CaSn4O9 crystallizes in the tetragonal P4/n space group. Ca(1) is bonded in a 8-coordinate geometry to four equivalent O(2) and four equivalent O(3) atoms. Sn(1) is bonded in a 6-coordinate geometry to one O(1), two equivalent O(2), and three equivalent O(3) atoms. There are three inequivalent O sites. In the first O site, O(1) is bonded to four equivalent Sn(1) atoms to form a mixture of edge and corner-sharing OSn4 tetrahedra. In the second O site, O(2) is bonded in a trigonal planar geometry to one Ca(1) and two equivalent Sn(1) atoms. In the third O site, O(3) is bonded to one Ca(1) and three equivalent Sn(1) atoms to form OCaSn3 tetrahedra that share  a cornercorner with one O(1)Sn4 tetrahedra, corners with three equivalent O(3)CaSn3 tetrahedra,  an edgeedge with one O(1)Sn4 tetrahedra, and edges with three equivalent O(3)CaSn3 tetrahedra.
CaSn4O9 crystallizes in the tetragonal P4/n space group. Ca(1) is bonded in a 8-coordinate geometry to four equivalent O(2) and four equivalent O(3) atoms. All Ca(1)-O(2) bond lengths are 2.40 Å. All Ca(1)-O(3) bond lengths are 2.48 Å. Sn(1) is bonded in a 6-coordinate geometry to one O(1), two equivalent O(2), and three equivalent O(3) atoms. The Sn(1)-O(1) bond length is 2.15 Å. There is one shorter (2.06 Å) and one longer (2.09 Å) Sn(1)-O(2) bond length. There are a spread of Sn(1)-O(3) bond distances ranging from 2.07-2.25 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded to four equivalent Sn(1) atoms to form a mixture of edge and corner-sharing OSn4 tetrahedra. In the second O site, O(2) is bonded in a trigonal planar geometry to one Ca(1) and two equivalent Sn(1) atoms. In the third O site, O(3) is bonded to one Ca(1) and three equivalent Sn(1) atoms to form OCaSn3 tetrahedra that share  a cornercorner with one O(1)Sn4 tetrahedra, corners with three equivalent O(3)CaSn3 tetrahedra,  an edgeedge with one O(1)Sn4 tetrahedra, and edges with three equivalent O(3)CaSn3 tetrahedra.