MgSi2 crystallizes in the triclinic P1 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Si(1), one Si(2), two equivalent Si(3), and two equivalent Si(4) atoms to form distorted MgSi6 octahedra that share  a cornercorner with one Si(4)Mg3Si3 octahedra, corners with two equivalent Si(8)Mg3Si3 octahedra, and edges with two equivalent Mg(1)Si6 octahedra. The corner-sharing octahedral tilt angles range from 23-116°. In the second Mg site, Mg(2) is bonded in a 6-coordinate geometry to one Si(2), one Si(5), one Si(6), one Si(8), and two equivalent Si(7) atoms. In the third Mg site, Mg(3) is bonded in a 4-coordinate geometry to one Si(1), one Si(3), one Si(4), and two equivalent Si(2) atoms. In the fourth Mg site, Mg(4) is bonded in a 7-coordinate geometry to one Si(3), one Si(5), one Si(7), two equivalent Si(6), and two equivalent Si(8) atoms. There are eight inequivalent Si sites. In the first Si site, Si(1) is bonded in a 5-coordinate geometry to one Mg(1), one Mg(3), one Si(4), and two equivalent Si(8) atoms. In the second Si site, Si(2) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(2), two equivalent Mg(3), one Si(6), and two equivalent Si(7) atoms. In the third Si site, Si(3) is bonded in a 7-coordinate geometry to one Mg(3), one Mg(4), two equivalent Mg(1), one Si(7), and two equivalent Si(6) atoms. In the fourth Si site, Si(4) is bonded to one Mg(3), two equivalent Mg(1), one Si(1), and two equivalent Si(5) atoms to form distorted SiMg3Si3 octahedra that share  a cornercorner with one Mg(1)Si6 octahedra, corners with four equivalent Si(8)Mg3Si3 octahedra, and edges with two equivalent Si(4)Mg3Si3 octahedra. The corner-sharing octahedral tilt angles range from 62-116°. In the fifth Si site, Si(5) is bonded in a 5-coordinate geometry to one Mg(2), one Mg(4), one Si(8), and two equivalent Si(4) atoms. In the sixth Si site, Si(6) is bonded in a 6-coordinate geometry to one Mg(2), two equivalent Mg(4), one Si(2), and two equivalent Si(3) atoms. In the seventh Si site, Si(7) is bonded in a 6-coordinate geometry to one Mg(4), two equivalent Mg(2), one Si(3), and two equivalent Si(2) atoms. In the eighth Si site, Si(8) is bonded to one Mg(2), two equivalent Mg(4), one Si(5), and two equivalent Si(1) atoms to form distorted SiMg3Si3 octahedra that share corners with two equivalent Mg(1)Si6 octahedra, corners with four equivalent Si(4)Mg3Si3 octahedra, and edges with two equivalent Si(8)Mg3Si3 octahedra. The corner-sharing octahedral tilt angles range from 23-81°.
