Al2Si2O7 crystallizes in the monoclinic C2/c space group. Al(1) is bonded to one O(1), two equivalent O(2), and two equivalent O(4) atoms to form AlO5 trigonal bipyramids that share corners with five equivalent Si(1)O4 tetrahedra and edges with two equivalent Al(1)O5 trigonal bipyramids. Si(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SiO4 tetrahedra that share  a cornercorner with one Si(1)O4 tetrahedra and corners with five equivalent Al(1)O5 trigonal bipyramids. There are four inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to one Al(1) and one Si(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Al(1) and one Si(1) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to two equivalent Si(1) atoms. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to two equivalent Al(1) and one Si(1) atom.
Al2Si2O7 crystallizes in the monoclinic C2/c space group. Al(1) is bonded to one O(1), two equivalent O(2), and two equivalent O(4) atoms to form AlO5 trigonal bipyramids that share corners with five equivalent Si(1)O4 tetrahedra and edges with two equivalent Al(1)O5 trigonal bipyramids. The Al(1)-O(1) bond length is 1.74 Å. There is one shorter (1.82 Å) and one longer (1.90 Å) Al(1)-O(2) bond length. There is one shorter (1.82 Å) and one longer (1.91 Å) Al(1)-O(4) bond length. Si(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SiO4 tetrahedra that share  a cornercorner with one Si(1)O4 tetrahedra and corners with five equivalent Al(1)O5 trigonal bipyramids. The Si(1)-O(1) bond length is 1.59 Å. The Si(1)-O(2) bond length is 1.65 Å. The Si(1)-O(3) bond length is 1.62 Å. The Si(1)-O(4) bond length is 1.65 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to one Al(1) and one Si(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Al(1) and one Si(1) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to two equivalent Si(1) atoms. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to two equivalent Al(1) and one Si(1) atom.