Al2SiO5 crystallizes in the monoclinic C2/c space group. Al(1) is bonded to one O(1), two equivalent O(3), and three equivalent O(2) atoms to form distorted AlO6 octahedra that share corners with three equivalent Si(1)O6 octahedra, corners with four equivalent Al(1)O6 octahedra,  an edgeedge with one Al(1)O6 octahedra, edges with two equivalent Si(1)O6 octahedra, and  a faceface with one Al(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-59°. Si(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form SiO6 octahedra that share corners with two equivalent Si(1)O6 octahedra, corners with six equivalent Al(1)O6 octahedra, and edges with four equivalent Al(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-56°. There are three inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Al(1) and two equivalent Si(1) atoms to form distorted OAl2Si2 trigonal pyramids that share corners with two equivalent O(1)Al2Si2 trigonal pyramids, corners with two equivalent O(2)Al3Si trigonal pyramids, and edges with four equivalent O(2)Al3Si trigonal pyramids. In the second O site, O(2) is bonded to three equivalent Al(1) and one Si(1) atom to form distorted OAl3Si trigonal pyramids that share  a cornercorner with one O(1)Al2Si2 trigonal pyramid, corners with three equivalent O(2)Al3Si trigonal pyramids, edges with two equivalent O(1)Al2Si2 trigonal pyramids, and edges with two equivalent O(2)Al3Si trigonal pyramids. In the third O site, O(3) is bonded in a trigonal planar geometry to two equivalent Al(1) and one Si(1) atom.
Al2SiO5 crystallizes in the monoclinic C2/c space group. Al(1) is bonded to one O(1), two equivalent O(3), and three equivalent O(2) atoms to form distorted AlO6 octahedra that share corners with three equivalent Si(1)O6 octahedra, corners with four equivalent Al(1)O6 octahedra,  an edgeedge with one Al(1)O6 octahedra, edges with two equivalent Si(1)O6 octahedra, and  a faceface with one Al(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-59°. The Al(1)-O(1) bond length is 2.02 Å. There is one shorter (1.80 Å) and one longer (1.83 Å) Al(1)-O(3) bond length. There are a spread of Al(1)-O(2) bond distances ranging from 1.92-2.09 Å. Si(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form SiO6 octahedra that share corners with two equivalent Si(1)O6 octahedra, corners with six equivalent Al(1)O6 octahedra, and edges with four equivalent Al(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-56°. Both Si(1)-O(1) bond lengths are 1.90 Å. Both Si(1)-O(2) bond lengths are 1.80 Å. Both Si(1)-O(3) bond lengths are 1.76 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Al(1) and two equivalent Si(1) atoms to form distorted OAl2Si2 trigonal pyramids that share corners with two equivalent O(1)Al2Si2 trigonal pyramids, corners with two equivalent O(2)Al3Si trigonal pyramids, and edges with four equivalent O(2)Al3Si trigonal pyramids. In the second O site, O(2) is bonded to three equivalent Al(1) and one Si(1) atom to form distorted OAl3Si trigonal pyramids that share  a cornercorner with one O(1)Al2Si2 trigonal pyramid, corners with three equivalent O(2)Al3Si trigonal pyramids, edges with two equivalent O(1)Al2Si2 trigonal pyramids, and edges with two equivalent O(2)Al3Si trigonal pyramids. In the third O site, O(3) is bonded in a trigonal planar geometry to two equivalent Al(1) and one Si(1) atom.