CaCr4(HgO4)2 crystallizes in the trigonal R-3m space group. Ca(1) is bonded to six equivalent O(2) atoms to form distorted CaO6 octahedra that share edges with six equivalent Cr(1)O6 octahedra. There are two inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to two equivalent O(1) and four equivalent O(2) atoms to form CrO6 octahedra that share edges with two equivalent Ca(1)O6 octahedra and edges with four equivalent Cr(1)O6 octahedra. In the second Cr site, Cr(2) is bonded in a 6-coordinate geometry to six equivalent O(2) atoms. Hg(1) is bonded in a 4-coordinate geometry to one O(1) and three equivalent O(2) atoms. There are two inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Cr(1) and one Hg(1) atom to form OCr3Hg tetrahedra that share corners with three equivalent O(1)Cr3Hg tetrahedra, corners with nine equivalent O(2)CaCr3Hg trigonal bipyramids, and edges with three equivalent O(2)CaCr3Hg trigonal bipyramids. In the second O site, O(2) is bonded to one Ca(1), one Cr(2), two equivalent Cr(1), and one Hg(1) atom to form distorted OCaCr3Hg trigonal bipyramids that share corners with three equivalent O(1)Cr3Hg tetrahedra, corners with six equivalent O(2)CaCr3Hg trigonal bipyramids,  an edgeedge with one O(1)Cr3Hg tetrahedra, and edges with six equivalent O(2)CaCr3Hg trigonal bipyramids.
CaCr4(HgO4)2 crystallizes in the trigonal R-3m space group. Ca(1) is bonded to six equivalent O(2) atoms to form distorted CaO6 octahedra that share edges with six equivalent Cr(1)O6 octahedra. All Ca(1)-O(2) bond lengths are 2.37 Å. There are two inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to two equivalent O(1) and four equivalent O(2) atoms to form CrO6 octahedra that share edges with two equivalent Ca(1)O6 octahedra and edges with four equivalent Cr(1)O6 octahedra. Both Cr(1)-O(1) bond lengths are 1.99 Å. All Cr(1)-O(2) bond lengths are 2.10 Å. In the second Cr site, Cr(2) is bonded in a 6-coordinate geometry to six equivalent O(2) atoms. All Cr(2)-O(2) bond lengths are 2.28 Å. Hg(1) is bonded in a 4-coordinate geometry to one O(1) and three equivalent O(2) atoms. The Hg(1)-O(1) bond length is 2.15 Å. All Hg(1)-O(2) bond lengths are 2.54 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Cr(1) and one Hg(1) atom to form OCr3Hg tetrahedra that share corners with three equivalent O(1)Cr3Hg tetrahedra, corners with nine equivalent O(2)CaCr3Hg trigonal bipyramids, and edges with three equivalent O(2)CaCr3Hg trigonal bipyramids. In the second O site, O(2) is bonded to one Ca(1), one Cr(2), two equivalent Cr(1), and one Hg(1) atom to form distorted OCaCr3Hg trigonal bipyramids that share corners with three equivalent O(1)Cr3Hg tetrahedra, corners with six equivalent O(2)CaCr3Hg trigonal bipyramids,  an edgeedge with one O(1)Cr3Hg tetrahedra, and edges with six equivalent O(2)CaCr3Hg trigonal bipyramids.