MgTa2(FeO4)2 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(7), and one O(8) atom to form distorted MgO6 octahedra that share corners with two equivalent Ta(2)O6 octahedra, edges with two equivalent Ta(1)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, and  a faceface with one Ta(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-49°. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(3), one O(4), one O(5), one O(6), and one O(7) atom to form TaO6 octahedra that share corners with four equivalent Ta(2)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-56°. In the second Ta site, Ta(2) is bonded to one O(1), one O(2), one O(4), one O(5), one O(7), and one O(8) atom to form distorted TaO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with four equivalent Ta(1)O6 octahedra, corners with four equivalent Fe(1)O6 octahedra, and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 34-56°. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(2), one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form FeO6 octahedra that share corners with four equivalent Ta(2)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 34-53°. In the second Fe site, Fe(2) is bonded in a 4-coordinate geometry to one O(2), one O(3), one O(6), one O(7), and one O(8) atom. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Mg(1), one Ta(1), and one Ta(2) atom. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Ta(2), one Fe(1), and one Fe(2) atom. In the third O site, O(3) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Ta(1), one Fe(1), and one Fe(2) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(2), and one Fe(1) atom. In the fifth O site, O(5) is bonded to one Mg(1), one Ta(1), one Ta(2), and one Fe(1) atom to form OMgTa2Fe trigonal pyramids that share  a cornercorner with one O(7)MgTa2Fe trigonal pyramid,  a cornercorner with one O(8)MgTaFe2 trigonal pyramid,  an edgeedge with one O(7)MgTa2Fe trigonal pyramid, and  an edgeedge with one O(8)MgTaFe2 trigonal pyramid. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Ta(1), one Fe(1), and one Fe(2) atom. In the seventh O site, O(7) is bonded to one Mg(1), one Ta(1), one Ta(2), and one Fe(2) atom to form distorted OMgTa2Fe trigonal pyramids that share  a cornercorner with one O(5)MgTa2Fe trigonal pyramid,  a cornercorner with one O(8)MgTaFe2 trigonal pyramid,  an edgeedge with one O(5)MgTa2Fe trigonal pyramid, and  an edgeedge with one O(8)MgTaFe2 trigonal pyramid. In the eighth O site, O(8) is bonded to one Mg(1), one Ta(2), one Fe(1), and one Fe(2) atom to form a mixture of distorted corner and edge-sharing OMgTaFe2 trigonal pyramids.
MgTa2(FeO4)2 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(7), and one O(8) atom to form distorted MgO6 octahedra that share corners with two equivalent Ta(2)O6 octahedra, edges with two equivalent Ta(1)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, and  a faceface with one Ta(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-49°. The Mg(1)-O(1) bond length is 2.09 Å. The Mg(1)-O(2) bond length is 2.10 Å. The Mg(1)-O(3) bond length is 1.98 Å. The Mg(1)-O(5) bond length is 2.00 Å. The Mg(1)-O(7) bond length is 2.20 Å. The Mg(1)-O(8) bond length is 2.13 Å. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(3), one O(4), one O(5), one O(6), and one O(7) atom to form TaO6 octahedra that share corners with four equivalent Ta(2)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-56°. The Ta(1)-O(1) bond length is 2.17 Å. The Ta(1)-O(3) bond length is 1.98 Å. The Ta(1)-O(4) bond length is 2.18 Å. The Ta(1)-O(5) bond length is 2.03 Å. The Ta(1)-O(6) bond length is 1.87 Å. The Ta(1)-O(7) bond length is 2.00 Å. In the second Ta site, Ta(2) is bonded to one O(1), one O(2), one O(4), one O(5), one O(7), and one O(8) atom to form distorted TaO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with four equivalent Ta(1)O6 octahedra, corners with four equivalent Fe(1)O6 octahedra, and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 34-56°. The Ta(2)-O(1) bond length is 1.99 Å. The Ta(2)-O(2) bond length is 1.88 Å. The Ta(2)-O(4) bond length is 1.90 Å. The Ta(2)-O(5) bond length is 2.29 Å. The Ta(2)-O(7) bond length is 2.30 Å. The Ta(2)-O(8) bond length is 2.00 Å. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(2), one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form FeO6 octahedra that share corners with four equivalent Ta(2)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 34-53°. The Fe(1)-O(2) bond length is 2.17 Å. The Fe(1)-O(3) bond length is 2.27 Å. The Fe(1)-O(4) bond length is 2.17 Å. The Fe(1)-O(5) bond length is 2.22 Å. The Fe(1)-O(6) bond length is 2.16 Å. The Fe(1)-O(8) bond length is 2.10 Å. In the second Fe site, Fe(2) is bonded in a 4-coordinate geometry to one O(2), one O(3), one O(6), one O(7), and one O(8) atom. The Fe(2)-O(2) bond length is 2.53 Å. The Fe(2)-O(3) bond length is 2.14 Å. The Fe(2)-O(6) bond length is 2.03 Å. The Fe(2)-O(7) bond length is 2.04 Å. The Fe(2)-O(8) bond length is 2.06 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Mg(1), one Ta(1), and one Ta(2) atom. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Ta(2), one Fe(1), and one Fe(2) atom. In the third O site, O(3) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Ta(1), one Fe(1), and one Fe(2) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(2), and one Fe(1) atom. In the fifth O site, O(5) is bonded to one Mg(1), one Ta(1), one Ta(2), and one Fe(1) atom to form OMgTa2Fe trigonal pyramids that share  a cornercorner with one O(7)MgTa2Fe trigonal pyramid,  a cornercorner with one O(8)MgTaFe2 trigonal pyramid,  an edgeedge with one O(7)MgTa2Fe trigonal pyramid, and  an edgeedge with one O(8)MgTaFe2 trigonal pyramid. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Ta(1), one Fe(1), and one Fe(2) atom. In the seventh O site, O(7) is bonded to one Mg(1), one Ta(1), one Ta(2), and one Fe(2) atom to form distorted OMgTa2Fe trigonal pyramids that share  a cornercorner with one O(5)MgTa2Fe trigonal pyramid,  a cornercorner with one O(8)MgTaFe2 trigonal pyramid,  an edgeedge with one O(5)MgTa2Fe trigonal pyramid, and  an edgeedge with one O(8)MgTaFe2 trigonal pyramid. In the eighth O site, O(8) is bonded to one Mg(1), one Ta(2), one Fe(1), and one Fe(2) atom to form a mixture of distorted corner and edge-sharing OMgTaFe2 trigonal pyramids.