K4MgNb4(Si2O7)4 crystallizes in the monoclinic Cm space group. There are three inequivalent K sites. In the first K site, K(1) is bonded in a 11-coordinate geometry to one O(1), one O(10), one O(13), one O(16), one O(17), one O(2), one O(3), one O(4), one O(5), one O(7), and one O(9) atom. In the second K site, K(2) is bonded in a 7-coordinate geometry to one Mg(1), one O(11), one O(14), two equivalent O(1), and two equivalent O(7) atoms. In the third K site, K(3) is bonded in a 12-coordinate geometry to one O(11), one O(14), two equivalent O(16), two equivalent O(17), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. Mg(1) is bonded in a 4-coordinate geometry to one K(2), two equivalent O(13), and two equivalent O(2) atoms. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(13), one O(15), one O(16), one O(17), and one O(3) atom to form NbO6 octahedra that share corners with two equivalent Nb(2)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra,  a cornercorner with one Si(5)O4 tetrahedra, and corners with two equivalent Si(4)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 4-6°. In the second Nb site, Nb(2) is bonded to one O(15), one O(16), one O(2), one O(4), one O(5), and one O(7) atom to form NbO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra,  a cornercorner with one Si(2)O4 tetrahedra,  a cornercorner with one Si(6)O4 tetrahedra, and corners with two equivalent Si(3)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 4-6°. There are six inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(8), one O(9), and two equivalent O(3) atoms to form SiO4 tetrahedra that share corners with two equivalent Nb(1)O6 octahedra,  a cornercorner with one Si(5)O4 tetrahedra, and  a cornercorner with one Si(6)O4 tetrahedra. The corner-sharing octahedral tilt angles are 22°. In the second Si site, Si(2) is bonded to one O(10), one O(12), and two equivalent O(2) atoms to form SiO4 tetrahedra that share corners with two equivalent Nb(2)O6 octahedra,  a cornercorner with one Si(5)O4 tetrahedra, and  a cornercorner with one Si(6)O4 tetrahedra. The corner-sharing octahedral tilt angles are 39°. In the third Si site, Si(3) is bonded to one O(14), one O(4), one O(6), and one O(7) atom to form SiO4 tetrahedra that share corners with two equivalent Nb(2)O6 octahedra,  a cornercorner with one Si(3)O4 tetrahedra, and  a cornercorner with one Si(4)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 23-26°. In the fourth Si site, Si(4) is bonded to one O(1), one O(11), one O(17), and one O(6) atom to form SiO4 tetrahedra that share corners with two equivalent Nb(1)O6 octahedra,  a cornercorner with one Si(3)O4 tetrahedra, and  a cornercorner with one Si(4)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 24-28°. In the fifth Si site, Si(5) is bonded to one O(12), one O(9), and two equivalent O(13) atoms to form SiO4 tetrahedra that share corners with two equivalent Nb(1)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 39°. In the sixth Si site, Si(6) is bonded to one O(10), one O(8), and two equivalent O(5) atoms to form SiO4 tetrahedra that share corners with two equivalent Nb(2)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 21°. There are seventeen inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one K(1), one K(2), one Nb(1), and one Si(4) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one K(1), one Mg(1), one Nb(2), and one Si(2) atom. In the third O site, O(3) is bonded in a 2-coordinate geometry to one K(1), one K(3), one Nb(1), and one Si(1) atom. In the fourth O site, O(4) is bonded in a distorted bent 150 degrees geometry to one K(1), one K(3), one Nb(2), and one Si(3) atom. In the fifth O site, O(5) is bonded in a 2-coordinate geometry to one K(1), one K(3), one Nb(2), and one Si(6) atom. In the sixth O site, O(6) is bonded in a linear geometry to one Si(3) and one Si(4) atom. