Al2SiO5 crystallizes in the orthorhombic Pnnm space group. There are two inequivalent Al sites. In the first Al site, Al(1) 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 four equivalent Si(1)O4 tetrahedra, corners with four equivalent Al(2)O5 trigonal bipyramids, and edges with two equivalent Al(1)O6 octahedra. In the second Al site, Al(2) is bonded to one O(3), two equivalent O(1), and two equivalent O(2) atoms to form AlO5 trigonal bipyramids that share corners with four equivalent Al(1)O6 octahedra, corners with four equivalent Si(1)O4 tetrahedra, and  an edgeedge with one Al(2)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 49-58°. Si(1) is bonded to one O(2), one O(4), and two equivalent O(1) atoms to form SiO4 tetrahedra that share corners with four equivalent Al(1)O6 octahedra and corners with four equivalent Al(2)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 55-69°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Al(1), one Al(2), and one Si(1) atom. In the second O site, O(2) is bonded in a trigonal planar geometry to two equivalent Al(2) and one Si(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Al(2) and two equivalent Al(1) atoms. In the fourth O site, O(4) is bonded in a trigonal planar geometry to two equivalent Al(1) and one Si(1) atom.
Al2SiO5 crystallizes in the orthorhombic Pnnm space group. There are two inequivalent Al sites. In the first Al site, Al(1) 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 four equivalent Si(1)O4 tetrahedra, corners with four equivalent Al(2)O5 trigonal bipyramids, and edges with two equivalent Al(1)O6 octahedra. Both Al(1)-O(1) bond lengths are 2.07 Å. Both Al(1)-O(3) bond lengths are 1.83 Å. Both Al(1)-O(4) bond lengths are 1.89 Å. In the second Al site, Al(2) is bonded to one O(3), two equivalent O(1), and two equivalent O(2) atoms to form AlO5 trigonal bipyramids that share corners with four equivalent Al(1)O6 octahedra, corners with four equivalent Si(1)O4 tetrahedra, and  an edgeedge with one Al(2)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 49-58°. The Al(2)-O(3) bond length is 1.81 Å. Both Al(2)-O(1) bond lengths are 1.81 Å. There is one shorter (1.84 Å) and one longer (1.89 Å) Al(2)-O(2) bond length. Si(1) is bonded to one O(2), one O(4), and two equivalent O(1) atoms to form SiO4 tetrahedra that share corners with four equivalent Al(1)O6 octahedra and corners with four equivalent Al(2)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 55-69°. The Si(1)-O(2) bond length is 1.62 Å. The Si(1)-O(4) bond length is 1.64 Å. Both Si(1)-O(1) bond lengths are 1.63 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Al(1), one Al(2), and one Si(1) atom. In the second O site, O(2) is bonded in a trigonal planar geometry to two equivalent Al(2) and one Si(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Al(2) and two equivalent Al(1) atoms. In the fourth O site, O(4) is bonded in a trigonal planar geometry to two equivalent Al(1) and one Si(1) atom.