Mg3(AsO4)2 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), one O(5), one O(6), and two equivalent O(2) atoms to form distorted MgO5 square pyramids that share corners with three equivalent As(1)O4 tetrahedra, corners with three equivalent Mg(2)O5 trigonal bipyramids,  an edgeedge with one Mg(1)O5 square pyramid, and  an edgeedge with one As(2)O4 tetrahedra. In the second Mg site, Mg(2) is bonded to one O(1), one O(3), one O(5), one O(6), and one O(7) atom to form distorted MgO5 trigonal bipyramids that share  a cornercorner with one Mg(3)O5 square pyramid, corners with three equivalent Mg(1)O5 square pyramids, corners with two equivalent As(1)O4 tetrahedra, and corners with three equivalent As(2)O4 tetrahedra. In the third Mg site, Mg(3) is bonded to one O(7), two equivalent O(4), and two equivalent O(8) atoms to form distorted MgO5 square pyramids that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra,  a cornercorner with one Mg(2)O5 trigonal bipyramid, and edges with two equivalent Mg(3)O5 square pyramids. 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 Mg(3)O5 square pyramids, corners with three equivalent Mg(1)O5 square pyramids, and corners with two equivalent Mg(2)O5 trigonal bipyramids. In the second As site, As(2) is bonded to one O(5), one O(6), one O(7), and one O(8) atom to form AsO4 tetrahedra that share corners with three equivalent Mg(3)O5 square pyramids, corners with three equivalent Mg(2)O5 trigonal bipyramids, and  an edgeedge with one Mg(1)O5 square pyramid. There are eight inequivalent O sites. In the first O site, O(7) is bonded in a trigonal planar geometry to one Mg(2), one Mg(3), and one As(2) atom. In the second O site, O(8) is bonded in a distorted trigonal planar geometry to two equivalent Mg(3) and one As(2) atom. In the third O site, O(1) is bonded in a trigonal planar geometry to one Mg(1), one Mg(2), and one As(1) atom. In the fourth O site, O(2) is bonded in a trigonal planar geometry to two equivalent Mg(1) and one As(1) atom. In the fifth O site, O(3) is bonded in a bent 120 degrees geometry to one Mg(2) and one As(1) atom. In the sixth O site, O(4) is bonded in a trigonal planar geometry to two equivalent Mg(3) and one As(1) atom. In the seventh O site, O(5) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one As(2) atom. In the eighth O site, O(6) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one As(2) atom.
Mg3(AsO4)2 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), one O(5), one O(6), and two equivalent O(2) atoms to form distorted MgO5 square pyramids that share corners with three equivalent As(1)O4 tetrahedra, corners with three equivalent Mg(2)O5 trigonal bipyramids,  an edgeedge with one Mg(1)O5 square pyramid, and  an edgeedge with one As(2)O4 tetrahedra. The Mg(1)-O(1) bond length is 2.05 Å. The Mg(1)-O(5) bond length is 2.08 Å. The Mg(1)-O(6) bond length is 2.25 Å. There is one shorter (2.06 Å) and one longer (2.08 Å) Mg(1)-O(2) bond length. In the second Mg site, Mg(2) is bonded to one O(1), one O(3), one O(5), one O(6), and one O(7) atom to form distorted MgO5 trigonal bipyramids that share  a cornercorner with one Mg(3)O5 square pyramid, corners with three equivalent Mg(1)O5 square pyramids, corners with two equivalent As(1)O4 tetrahedra, and corners with three equivalent As(2)O4 tetrahedra. The Mg(2)-O(1) bond length is 2.27 Å. The Mg(2)-O(3) bond length is 2.01 Å. The Mg(2)-O(5) bond length is 1.94 Å. The Mg(2)-O(6) bond length is 1.96 Å. The Mg(2)-O(7) bond length is 2.37 Å. In the third Mg site, Mg(3) is bonded to one O(7), two equivalent O(4), and two equivalent O(8) atoms to form distorted MgO5 square pyramids that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra,  a cornercorner with one Mg(2)O5 trigonal bipyramid, and edges with two equivalent Mg(3)O5 square pyramids. The Mg(3)-O(7) bond length is 2.06 Å. There is one shorter (2.00 Å) and one longer (2.14 Å) Mg(3)-O(4) bond length. There is one shorter (2.00 Å) and one longer (2.31 Å) Mg(3)-O(8) bond length. 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 Mg(3)O5 square pyramids, corners with three equivalent Mg(1)O5 square pyramids, and corners with two equivalent Mg(2)O5 trigonal bipyramids. The As(1)-O(1) bond length is 1.73 Å. The As(1)-O(2) bond length is 1.73 Å. The As(1)-O(3) bond length is 1.70 Å. The As(1)-O(4) bond length is 1.72 Å. In the second As site, As(2) is bonded to one O(5), one O(6), one O(7), and one O(8) atom to form AsO4 tetrahedra that share corners with three equivalent Mg(3)O5 square pyramids, corners with three equivalent Mg(2)O5 trigonal bipyramids, and  an edgeedge with one Mg(1)O5 square pyramid. The As(2)-O(5) bond length is 1.72 Å. The As(2)-O(6) bond length is 1.72 Å. The As(2)-O(7) bond length is 1.73 Å. The As(2)-O(8) bond length is 1.71 Å. There are eight inequivalent O sites. In the first O site, O(7) is bonded in a trigonal planar geometry to one Mg(2), one Mg(3), and one As(2) atom. In the second O site, O(8) is bonded in a distorted trigonal planar geometry to two equivalent Mg(3) and one As(2) atom. In the third O site, O(1) is bonded in a trigonal planar geometry to one Mg(1), one Mg(2), and one As(1) atom. In the fourth O site, O(2) is bonded in a trigonal planar geometry to two equivalent Mg(1) and one As(1) atom. In the fifth O site, O(3) is bonded in a bent 120 degrees geometry to one Mg(2) and one As(1) atom. In the sixth O site, O(4) is bonded in a trigonal planar geometry to two equivalent Mg(3) and one As(1) atom. In the seventh O site, O(5) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one As(2) atom. In the eighth O site, O(6) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one As(2) atom.