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one K(1), one K(2), one Nb(2), and one Si(3) atom. In the eighth O site, O(8) is bonded in a linear geometry to one Si(1) and one Si(6) atom. In the ninth O site, O(9) is bonded in a distorted bent 150 degrees geometry to two equivalent K(1), one Si(1), and one Si(5) atom. In the tenth O site, O(10) is bonded in a distorted bent 150 degrees geometry to two equivalent K(1), one Si(2), and one Si(6) atom. In the eleventh O site, O(11) is bonded in a 4-coordinate geometry to one K(2), one K(3), and two equivalent Si(4) atoms. In the twelfth O site, O(12) is bonded in a linear geometry to one Si(2) and one Si(5) atom. In the thirteenth O site, O(13) is bonded in a 3-coordinate geometry to one K(1), one Mg(1), one Nb(1), and one Si(5) atom. In the fourteenth O site, O(14) is bonded in a 4-coordinate geometry to one K(2), one K(3), and two equivalent Si(3) atoms. In the fifteenth O site, O(15) is bonded in a linear geometry to one Nb(1) and one Nb(2) atom. In the sixteenth O site, O(16) is bonded in a distorted linear geometry to one K(1), one K(3), one Nb(1), and one Nb(2) atom. In the seventeenth O site, O(17) is bonded in a distorted bent 150 degrees geometry to one K(1), one K(3), one Nb(1), and one Si(4) atom.
K4MgNb4(Si2O7)4 crystallizes in the monoclinic Cm space group. There are three inequivalent K sites. In the first K site, K(1) is bonded in a 11-coordinate geometry to one O(1), one O(10), one O(13), one O(16), one O(17), one O(2), one O(3), one O(4), one O(5), one O(7), and one O(9) atom. The K(1)-O(1) bond length is 2.76 Å. The K(1)-O(10) bond length is 3.04 Å. The K(1)-O(13) bond length is 3.23 Å. The K(1)-O(16) bond length is 2.92 Å. The K(1)-O(17) bond length is 2.88 Å. The K(1)-O(2) bond length is 3.20 Å. The K(1)-O(3) bond length is 3.00 Å. The K(1)-O(4) bond length is 2.87 Å. The K(1)-O(5) bond length is 3.01 Å. The K(1)-O(7) bond length is 2.78 Å. The K(1)-O(9) bond length is 3.06 Å. In the second K site, K(2) is bonded in a 7-coordinate geometry to one Mg(1), one O(11), one O(14), two equivalent O(1), and two equivalent O(7) atoms. The K(2)-Mg(1) bond length is 2.68 Å. The K(2)-O(11) bond length is 2.57 Å. The K(2)-O(14) bond length is 2.57 Å. Both K(2)-O(1) bond lengths are 2.89 Å. Both K(2)-O(7) bond lengths are 2.87 Å. In the third K site, K(3) is bonded in a 12-coordinate geometry to one O(11), one O(14), two equivalent O(16), two equivalent O(17), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. The K(3)-O(11) bond length is 2.85 Å. The K(3)-O(14) bond length is 2.86 Å. Both K(3)-O(16) bond lengths are 3.28 Å. Both K(3)-O(17) bond lengths are 3.18 Å. Both K(3)-O(3) bond lengths are 2.89 Å. Both K(3)-O(4) bond lengths are 3.20 Å. Both K(3)-O(5) bond lengths are 2.88 Å. Mg(1) is bonded in a 4-coordinate geometry to one K(2), two equivalent O(13), and two equivalent O(2) atoms. Both Mg(1)-O(13) bond lengths are 2.06 Å. Both Mg(1)-O(2) bond lengths are 2.06 Å. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(13), one O(15), one O(16), one O(17), and one O(3) atom to form NbO6 octahedra that share corners with two equivalent Nb(2)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra,  a cornercorner with one Si(5)O4 tetrahedra, and corners with two equivalent Si(4)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 4-6°. The Nb(1)-O(1) bond length is 2.08 Å. The Nb(1)-O(13) bond length is 2.18 Å. The Nb(1)-O(15) bond length is 2.00 Å. The Nb(1)-O(16) bond length is 2.02 Å. The Nb(1)-O(17) bond length is 2.04 Å. The Nb(1)-O(3) bond length is 2.04 Å. In the second Nb site, Nb(2) is bonded to one O(15), one O(16), one O(2), one O(4), one O(5), and one O(7) atom to form NbO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra,  a cornercorner with one Si(2)O4 tetrahedra,  a cornercorner with one Si(6)O4 tetrahedra, and corners with two equivalent Si(3)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 4-6°. The Nb(2)-O(15) bond length is 2.05 Å. The Nb(2)-O(16) bond length is 1.98 Å. The Nb(2)-O(2) bond length is 2.18 Å. The Nb(2)-O(4) bond length is 2.04 Å. The Nb(2)-O(5) bond length is 2.03 Å. The Nb(2)-O(7) bond length is 2.08 Å. There are six inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(8), one O(9), and two equivalent O(3) atoms to form SiO4 tetrahedra that share corners with two equivalent Nb(1)O6 octahedra,  a cornercorner with one Si(5)O4 tetrahedra, and  a cornercorner with one Si(6)O4 tetrahedra. The