K3Mg4(Si2O7)2 crystallizes in the monoclinic Pm space group. There are three inequivalent K sites. In the first K site, K(1) is bonded in a 9-coordinate geometry to two equivalent O(1), two equivalent O(2), two equivalent O(3), and three equivalent O(7) atoms. In the second K site, K(2) is bonded in a 9-coordinate geometry to two equivalent O(1), two equivalent O(2), two equivalent O(3), and three equivalent O(7) atoms. In the third K site, K(3) is bonded in a 9-coordinate geometry to two equivalent O(4), two equivalent O(5), two equivalent O(6), and three equivalent O(8) atoms. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form MgO6 octahedra that share corners with three equivalent Si(1)O4 tetrahedra, corners with three equivalent Si(2)O4 tetrahedra, and edges with three equivalent Mg(2)O6 octahedra. In the second Mg site, Mg(2) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form MgO6 octahedra that share corners with three equivalent Si(1)O4 tetrahedra, corners with three equivalent Si(2)O4 tetrahedra, and edges with three equivalent Mg(1)O6 octahedra. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(2), one O(3), and one O(7) atom to form SiO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra, corners with three equivalent Mg(2)O6 octahedra, and  a cornercorner with one Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 54-55°. In the second Si site, Si(2) is bonded to one O(4), one O(5), one O(6), and one O(8) atom to form SiO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra, corners with three equivalent Mg(2)O6 octahedra, and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-54°. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one K(1), one K(2), one Mg(1), one Mg(2), and one Si(1) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one K(1), one K(2), one Mg(1), one Mg(2), and one Si(1) atom. In the third O site, O(3) is bonded in a 5-coordinate geometry to one K(1), one K(2), one Mg(1), one Mg(2), and one Si(1) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one K(3), one Mg(1), one Mg(2), and one Si(2) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one K(3), one Mg(1), one Mg(2), and one Si(2) atom. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to one K(3), one Mg(1), one Mg(2), and one Si(2) atom. In the seventh O site, O(7) is bonded in a linear geometry to three equivalent K(1), three equivalent K(2), and two equivalent Si(1) atoms. In the eighth O site, O(8) is bonded in a linear geometry to three equivalent K(3) and two equivalent Si(2) atoms.
K3Mg4(Si2O7)2 crystallizes in the monoclinic Pm space group. There are three inequivalent K sites. In the first K site, K(1) is bonded in a 9-coordinate geometry to two equivalent O(1), two equivalent O(2), two equivalent O(3), and three equivalent O(7) atoms. Both K(1)-O(1) bond lengths are 2.77 Å. Both K(1)-O(2) bond lengths are 2.75 Å. Both K(1)-O(3) bond lengths are 2.74 Å. There are a spread of K(1)-O(7) bond distances ranging from 2.93-2.98 Å. In the second K site, K(2) is bonded in a 9-coordinate geometry to two equivalent O(1), two equivalent O(2), two equivalent O(3), and three equivalent O(7) atoms. Both K(2)-O(1) bond lengths are 2.75 Å. Both K(2)-O(2) bond lengths are 2.77 Å. Both K(2)-O(3) bond lengths are 2.74 Å. There are a spread of K(2)-O(7) bond distances ranging from 2.93-2.98 Å. In the third K site, K(3) is bonded in a 9-coordinate geometry to two equivalent O(4), two equivalent O(5), two equivalent O(6), and three equivalent O(8) atoms. Both K(3)-O(4) bond lengths are 2.78 Å. Both K(3)-O(5) bond lengths are 2.80 Å. Both K(3)-O(6) bond lengths are 2.76 Å. There are a spread of K(3)-O(8) bond distances ranging from 2.93-3.01 Å. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form MgO6 octahedra that share corners with three equivalent Si(1)O4 tetrahedra, corners with three equivalent Si(2)O4 tetrahedra, and edges with three equivalent Mg(2)O6 octahedra. The Mg(1)-O(1) bond length is 2.18 Å. The Mg(1)-O(2) bond length is 2.19 Å. The Mg(1)-O(3) bond length is 2.20 Å. The Mg(1)-O(4) bond length is 2.10 Å. The Mg(1)-O(5) bond length is 2.10 Å. The Mg(1)-O(6) bond length is 2.09 Å. In the second Mg site, Mg(2) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form MgO6 octahedra that share corners with three equivalent Si(1)O4 tetrahedra, corners with three equivalent Si(2)O4 tetrahedra, and edges with three equivalent Mg(1)O6 octahedra. The Mg(2)-O(1) bond length is 2.14 Å. The Mg(2)-O(2) bond length is 2.13 Å. The Mg(2)-O(3) bond length is 2.14 Å. The Mg(2)-O(4) bond length is 2.12 Å. The Mg(2)-O(5) bond length is 2.11 Å. The Mg(2)-O(6) bond length is 2.12 Å. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(2), one O(3), and one O(7) atom to form SiO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra, corners with three equivalent Mg(2)O6 octahedra, and  a cornercorner with one Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 54-55°. The Si(1)-O(1) bond length is 1.65 Å. The Si(1)-O(2) bond length is 1.64 Å. The Si(1)-O(3) bond length is 1.64 Å. The Si(1)-O(7) bond length is 1.72 Å. In the second Si site, Si(2) is bonded to one O(4), one O(5), one O(6), and one O(8) atom to form SiO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra, corners with three equivalent Mg(2)O6 octahedra, and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-54°. The Si(2)-O(4) bond length is 1.65 Å. The Si(2)-O(5) bond length is 1.64 Å. The Si(2)-O(6) bond length is 1.65 Å. The Si(2)-O(8) bond length is 1.67 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one K(1), one K(2), one Mg(1), one Mg(2), and one Si(1) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one K(1), one K(2), one Mg(1), one Mg(2), and one Si(1) atom. In the third O site, O(3) is bonded in a 5-coordinate geometry to one K(1), one K(2), one Mg(1), one Mg(2), and one Si(1) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one K(3), one Mg(1), one Mg(2), and one Si(2) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one K(3), one Mg(1), one Mg(2), and one Si(2) atom. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to one K(3), one Mg(1), one Mg(2), and one Si(2) atom. In the seventh O site, O(7) is bonded in a linear geometry to three equivalent K(1), three equivalent K(2), and two equivalent Si(1) atoms. In the eighth O site, O(8) is bonded in a linear geometry to three equivalent K(3) and two equivalent Si(2) atoms.