MgAl2SiO7 crystallizes in the triclinic P-1 space group. Mg(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form distorted MgO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, edges with three equivalent Mg(1)O6 octahedra, and edges with three equivalent Al(3)O6 octahedra. There are four inequivalent Al sites. In the first Al site, Al(1) is bonded to two equivalent O(5), two equivalent O(6), and two equivalent O(7) atoms to form AlO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, edges with two equivalent Al(2)O6 octahedra, and edges with two equivalent Al(4)O6 octahedra. In the second Al site, Al(2) is bonded to two equivalent O(2), two equivalent O(5), and two equivalent O(6) atoms to form AlO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, edges with two equivalent Al(1)O6 octahedra, and edges with two equivalent Al(4)O6 octahedra. In the third Al site, Al(3) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form AlO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra and edges with six equivalent Mg(1)O6 octahedra. In the fourth Al site, Al(4) is bonded to two equivalent O(2), two equivalent O(5), and two equivalent O(7) atoms to form AlO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, edges with two equivalent Al(1)O6 octahedra, and edges with two equivalent Al(2)O6 octahedra. Si(1) is bonded to one O(1), one O(2), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(3)O6 octahedra, corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Al(1)O6 octahedra, corners with two equivalent Al(2)O6 octahedra, and corners with two equivalent Al(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 53-60°. There are seven inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Mg(1), one Al(3), and one Si(1) atom to form a mixture of distorted corner and edge-sharing OMg2AlSi tetrahedra. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Al(2), one Al(4), and one Si(1) atom. In the third O site, O(3) is bonded in a trigonal non-coplanar geometry to two equivalent Mg(1) and one Al(3) atom. In the fourth O site, O(4) is bonded in a trigonal non-coplanar geometry to two equivalent Mg(1) and one Al(3) atom. In the fifth O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to one Al(1), one Al(2), and one Al(4) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Al(1), one Al(2), and one Si(1) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to one Al(1), one Al(4), and one Si(1) atom.
MgAl2SiO7 crystallizes in the triclinic P-1 space group. Mg(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form distorted MgO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, edges with three equivalent Mg(1)O6 octahedra, and edges with three equivalent Al(3)O6 octahedra. There is one shorter (2.12 Å) and one longer (2.16 Å) Mg(1)-O(1) bond length. There is one shorter (2.11 Å) and one longer (2.16 Å) Mg(1)-O(3) bond length. There is one shorter (2.08 Å) and one longer (2.16 Å) Mg(1)-O(4) bond length. There are four inequivalent Al sites. In the first Al site, Al(1) is bonded to two equivalent O(5), two equivalent O(6), and two equivalent O(7) atoms to form AlO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, edges with two equivalent Al(2)O6 octahedra, and edges with two equivalent Al(4)O6 octahedra. Both Al(1)-O(5) bond lengths are 1.81 Å. Both Al(1)-O(6) bond lengths are 1.96 Å. Both Al(1)-O(7) bond lengths are 1.98 Å. In the second Al site, Al(2) is bonded to two equivalent O(2), two equivalent O(5), and two equivalent O(6) atoms to form AlO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, edges with two equivalent Al(1)O6 octahedra, and edges with two equivalent Al(4)O6 octahedra. Both Al(2)-O(2) bond lengths are 1.99 Å. Both Al(2)-O(5) bond lengths are 1.81 Å. Both Al(2)-O(6) bond lengths are 1.95 Å. In the third Al site, Al(3) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form AlO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra and edges with six equivalent Mg(1)O6 octahedra. Both Al(3)-O(1) bond lengths are 2.07 Å. Both Al(3)-O(3) bond lengths are 1.88 Å. Both Al(3)-O(4) bond lengths are 1.87 Å. In the fourth Al site, Al(4) is bonded to two equivalent O(2), two equivalent O(5), and two equivalent O(7) atoms to form AlO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, edges with two equivalent Al(1)O6 octahedra, and edges with two equivalent Al(2)O6 octahedra. Both Al(4)-O(2) bond lengths are 1.97 Å. Both Al(4)-O(5) bond lengths are 1.82 Å. Both Al(4)-O(7) bond lengths are 1.96 Å. Si(1) is bonded to one O(1), one O(2), one O(6), and one O(7) atom to form SiO4 tetrahedra that share  a cornercorner with one Al(3)O6 octahedra, corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Al(1)O6 octahedra, corners with two equivalent Al(2)O6 octahedra, and corners with two equivalent Al(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 53-60°. The Si(1)-O(1) bond length is 1.66 Å. The Si(1)-O(2) bond length is 1.66 Å. The Si(1)-O(6) bond length is 1.65 Å. The Si(1)-O(7) bond length is 1.65 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Mg(1), one Al(3), and one Si(1) atom to form a mixture of distorted corner and edge-sharing OMg2AlSi tetrahedra. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Al(2), one Al(4), and one Si(1) atom. In the third O site, O(3) is bonded in a trigonal non-coplanar geometry to two equivalent Mg(1) and one Al(3) atom. In the fourth O site, O(4) is bonded in a trigonal non-coplanar geometry to two equivalent Mg(1) and one Al(3) atom. In the fifth O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to one Al(1), one Al(2), and one Al(4) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Al(1), one Al(2), and one Si(1) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to one Al(1), one Al(4), and one Si(1) atom.