corner-sharing octahedral tilt angles are 22°. The Si(1)-O(8) bond length is 1.61 Å. The Si(1)-O(9) bond length is 1.68 Å. Both Si(1)-O(3) bond lengths are 1.61 Å. In the second Si site, Si(2) is bonded to one O(10), one O(12), and two equivalent O(2) atoms to form SiO4 tetrahedra that share corners with two equivalent Nb(2)O6 octahedra,  a cornercorner with one Si(5)O4 tetrahedra, and  a cornercorner with one Si(6)O4 tetrahedra. The corner-sharing octahedral tilt angles are 39°. The Si(2)-O(10) bond length is 1.66 Å. The Si(2)-O(12) bond length is 1.60 Å. Both Si(2)-O(2) bond lengths are 1.64 Å. In the third Si site, Si(3) is bonded to one O(14), one O(4), one O(6), and one O(7) atom to form SiO4 tetrahedra that share corners with two equivalent Nb(2)O6 octahedra,  a cornercorner with one Si(3)O4 tetrahedra, and  a cornercorner with one Si(4)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 23-26°. The Si(3)-O(14) bond length is 1.67 Å. The Si(3)-O(4) bond length is 1.62 Å. The Si(3)-O(6) bond length is 1.60 Å. The Si(3)-O(7) bond length is 1.63 Å. In the fourth Si site, Si(4) is bonded to one O(1), one O(11), one O(17), and one O(6) atom to form SiO4 tetrahedra that share corners with two equivalent Nb(1)O6 octahedra,  a cornercorner with one Si(3)O4 tetrahedra, and  a cornercorner with one Si(4)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 24-28°. The Si(4)-O(1) bond length is 1.63 Å. The Si(4)-O(11) bond length is 1.67 Å. The Si(4)-O(17) bond length is 1.62 Å. The Si(4)-O(6) bond length is 1.60 Å. In the fifth Si site, Si(5) is bonded to one O(12), one O(9), and two equivalent O(13) atoms to form SiO4 tetrahedra that share corners with two equivalent Nb(1)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 39°. The Si(5)-O(12) bond length is 1.60 Å. The Si(5)-O(9) bond length is 1.66 Å. Both Si(5)-O(13) bond lengths are 1.64 Å. In the sixth Si site, Si(6) is bonded to one O(10), one O(8), and two equivalent O(5) atoms to form SiO4 tetrahedra that share corners with two equivalent Nb(2)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 21°. The Si(6)-O(10) bond length is 1.68 Å. The Si(6)-O(8) bond length is 1.61 Å. Both Si(6)-O(5) bond lengths are 1.61 Å. There are seventeen inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to one K(1), one K(2), one Nb(1), and one Si(4) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one K(1), one Mg(1), one Nb(2), and one Si(2) atom. In the third O site, O(3) is bonded in a 2-coordinate geometry to one K(1), one K(3), one Nb(1), and one Si(1) atom. In the fourth O site, O(4) is bonded in a distorted bent 150 degrees geometry to one K(1), one K(3), one Nb(2), and one Si(3) atom. In the fifth O site, O(5) is bonded in a 2-coordinate geometry to one K(1), one K(3), one Nb(2), and one Si(6) atom. In the sixth O site, O(6) is bonded in a linear geometry to one Si(3) and one Si(4) atom. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one K(1), one K(2), one Nb(2), and one Si(3) atom. In the eighth O site, O(8) is bonded in a linear geometry to one Si(1) and one Si(6) atom. In the ninth O site, O(9) is bonded in a distorted bent 150 degrees geometry to two equivalent K(1), one Si(1), and one Si(5) atom. In the tenth O site, O(10) is bonded in a distorted bent 150 degrees geometry to two equivalent K(1), one Si(2), and one Si(6) atom. In the eleventh O site, O(11) is bonded in a 4-coordinate geometry to one K(2), one K(3), and two equivalent Si(4) atoms. In the twelfth O site, O(12) is bonded in a linear geometry to one Si(2) and one Si(5) atom. In the thirteenth O site, O(13) is bonded in a 3-coordinate geometry to one K(1), one Mg(1), one Nb(1), and one Si(5) atom. In the fourteenth O site, O(14) is bonded in a 4-coordinate geometry to one K(2), one K(3), and two equivalent Si(3) atoms. In the fifteenth O site, O(15) is bonded in a linear geometry to one Nb(1) and one Nb(2) atom. In the sixteenth O site, O(16) is bonded in a distorted linear geometry to one K(1), one K(3), one Nb(1), and one Nb(2) atom. In the seventeenth O site, O(17) is bonded in a distorted bent 150 degrees geometry to one K(1), one K(3), one Nb(1), and one Si(4) atom.