RbMgCr3O8 crystallizes in the monoclinic Cm space group. Rb(1) is bonded in a 6-coordinate geometry to one O(5), one O(6), two equivalent O(1), and two equivalent O(2) atoms. Mg(1) is bonded to one O(3), one O(6), and two equivalent O(1) atoms to form distorted MgO4 trigonal pyramids that share corners with three equivalent Cr(3)O6 octahedra, corners with two equivalent Cr(1)O4 tetrahedra, and  an edgeedge with one Cr(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 56-64°. There are three inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(3), one O(6), and two equivalent O(2) atoms to form CrO4 tetrahedra that share corners with three equivalent Cr(3)O6 octahedra and corners with two equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 39-47°. In the second Cr site, Cr(2) is bonded to one O(4), one O(5), and two equivalent O(1) atoms to form CrO4 tetrahedra that share corners with three equivalent Cr(3)O6 octahedra and  an edgeedge with one Mg(1)O4 trigonal pyramid. The corner-sharing octahedral tilt angles range from 44-53°. In the third Cr site, Cr(3) is bonded to one O(3), one O(4), two equivalent O(1), and two equivalent O(2) atoms to form CrO6 octahedra that share corners with three equivalent Cr(1)O4 tetrahedra, corners with three equivalent Cr(2)O4 tetrahedra, and corners with three equivalent Mg(1)O4 trigonal pyramids. There are six inequivalent O sites. In the first O site, O(1) is bonded to one Rb(1), one Mg(1), one Cr(2), and one Cr(3) atom to form distorted edge-sharing ORbMgCr2 tetrahedra. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Rb(1), one Cr(1), and one Cr(3) atom. In the third O site, O(3) is bonded in a trigonal planar geometry to one Mg(1), one Cr(1), and one Cr(3) atom. In the fourth O site, O(4) is bonded in a distorted bent 150 degrees geometry to one Cr(2) and one Cr(3) atom. In the fifth O site, O(5) is bonded in a distorted single-bond geometry to one Rb(1) and one Cr(2) atom. In the sixth O site, O(6) is bonded in a 2-coordinate geometry to one Rb(1), one Mg(1), and one Cr(1) atom.
RbMgCr3O8 crystallizes in the monoclinic Cm space group. Rb(1) is bonded in a 6-coordinate geometry to one O(5), one O(6), two equivalent O(1), and two equivalent O(2) atoms. The Rb(1)-O(5) bond length is 2.75 Å. The Rb(1)-O(6) bond length is 2.89 Å. Both Rb(1)-O(1) bond lengths are 2.74 Å. Both Rb(1)-O(2) bond lengths are 2.63 Å. Mg(1) is bonded to one O(3), one O(6), and two equivalent O(1) atoms to form distorted MgO4 trigonal pyramids that share corners with three equivalent Cr(3)O6 octahedra, corners with two equivalent Cr(1)O4 tetrahedra, and  an edgeedge with one Cr(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 56-64°. The Mg(1)-O(3) bond length is 2.09 Å. The Mg(1)-O(6) bond length is 2.25 Å. Both Mg(1)-O(1) bond lengths are 2.02 Å. There are three inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(3), one O(6), and two equivalent O(2) atoms to form CrO4 tetrahedra that share corners with three equivalent Cr(3)O6 octahedra and corners with two equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 39-47°. The Cr(1)-O(3) bond length is 1.85 Å. The Cr(1)-O(6) bond length is 1.72 Å. Both Cr(1)-O(2) bond lengths are 1.72 Å. In the second Cr site, Cr(2) is bonded to one O(4), one O(5), and two equivalent O(1) atoms to form CrO4 tetrahedra that share corners with three equivalent Cr(3)O6 octahedra and  an edgeedge with one Mg(1)O4 trigonal pyramid. The corner-sharing octahedral tilt angles range from 44-53°. The Cr(2)-O(4) bond length is 1.68 Å. The Cr(2)-O(5) bond length is 1.63 Å. Both Cr(2)-O(1) bond lengths are 1.83 Å. In the third Cr site, Cr(3) is bonded to one O(3), one O(4), two equivalent O(1), and two equivalent O(2) atoms to form CrO6 octahedra that share corners with three equivalent Cr(1)O4 tetrahedra, corners with three equivalent Cr(2)O4 tetrahedra, and corners with three equivalent Mg(1)O4 trigonal pyramids. The Cr(3)-O(3) bond length is 2.06 Å. The Cr(3)-O(4) bond length is 2.02 Å. Both Cr(3)-O(1) bond lengths are 2.04 Å. Both Cr(3)-O(2) bond lengths are 2.01 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded to one Rb(1), one Mg(1), one Cr(2), and one Cr(3) atom to form distorted edge-sharing ORbMgCr2 tetrahedra. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Rb(1), one Cr(1), and one Cr(3) atom. In the third O site, O(3) is bonded in a trigonal planar geometry to one Mg(1), one Cr(1), and one Cr(3) atom. In the fourth O site, O(4) is bonded in a distorted bent 150 degrees geometry to one Cr(2) and one Cr(3) atom. In the fifth O site, O(5) is bonded in a distorted single-bond geometry to one Rb(1) and one Cr(2) atom. In the sixth O site, O(6) is bonded in a 2-coordinate geometry to one Rb(1), one Mg(1), and one Cr(1) atom.