Sr6Mg(W2O9)2 crystallizes in the trigonal R32 space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to two equivalent O(3), two equivalent O(4), four equivalent O(1), and four equivalent O(2) atoms to form distorted SrO12 cuboctahedra that share corners with six equivalent Sr(1)O12 cuboctahedra, corners with two equivalent W(1)O6 octahedra, corners with two equivalent W(2)O6 octahedra, faces with two equivalent Sr(1)O12 cuboctahedra, faces with two equivalent W(1)O6 octahedra, and faces with two equivalent W(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 16-22°. In the second Sr site, Sr(2) is bonded in a 10-coordinate geometry to two equivalent O(1), two equivalent O(3), two equivalent O(4), and four equivalent O(2) atoms. Mg(1) is bonded in a trigonal planar geometry to three equivalent O(3) atoms. There are two inequivalent W sites. In the first W site, W(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted WO6 octahedra that share corners with three equivalent Sr(1)O12 cuboctahedra, faces with three equivalent Sr(1)O12 cuboctahedra, and  a faceface with one W(2)O6 octahedra. In the second W site, W(2) is bonded to three equivalent O(1) and three equivalent O(4) atoms to form WO6 octahedra that share corners with three equivalent Sr(1)O12 cuboctahedra, faces with three equivalent Sr(1)O12 cuboctahedra,  a faceface with one W(1)O6 octahedra, and  a faceface with one W(2)O6 octahedra. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 2-coordinate geometry to one Sr(2), two equivalent Sr(1), one W(1), and one W(2) atom. In the second O site, O(2) is bonded in a 1-coordinate geometry to two equivalent Sr(1), two equivalent Sr(2), and one W(1) atom. In the third O site, O(3) is bonded in a distorted single-bond geometry to two equivalent Sr(1), two equivalent Sr(2), and one Mg(1) atom. In the fourth O site, O(4) is bonded in a 6-coordinate geometry to two equivalent Sr(1), two equivalent Sr(2), and two equivalent W(2) atoms.
Sr6Mg(W2O9)2 crystallizes in the trigonal R32 space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to two equivalent O(3), two equivalent O(4), four equivalent O(1), and four equivalent O(2) atoms to form distorted SrO12 cuboctahedra that share corners with six equivalent Sr(1)O12 cuboctahedra, corners with two equivalent W(1)O6 octahedra, corners with two equivalent W(2)O6 octahedra, faces with two equivalent Sr(1)O12 cuboctahedra, faces with two equivalent W(1)O6 octahedra, and faces with two equivalent W(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 16-22°. Both Sr(1)-O(3) bond lengths are 2.79 Å. Both Sr(1)-O(4) bond lengths are 3.01 Å. There are two shorter (2.94 Å) and two longer (3.23 Å) Sr(1)-O(1) bond lengths. There are two shorter (2.69 Å) and two longer (3.08 Å) Sr(1)-O(2) bond lengths. In the second Sr site, Sr(2) is bonded in a 10-coordinate geometry to two equivalent O(1), two equivalent O(3), two equivalent O(4), and four equivalent O(2) atoms. Both Sr(2)-O(1) bond lengths are 2.98 Å. Both Sr(2)-O(3) bond lengths are 2.60 Å. Both Sr(2)-O(4) bond lengths are 3.18 Å. There are two shorter (2.74 Å) and two longer (3.13 Å) Sr(2)-O(2) bond lengths. Mg(1) is bonded in a trigonal planar geometry to three equivalent O(3) atoms. All Mg(1)-O(3) bond lengths are 1.95 Å. There are two inequivalent W sites. In the first W site, W(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted WO6 octahedra that share corners with three equivalent Sr(1)O12 cuboctahedra, faces with three equivalent Sr(1)O12 cuboctahedra, and  a faceface with one W(2)O6 octahedra. All W(1)-O(1) bond lengths are 2.16 Å. All W(1)-O(2) bond lengths are 1.83 Å. In the second W site, W(2) is bonded to three equivalent O(1) and three equivalent O(4) atoms to form WO6 octahedra that share corners with three equivalent Sr(1)O12 cuboctahedra, faces with three equivalent Sr(1)O12 cuboctahedra,  a faceface with one W(1)O6 octahedra, and  a faceface with one W(2)O6 octahedra. All W(2)-O(1) bond lengths are 1.96 Å. All W(2)-O(4) bond lengths are 2.07 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 2-coordinate geometry to one Sr(2), two equivalent Sr(1), one W(1), and one W(2) atom. In the second O site, O(2) is bonded in a 1-coordinate geometry to two equivalent Sr(1), two equivalent Sr(2), and one W(1) atom. In the third O site, O(3) is bonded in a distorted single-bond geometry to two equivalent Sr(1), two equivalent Sr(2), and one Mg(1) atom. In the fourth O site, O(4) is bonded in a 6-coordinate geometry to two equivalent Sr(1), two equivalent Sr(2), and two equivalent W(2) atoms.