Ca2Ga2Si(HO4)2 crystallizes in the tetragonal I-4 space group. Ca(1) is bonded to one O(1), one O(2), one O(4), and three equivalent O(3) atoms to form CaO6 octahedra that share  a cornercorner with one Si(1)O4 tetrahedra,  a cornercorner with one Si(2)O4 tetrahedra, corners with four equivalent Ga(1)O4 tetrahedra, and edges with three equivalent Ca(1)O6 octahedra. Ga(1) is bonded to one O(2), one O(4), and two equivalent O(1) atoms to form GaO4 tetrahedra that share corners with four equivalent Ca(1)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra,  a cornercorner with one Si(2)O4 tetrahedra, and corners with two equivalent Ga(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 64-69°. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to four equivalent O(2) atoms to form SiO4 tetrahedra that share corners with four equivalent Ca(1)O6 octahedra and corners with four equivalent Ga(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 60°. In the second Si site, Si(2) is bonded to four equivalent O(4) atoms to form SiO4 tetrahedra that share corners with four equivalent Ca(1)O6 octahedra and corners with four equivalent Ga(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 58°. H(1) is bonded in a single-bond geometry to one O(3) atom. There are four inequivalent O sites. In the first O site, O(4) is bonded in a distorted trigonal planar geometry to one Ca(1), one Ga(1), and one Si(2) atom. In the second O site, O(1) is bonded in a trigonal planar geometry to one Ca(1) and two equivalent Ga(1) atoms. In the third O site, O(2) is bonded in a distorted trigonal planar geometry to one Ca(1), one Ga(1), and one Si(1) atom. In the fourth O site, O(3) is bonded in a distorted single-bond geometry to three equivalent Ca(1) and one H(1) atom.
Ca2Ga2Si(HO4)2 crystallizes in the tetragonal I-4 space group. Ca(1) is bonded to one O(1), one O(2), one O(4), and three equivalent O(3) atoms to form CaO6 octahedra that share  a cornercorner with one Si(1)O4 tetrahedra,  a cornercorner with one Si(2)O4 tetrahedra, corners with four equivalent Ga(1)O4 tetrahedra, and edges with three equivalent Ca(1)O6 octahedra. The Ca(1)-O(1) bond length is 2.42 Å. The Ca(1)-O(2) bond length is 2.48 Å. The Ca(1)-O(4) bond length is 2.50 Å. There are two shorter (2.31 Å) and one longer (2.35 Å) Ca(1)-O(3) bond length. Ga(1) is bonded to one O(2), one O(4), and two equivalent O(1) atoms to form GaO4 tetrahedra that share corners with four equivalent Ca(1)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra,  a cornercorner with one Si(2)O4 tetrahedra, and corners with two equivalent Ga(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 64-69°. The Ga(1)-O(2) bond length is 1.89 Å. The Ga(1)-O(4) bond length is 1.88 Å. Both Ga(1)-O(1) bond lengths are 1.84 Å. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to four equivalent O(2) atoms to form SiO4 tetrahedra that share corners with four equivalent Ca(1)O6 octahedra and corners with four equivalent Ga(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 60°. All Si(1)-O(2) bond lengths are 1.65 Å. In the second Si site, Si(2) is bonded to four equivalent O(4) atoms to form SiO4 tetrahedra that share corners with four equivalent Ca(1)O6 octahedra and corners with four equivalent Ga(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 58°. All Si(2)-O(4) bond lengths are 1.65 Å. H(1) is bonded in a single-bond geometry to one O(3) atom. The H(1)-O(3) bond length is 0.97 Å. There are four inequivalent O sites. In the first O site, O(4) is bonded in a distorted trigonal planar geometry to one Ca(1), one Ga(1), and one Si(2) atom. In the second O site, O(1) is bonded in a trigonal planar geometry to one Ca(1) and two equivalent Ga(1) atoms. In the third O site, O(2) is bonded in a distorted trigonal planar geometry to one Ca(1), one Ga(1), and one Si(1) atom. In the fourth O site, O(3) is bonded in a distorted single-bond geometry to three equivalent Ca(1) and one H(1) atom.