RbMgFe(SeO4)2 crystallizes in the triclinic P1 space group. Rb(1) is bonded in a 9-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(5), one O(7), one O(8), and two equivalent O(6) atoms. Mg(1) is bonded to one O(4), one O(6), one O(7), and one O(8) atom to form MgO4 tetrahedra that share  a cornercorner with one Se(1)O4 tetrahedra and corners with three equivalent Fe(1)O5 trigonal bipyramids. Fe(1) is bonded to one O(2), one O(3), one O(4), one O(7), and one O(8) atom to form distorted FeO5 trigonal bipyramids that share corners with two equivalent Se(1)O4 tetrahedra and corners with three equivalent Mg(1)O4 tetrahedra. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one O(1), one O(2), one O(3), and one O(6) atom to form SeO4 tetrahedra that share  a cornercorner with one Mg(1)O4 tetrahedra and corners with two equivalent Fe(1)O5 trigonal bipyramids. In the second Se site, Se(2) is bonded in a distorted trigonal non-coplanar geometry to one O(4), one O(5), and one O(8) atom. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 120 degrees geometry to one Rb(1) and one Se(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Rb(1), one Fe(1), and one Se(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Rb(1), one Fe(1), and one Se(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Rb(1), one Mg(1), one Fe(1), and one Se(2) atom. In the fifth O site, O(5) is bonded in a distorted single-bond geometry to one Rb(1) and one Se(2) atom. In the sixth O site, O(6) is bonded in a distorted linear geometry to two equivalent Rb(1), one Mg(1), and one Se(1) atom. In the seventh O site, O(7) is bonded in a distorted bent 120 degrees geometry to one Rb(1), one Mg(1), and one Fe(1) atom. In the eighth O site, O(8) is bonded in a 4-coordinate geometry to one Rb(1), one Mg(1), one Fe(1), and one Se(2) atom.
RbMgFe(SeO4)2 crystallizes in the triclinic P1 space group. Rb(1) is bonded in a 9-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(5), one O(7), one O(8), and two equivalent O(6) atoms. The Rb(1)-O(1) bond length is 2.84 Å. The Rb(1)-O(2) bond length is 2.93 Å. The Rb(1)-O(3) bond length is 2.82 Å. The Rb(1)-O(4) bond length is 3.12 Å. The Rb(1)-O(5) bond length is 3.08 Å. The Rb(1)-O(7) bond length is 3.16 Å. The Rb(1)-O(8) bond length is 3.04 Å. There is one shorter (3.08 Å) and one longer (3.30 Å) Rb(1)-O(6) bond length. Mg(1) is bonded to one O(4), one O(6), one O(7), and one O(8) atom to form MgO4 tetrahedra that share  a cornercorner with one Se(1)O4 tetrahedra and corners with three equivalent Fe(1)O5 trigonal bipyramids. The Mg(1)-O(4) bond length is 2.09 Å. The Mg(1)-O(6) bond length is 2.03 Å. The Mg(1)-O(7) bond length is 1.93 Å. The Mg(1)-O(8) bond length is 2.07 Å. Fe(1) is bonded to one O(2), one O(3), one O(4), one O(7), and one O(8) atom to form distorted FeO5 trigonal bipyramids that share corners with two equivalent Se(1)O4 tetrahedra and corners with three equivalent Mg(1)O4 tetrahedra. The Fe(1)-O(2) bond length is 2.08 Å. The Fe(1)-O(3) bond length is 2.02 Å. The Fe(1)-O(4) bond length is 2.03 Å. The Fe(1)-O(7) bond length is 1.78 Å. The Fe(1)-O(8) bond length is 2.26 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one O(1), one O(2), one O(3), and one O(6) atom to form SeO4 tetrahedra that share  a cornercorner with one Mg(1)O4 tetrahedra and corners with two equivalent Fe(1)O5 trigonal bipyramids. The Se(1)-O(1) bond length is 1.64 Å. The Se(1)-O(2) bond length is 1.68 Å. The Se(1)-O(3) bond length is 1.69 Å. The Se(1)-O(6) bond length is 1.68 Å. In the second Se site, Se(2) is bonded in a distorted trigonal non-coplanar geometry to one O(4), one O(5), and one O(8) atom. The Se(2)-O(4) bond length is 1.89 Å. The Se(2)-O(5) bond length is 1.66 Å. The Se(2)-O(8) bond length is 1.82 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 120 degrees geometry to one Rb(1) and one Se(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Rb(1), one Fe(1), and one Se(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Rb(1), one Fe(1), and one Se(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Rb(1), one Mg(1), one Fe(1), and one Se(2) atom. In the fifth O site, O(5) is bonded in a distorted single-bond geometry to one Rb(1) and one Se(2) atom. In the sixth O site, O(6) is bonded in a distorted linear geometry to two equivalent Rb(1), one Mg(1), and one Se(1) atom. In the seventh O site, O(7) is bonded in a distorted bent 120 degrees geometry to one Rb(1), one Mg(1), and one Fe(1) atom. In the eighth O site, O(8) is bonded in a 4-coordinate geometry to one Rb(1), one Mg(1), one Fe(1), and one Se(2) atom.