornercorner with one Mg(1)O6 octahedra, corners with three equivalent As(1)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra,  an edgeedge with one As(1)O4 tetrahedra, and  an edgeedge with one Mg(2)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 51°. 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 three equivalent Mg(1)O6 octahedra, corners with three equivalent Mg(2)O5 trigonal bipyramids, and  an edgeedge with one Mg(2)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 39-54°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one As(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Mg(2) and one As(1) atom. In the third O site, O(3) is bonded in a T-shaped geometry to one Mg(1), one Mg(2), and one As(1) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one As(1) atom.
Mg3(AsO4)2 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form MgO6 octahedra that share corners with six equivalent As(1)O4 tetrahedra, corners with two equivalent Mg(2)O5 trigonal bipyramids, and edges with two equivalent Mg(2)O5 trigonal bipyramids. Both Mg(1)-O(1) bond lengths are 2.15 Å. Both Mg(1)-O(3) bond lengths are 2.05 Å. Both Mg(1)-O(4) bond lengths are 2.13 Å. In the second Mg site, Mg(2) is bonded to one O(1), one O(3), one O(4), and two equivalent O(2) atoms to form MgO5 trigonal bipyramids that share  a cornercorner with one Mg(1)O6 octahedra, corners with three equivalent As(1)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra,  an edgeedge with one As(1)O4 tetrahedra, and  an edgeedge with one Mg(2)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 51°. The Mg(2)-O(1) bond length is 2.05 Å. The Mg(2)-O(3) bond length is 2.14 Å. The Mg(2)-O(4) bond length is 1.97 Å. There is one shorter (1.97 Å) and one longer (2.00 Å) Mg(2)-O(2) bond length. 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 three equivalent Mg(1)O6 octahedra, corners with three equivalent Mg(2)O5 trigonal bipyramids, and  an edgeedge with one Mg(2)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 39-54°. The As(1)-O(1) bond length is 1.71 Å. The As(1)-O(2) bond length is 1.69 Å. The As(1)-O(3) bond length is 1.70 Å. The As(1)-O(4) bond length is 1.68 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one As(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Mg(2) and one As(1) atom. In the third O site, O(3) is bonded in a T-shaped geometry to one Mg(1), one Mg(2), and one As(1) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one As(1) atom.