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(2), one O(3), one O(4), and one O(7) atom to form MgO5 trigonal bipyramids that share corners with two equivalent As(2)O4 tetrahedra, corners with three equivalent As(1)O4 tetrahedra,  a cornercorner with one Mg(2)O5 trigonal bipyramid, and  an edgeedge with one Mg(2)O5 trigonal bipyramid. In the second Mg site, Mg(2) is bonded to one O(1), one O(3), one O(4), one O(6), and one O(8) atom to form MgO5 trigonal bipyramids that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra,  a cornercorner with one Mg(1)O5 trigonal bipyramid, and  an edgeedge with one Mg(1)O5 trigonal bipyramid. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one O(2), one O(6), one O(7), and two equivalent O(5) atoms. 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(5) atom to form AsO4 tetrahedra that share corners with two equivalent Mg(2)O5 trigonal bipyramids and corners with three equivalent Mg(1)O5 trigonal bipyramids. In the second As site, As(2) is bonded to one O(4), one O(6), one O(7), and one O(8) atom to form AsO4 tetrahedra that share corners with two equivalent Mg(1)O5 trigonal bipyramids and corners with three equivalent Mg(2)O5 trigonal bipyramids. There are eight inequivalent O sites. In the first O site, O(7) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(3), and one As(2) atom. In the second O site, O(8) is bonded in a distorted bent 120 degrees geometry to one Mg(2) and one As(2) atom. In the third 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 fourth O site, O(2) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(3), and one As(1) atom. In the fifth O site, O(3) is bonded in a trigonal planar geometry to one Mg(1), one Mg(2), and one As(1) atom. In the sixth O site, O(4) is bonded in a trigonal planar geometry to one Mg(1), one Mg(2), and one As(2) atom. In the seventh O site, O(5) is bonded in a distorted trigonal planar geometry to two equivalent Mg(3) and one As(1) atom. In the eighth O site, O(6) is bonded in a trigonal planar geometry to one Mg(2), one Mg(3), 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(2), one O(3), one O(4), and one O(7) atom to form MgO5 trigonal bipyramids that share corners with two equivalent As(2)O4 tetrahedra, corners with three equivalent As(1)O4 tetrahedra,  a cornercorner with one Mg(2)O5 trigonal bipyramid, and  an edgeedge with one Mg(2)O5 trigonal bipyramid. The Mg(1)-O(1) bond length is 2.09 Å. The Mg(1)-O(2) bond length is 2.02 Å. The Mg(1)-O(3) bond length is 2.05 Å. The Mg(1)-O(4) bond length is 2.07 Å. The Mg(1)-O(7) bond length is 2.08 Å. In the second Mg site, Mg(2) is bonded to one O(1), one O(3), one O(4), one O(6), and one O(8) atom to form MgO5 trigonal bipyramids that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra,  a cornercorner with one Mg(1)O5 trigonal bipyramid, and  an edgeedge with one Mg(1)O5 trigonal bipyramid. The Mg(2)-O(1) bond length is 2.13 Å. The Mg(2)-O(3) bond length is 2.05 Å. The Mg(2)-O(4) bond length is 2.02 Å. The Mg(2)-O(6) bond length is 1.98 Å. The Mg(2)-O(8) bond length is 2.02 Å. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one O(2), one O(6), one O(7), and two equivalent O(5) atoms. The Mg(3)-O(2) bond length is 2.06 Å. The Mg(3)-O(6) bond length is 2.09 Å. The Mg(3)-O(7) bond length is 2.06 Å. There is one shorter (2.03 Å) and one longer (2.15 Å) Mg(3)-O(5) 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(5) atom to form AsO4 tetrahedra that share corners with two equivalent Mg(2)O5 trigonal bipyramids and corners with three equivalent Mg(1)O5 trigonal bipyramids. The As(1)-O(1) bond length is 1.72 Å. The As(1)-O(2) bond length is 1.73 Å. The As(1)-O(3) bond length is 1.72 Å. The As(1)-O(5) bond length is 1.73 Å. In the second As site, As(2) is bonded to one O(4), one O(6), one O(7), and one O(8) atom to form AsO4 tetrahedra that share corners with two equivalent Mg(1)O5 trigonal bipyramids and corners with three equivalent Mg(2)O5 trigonal bipyramids. The As(2)-O(4) bond length is 1.74 Å. The As(2)-O(6) bond length is 1.73 Å. The As(2)-O(7) bond length is 1.73 Å. The As(2)-O(8) bond length is 1.70 Å. There are eight inequivalent O sites. In the first O site, O(7) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(3), and one As(2) atom. In the second O site, O(8) is bonded in a distorted bent 120 degrees geometry to one Mg(2) and one As(2) atom. In the third 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 fourth O site, O(2) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(3), and one As(1) atom. In the fifth O site, O(3) is bonded in a trigonal planar geometry to one Mg(1), one Mg(2), and one As(1) atom. In the sixth O site, O(4) is bonded in a trigonal planar geometry to one Mg(1), one Mg(2), and one As(2) atom. In the seventh O site, O(5) is bonded in a distorted trigonal planar geometry to two equivalent Mg(3) and one As(1) atom. In the eighth O site, O(6) is bonded in a trigonal planar geometry to one Mg(2), one Mg(3), and one As(2) atom.