MgSi2 crystallizes in the triclinic P1 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Si(1), one Si(2), two equivalent Si(3), and two equivalent Si(4) atoms to form distorted MgSi6 octahedra that share  a cornercorner with one Si(4)Mg3Si3 octahedra, corners with two equivalent Si(8)Mg3Si3 octahedra, and edges with two equivalent Mg(1)Si6 octahedra. The corner-sharing octahedral tilt angles range from 23-116°. The Mg(1)-Si(1) bond length is 2.91 Å. The Mg(1)-Si(2) bond length is 2.97 Å. There is one shorter (2.80 Å) and one longer (2.88 Å) Mg(1)-Si(3) bond length. There is one shorter (2.85 Å) and one longer (2.89 Å) Mg(1)-Si(4) bond length. In the second Mg site, Mg(2) is bonded in a 6-coordinate geometry to one Si(2), one Si(5), one Si(6), one Si(8), and two equivalent Si(7) atoms. The Mg(2)-Si(2) bond length is 2.99 Å. The Mg(2)-Si(5) bond length is 2.72 Å. The Mg(2)-Si(6) bond length is 3.04 Å. The Mg(2)-Si(8) bond length is 2.83 Å. There is one shorter (2.83 Å) and one longer (2.87 Å) Mg(2)-Si(7) bond length. In the third Mg site, Mg(3) is bonded in a 4-coordinate geometry to one Si(1), one Si(3), one Si(4), and two equivalent Si(2) atoms. The Mg(3)-Si(1) bond length is 2.70 Å. The Mg(3)-Si(3) bond length is 3.21 Å. The Mg(3)-Si(4) bond length is 2.80 Å. There is one shorter (2.81 Å) and one longer (2.86 Å) Mg(3)-Si(2) bond length. In the fourth Mg site, Mg(4) is bonded in a 7-coordinate geometry to one Si(3), one Si(5), one Si(7), two equivalent Si(6), and two equivalent Si(8) atoms. The Mg(4)-Si(3) bond length is 2.90 Å. The Mg(4)-Si(5) bond length is 2.91 Å. The Mg(4)-Si(7) bond length is 2.85 Å. There is one shorter (2.79 Å) and one longer (2.93 Å) Mg(4)-Si(6) bond length. There is one shorter (2.81 Å) and one longer (2.91 Å) Mg(4)-Si(8) bond length. There are eight inequivalent Si sites. In the first Si site, Si(1) is bonded in a 5-coordinate geometry to one Mg(1), one Mg(3), one Si(4), and two equivalent Si(8) atoms. The Si(1)-Si(4) bond length is 2.42 Å. There is one shorter (2.39 Å) and one longer (2.40 Å) Si(1)-Si(8) bond length. In the second Si site, Si(2) is bonded in a 7-coordinate geometry to one Mg(1), one Mg(2), two equivalent Mg(3), one Si(6), and two equivalent Si(7) atoms. The Si(2)-Si(6) bond length is 2.34 Å. There is one shorter (2.36 Å) and one longer (2.37 Å) Si(2)-Si(7) bond length. In the third Si site, Si(3) is bonded in a 7-coordinate geometry to one Mg(3), one Mg(4), two equivalent Mg(1), one Si(7), and two equivalent Si(6) atoms. The Si(3)-Si(7) bond length is 2.35 Å. Both Si(3)-Si(6) bond lengths are 2.36 Å. In the fourth Si site, Si(4) is bonded to one Mg(3), two equivalent Mg(1), one Si(1), and two equivalent Si(5) atoms to form distorted SiMg3Si3 octahedra that share  a cornercorner with one Mg(1)Si6 octahedra, corners with four equivalent Si(8)Mg3Si3 octahedra, and edges with two equivalent Si(4)Mg3Si3 octahedra. The corner-sharing octahedral tilt angles range from 62-116°. There is one shorter (2.38 Å) and one longer (2.39 Å) Si(4)-Si(5) bond length. In the fifth Si site, Si(5) is bonded in a 5-coordinate geometry to one Mg(2), one Mg(4), one Si(8), and two equivalent Si(4) atoms. The Si(5)-Si(8) bond length is 2.43 Å. In the sixth Si site, Si(6) is bonded in a 6-coordinate geometry to one Mg(2), two equivalent Mg(4), one Si(2), and two equivalent Si(3) atoms. In the seventh Si site, Si(7) is bonded in a 6-coordinate geometry to one Mg(4), two equivalent Mg(2), one Si(3), and two equivalent Si(2) atoms. In the eighth Si site, Si(8) is bonded to one Mg(2), two equivalent Mg(4), one Si(5), and two equivalent Si(1) atoms to form distorted SiMg3Si3 octahedra that share corners with two equivalent Mg(1)Si6 octahedra, corners with four equivalent Si(4)Mg3Si3 octahedra, and edges with two equivalent Si(8)Mg3Si3 octahedra. The corner-sharing octahedral tilt angles range from 23-81°.