Mg2PO8 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 one O(3), one O(4), one O(6), one O(7), and two equivalent O(5) atoms to form distorted MgO6 octahedra that share corners with two equivalent P(1)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra, and edges with two equivalent Mg(2)O6 octahedra. In the second Mg site, Mg(2) is bonded to one O(1), one O(2), one O(3), one O(5), one O(6), and one O(8) atom to form distorted MgO6 octahedra that share corners with three equivalent P(1)O4 tetrahedra and edges with two equivalent Mg(1)O6 octahedra. P(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form PO4 tetrahedra that share corners with two equivalent Mg(1)O6 octahedra and corners with three equivalent Mg(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 39-61°. There are eight inequivalent O sites. In the first O site, O(6) is bonded in a bent 120 degrees geometry to one Mg(1) and one Mg(2) atom. In the second O site, O(7) is bonded in a single-bond geometry to one Mg(1) atom. In the third O site, O(8) is bonded in a single-bond geometry to one Mg(2) atom. In the fourth O site, O(1) is bonded in a bent 120 degrees geometry to one Mg(2) and one P(1) atom. In the fifth O site, O(2) is bonded in a bent 120 degrees geometry to one Mg(2) and one P(1) atom. In the sixth O site, O(3) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one P(1) atom. In the seventh O site, O(4) is bonded in a distorted bent 120 degrees geometry to one Mg(1) and one P(1) atom. In the eighth O site, O(5) is bonded in a trigonal non-coplanar geometry to one Mg(2) and two equivalent Mg(1) atoms.
Mg2PO8 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 one O(3), one O(4), one O(6), one O(7), and two equivalent O(5) atoms to form distorted MgO6 octahedra that share corners with two equivalent P(1)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra, and edges with two equivalent Mg(2)O6 octahedra. The Mg(1)-O(3) bond length is 2.08 Å. The Mg(1)-O(4) bond length is 2.00 Å. The Mg(1)-O(6) bond length is 2.09 Å. The Mg(1)-O(7) bond length is 2.18 Å. There is one shorter (2.04 Å) and one longer (2.21 Å) Mg(1)-O(5) bond length. In the second Mg site, Mg(2) is bonded to one O(1), one O(2), one O(3), one O(5), one O(6), and one O(8) atom to form distorted MgO6 octahedra that share corners with three equivalent P(1)O4 tetrahedra and edges with two equivalent Mg(1)O6 octahedra. The Mg(2)-O(1) bond length is 2.06 Å. The Mg(2)-O(2) bond length is 2.06 Å. The Mg(2)-O(3) bond length is 2.05 Å. The Mg(2)-O(5) bond length is 2.17 Å. The Mg(2)-O(6) bond length is 2.10 Å. The Mg(2)-O(8) bond length is 2.38 Å. P(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form PO4 tetrahedra that share corners with two equivalent Mg(1)O6 octahedra and corners with three equivalent Mg(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 39-61°. The P(1)-O(1) bond length is 1.55 Å. The P(1)-O(2) bond length is 1.55 Å. The P(1)-O(3) bond length is 1.54 Å. The P(1)-O(4) bond length is 1.56 Å. There are eight inequivalent O sites. In the first O site, O(6) is bonded in a bent 120 degrees geometry to one Mg(1) and one Mg(2) atom. In the second O site, O(7) is bonded in a single-bond geometry to one Mg(1) atom. In the third O site, O(8) is bonded in a single-bond geometry to one Mg(2) atom. In the fourth O site, O(1) is bonded in a bent 120 degrees geometry to one Mg(2) and one P(1) atom. In the fifth O site, O(2) is bonded in a bent 120 degrees geometry to one Mg(2) and one P(1) atom. In the sixth O site, O(3) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one P(1) atom. In the seventh O site, O(4) is bonded in a distorted bent 120 degrees geometry to one Mg(1) and one P(1) atom. In the eighth O site, O(5) is bonded in a trigonal non-coplanar geometry to one Mg(2) and two equivalent Mg(1) atoms.