MgSi2 crystallizes in the monoclinic C2/m space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 10-coordinate geometry to two equivalent Si(3), four equivalent Si(1), and four equivalent Si(2) atoms. In the second Mg site, Mg(2) is bonded in a 9-coordinate geometry to two equivalent Si(1), two equivalent Si(2), two equivalent Si(3), and three equivalent Si(4) atoms. In the third Mg site, Mg(3) is bonded to two equivalent Si(1) and four equivalent Si(4) atoms to form distorted MgSi6 octahedra that share edges with four equivalent Si(3)Mg3Si4 hexagonal pyramids and edges with two equivalent Mg(3)Si6 octahedra. There are four inequivalent Si sites. In the first Si site, Si(1) is bonded in a 7-coordinate geometry to one Mg(3), two equivalent Mg(1), two equivalent Mg(2), and two equivalent Si(3) atoms. In the second Si site, Si(2) is bonded in a 7-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), one Si(3), and two equivalent Si(2) atoms. In the third Si site, Si(3) is bonded to one Mg(1), two equivalent Mg(2), one Si(2), one Si(4), and two equivalent Si(1) atoms to form distorted SiMg3Si4 hexagonal pyramids that share  a cornercorner with one Si(3)Mg3Si4 hexagonal pyramid, edges with two equivalent Si(3)Mg3Si4 hexagonal pyramids, and edges with two equivalent Mg(3)Si6 octahedra. In the fourth Si site, Si(4) is bonded in a 8-coordinate geometry to two equivalent Mg(3), three equivalent Mg(2), one Si(3), and two equivalent Si(4) atoms.
MgSi2 crystallizes in the monoclinic C2/m space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 10-coordinate geometry to two equivalent Si(3), four equivalent Si(1), and four equivalent Si(2) atoms. Both Mg(1)-Si(3) bond lengths are 2.90 Å. All Mg(1)-Si(1) bond lengths are 2.89 Å. All Mg(1)-Si(2) bond lengths are 3.09 Å. In the second Mg site, Mg(2) is bonded in a 9-coordinate geometry to two equivalent Si(1), two equivalent Si(2), two equivalent Si(3), and three equivalent Si(4) atoms. Both Mg(2)-Si(1) bond lengths are 2.76 Å. Both Mg(2)-Si(2) bond lengths are 3.01 Å. Both Mg(2)-Si(3) bond lengths are 2.93 Å. There is one shorter (2.70 Å) and two longer (3.06 Å) Mg(2)-Si(4) bond lengths. In the third Mg site, Mg(3) is bonded to two equivalent Si(1) and four equivalent Si(4) atoms to form distorted MgSi6 octahedra that share edges with four equivalent Si(3)Mg3Si4 hexagonal pyramids and edges with two equivalent Mg(3)Si6 octahedra. Both Mg(3)-Si(1) bond lengths are 2.97 Å. All Mg(3)-Si(4) bond lengths are 2.88 Å. There are four inequivalent Si sites. In the first Si site, Si(1) is bonded in a 7-coordinate geometry to one Mg(3), two equivalent Mg(1), two equivalent Mg(2), and two equivalent Si(3) atoms. Both Si(1)-Si(3) bond lengths are 2.41 Å. In the second Si site, Si(2) is bonded in a 7-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), one Si(3), and two equivalent Si(2) atoms. The Si(2)-Si(3) bond length is 2.45 Å. Both Si(2)-Si(2) bond lengths are 2.41 Å. In the third Si site, Si(3) is bonded to one Mg(1), two equivalent Mg(2), one Si(2), one Si(4), and two equivalent Si(1) atoms to form distorted SiMg3Si4 hexagonal pyramids that share  a cornercorner with one Si(3)Mg3Si4 hexagonal pyramid, edges with two equivalent Si(3)Mg3Si4 hexagonal pyramids, and edges with two equivalent Mg(3)Si6 octahedra. The Si(3)-Si(4) bond length is 2.39 Å. In the fourth Si site, Si(4) is bonded in a 8-coordinate geometry to two equivalent Mg(3), three equivalent Mg(2), one Si(3), and two equivalent Si(4) atoms. Both Si(4)-Si(4) bond lengths are 2.44 Å.