Mg3Bi2O7 crystallizes in the orthorhombic Cmc2_1 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(3) and two equivalent O(4) atoms to form MgO4 trigonal pyramids that share corners with eight equivalent Bi(1)O6 octahedra and corners with two equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 64-82°. In the second Mg site, Mg(2) is bonded in a 4-coordinate geometry to one O(2), one O(3), and three equivalent O(1) atoms. Bi(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form BiO6 octahedra that share corners with five equivalent Bi(1)O6 octahedra and corners with four equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 47-64°. There are four inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Mg(2) and one Bi(1) atom to form OMg3Bi tetrahedra that share  a cornercorner with one O(4)Mg2Bi2 tetrahedra and corners with six equivalent O(1)Mg3Bi tetrahedra. In the second O site, O(2) is bonded in a trigonal planar geometry to one Mg(2) and two equivalent Bi(1) atoms. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Mg(1), one Mg(2), and two equivalent Bi(1) atoms. In the fourth O site, O(4) is bonded to two equivalent Mg(1) and two equivalent Bi(1) atoms to form distorted OMg2Bi2 tetrahedra that share corners with two equivalent O(4)Mg2Bi2 tetrahedra and corners with two equivalent O(1)Mg3Bi tetrahedra.
Mg3Bi2O7 crystallizes in the orthorhombic Cmc2_1 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(3) and two equivalent O(4) atoms to form MgO4 trigonal pyramids that share corners with eight equivalent Bi(1)O6 octahedra and corners with two equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 64-82°. Both Mg(1)-O(3) bond lengths are 2.03 Å. Both Mg(1)-O(4) bond lengths are 2.06 Å. In the second Mg site, Mg(2) is bonded in a 4-coordinate geometry to one O(2), one O(3), and three equivalent O(1) atoms. The Mg(2)-O(2) bond length is 2.05 Å. The Mg(2)-O(3) bond length is 2.69 Å. There are a spread of Mg(2)-O(1) bond distances ranging from 2.04-2.11 Å. Bi(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form BiO6 octahedra that share corners with five equivalent Bi(1)O6 octahedra and corners with four equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 47-64°. The Bi(1)-O(1) bond length is 2.23 Å. The Bi(1)-O(4) bond length is 2.41 Å. There is one shorter (2.20 Å) and one longer (2.26 Å) Bi(1)-O(2) bond length. There is one shorter (2.23 Å) and one longer (2.25 Å) Bi(1)-O(3) bond length. There are four inequivalent O sites. In the first O site, O(1) is bonded to three equivalent Mg(2) and one Bi(1) atom to form OMg3Bi tetrahedra that share  a cornercorner with one O(4)Mg2Bi2 tetrahedra and corners with six equivalent O(1)Mg3Bi tetrahedra. In the second O site, O(2) is bonded in a trigonal planar geometry to one Mg(2) and two equivalent Bi(1) atoms. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Mg(1), one Mg(2), and two equivalent Bi(1) atoms. In the fourth O site, O(4) is bonded to two equivalent Mg(1) and two equivalent Bi(1) atoms to form distorted OMg2Bi2 tetrahedra that share corners with two equivalent O(4)Mg2Bi2 tetrahedra and corners with two equivalent O(1)Mg3Bi tetrahedra.