CaAl2Si2O7(OH)2H2O crystallizes in the orthorhombic Cmcm space group. The structure consists of four water molecules inside a CaAl2Si2O7(OH)2 framework. In the CaAl2Si2O7(OH)2 framework, Ca(1) is bonded to one O(1) and four equivalent O(2) atoms to form distorted CaO5 trigonal bipyramids that share corners with four equivalent Al(1)O6 octahedra and corners with six equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 44°. Al(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form AlO6 octahedra that share corners with two equivalent Ca(1)O5 trigonal bipyramids, edges with two equivalent Al(1)O6 octahedra, and edges with two equivalent Si(1)O4 tetrahedra. Si(1) is bonded to one O(1), one O(3), and two equivalent O(2) atoms to form SiO4 tetrahedra that share  a cornercorner with one Si(1)O4 tetrahedra, corners with three equivalent Ca(1)O5 trigonal bipyramids, and edges with two equivalent Al(1)O6 octahedra. H(2) is bonded in a single-bond geometry to one O(4) atom. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Ca(1) and two equivalent Si(1) atoms. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Ca(1), one Al(1), and one Si(1) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to two equivalent Al(1) and one Si(1) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to two equivalent Al(1) and one H(2) atom.
CaAl2Si2O7(OH)2H2O crystallizes in the orthorhombic Cmcm space group. The structure consists of four water molecules inside a CaAl2Si2O7(OH)2 framework. In the CaAl2Si2O7(OH)2 framework, Ca(1) is bonded to one O(1) and four equivalent O(2) atoms to form distorted CaO5 trigonal bipyramids that share corners with four equivalent Al(1)O6 octahedra and corners with six equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 44°. The Ca(1)-O(1) bond length is 2.57 Å. All Ca(1)-O(2) bond lengths are 2.37 Å. Al(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form AlO6 octahedra that share corners with two equivalent Ca(1)O5 trigonal bipyramids, edges with two equivalent Al(1)O6 octahedra, and edges with two equivalent Si(1)O4 tetrahedra. Both Al(1)-O(2) bond lengths are 2.05 Å. Both Al(1)-O(3) bond lengths are 1.96 Å. Both Al(1)-O(4) bond lengths are 1.91 Å. Si(1) is bonded to one O(1), one O(3), and two equivalent O(2) atoms to form SiO4 tetrahedra that share  a cornercorner with one Si(1)O4 tetrahedra, corners with three equivalent Ca(1)O5 trigonal bipyramids, and edges with two equivalent Al(1)O6 octahedra. The Si(1)-O(1) bond length is 1.67 Å. The Si(1)-O(3) bond length is 1.66 Å. Both Si(1)-O(2) bond lengths are 1.63 Å. H(2) is bonded in a single-bond geometry to one O(4) atom. The H(2)-O(4) bond length is 0.97 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Ca(1) and two equivalent Si(1) atoms. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Ca(1), one Al(1), and one Si(1) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to two equivalent Al(1) and one Si(1) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to two equivalent Al(1) and one H(2) atom.