Mg6CuSi crystallizes in the orthorhombic Amm2 space group. There are nine inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(2)Mg10Cu2 cuboctahedra; corners with ten Mg(1,1,1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with two equivalent Mg(1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Si(1)Mg10 cuboctahedra; edges with three equivalent Cu(1)Mg10 cuboctahedra; edges with four equivalent Mg(4)Mg10Si2 cuboctahedra; edges with four equivalent Mg(3)Mg8Cu2Si2 cuboctahedra;  a faceface with one Cu(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Si(1)Mg10 cuboctahedra; faces with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(3,3,3)Mg8Cu2Si2 cuboctahedra. In the second Mg site, Mg(1) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(2)Mg10Cu2 cuboctahedra; corners with ten Mg(1,1,1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with two equivalent Mg(1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Si(1)Mg10 cuboctahedra; edges with three equivalent Cu(1)Mg10 cuboctahedra; edges with four equivalent Mg(4)Mg10Si2 cuboctahedra; edges with four equivalent Mg(3)Mg8Cu2Si2 cuboctahedra;  a faceface with one Cu(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Si(1)Mg10 cuboctahedra; faces with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(3,3)Mg8Cu2Si2 cuboctahedra. In the third Mg site, Mg(2) is bonded to two equivalent Mg(4); four Mg(1,1,1,1); four Mg(3,3); and two equivalent Cu(1) atoms to form distorted MgMg10Cu2 cuboctahedra that share corners with four equivalent Cu(1)Mg10 cuboctahedra; corners with six equivalent Mg(2)Mg10Cu2 cuboctahedra; corners with eight Mg(1,1,1,1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Cu(1)Mg10 cuboctahedra; edges with four equivalent Mg(4)Mg10Si2 cuboctahedra; edges with four Mg(1,1,1,1)Mg8Cu2Si2 cuboctahedra; edges with eight Mg(3,3,3)Mg8Cu2Si2 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Cu(1)Mg10 cuboctahedra; faces with four Mg(1,1,1,1)Mg8Cu2Si2 cuboctahedra; faces with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; and faces with six equivalent Si(1)Mg10 cuboctahedra. In the fourth Mg site, Mg(3) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(4)Mg10Si2 cuboctahedra; corners with ten Mg(3,3,3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(4)Mg10Si2 cuboctahedra; edges with two equivalent Mg(3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Cu(1)Mg10 cuboctahedra; edges with three equivalent Si(1)Mg10 cuboctahedra; edges with four equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with four equivalent Mg(1)Mg8Cu2Si2 cuboctahedra;  a faceface with one Si(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Cu(1)Mg10 cuboctahedra; faces with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(1,1,1)Mg8Cu2Si2 cuboctahedra. In the fifth Mg site, Mg(3) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(4)Mg10Si2 cuboctahedra; corners with ten Mg(3,3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(4)Mg10Si2 cuboctahedra; edges with two equivalent Mg(3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Cu(1)Mg10 cuboctahedra; edges with three equivalent Si(1)Mg10 cuboctahedra; edges with four equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with four equivalent Mg(1)Mg8Cu2Si2 cuboctahedra;  a faceface with one Si(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Cu(1)Mg10 cuboctahedra; faces with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(1,1,1)Mg8Cu2Si2 cuboctahedra. In the sixth Mg site, Mg(4) is bonded to two equivalent Mg(2); four Mg(1,1); four Mg(3,3,3); and two equivalent Si(1) atoms to form distorted MgMg10Si2 cuboctahedra that share corners with four equivalent Si(1)Mg10 cuboctahedra; corners with six equivalent Mg(4)Mg10Si2 cuboctahedra; corners with eight Mg(3,3,3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Si(1)Mg10 cuboctahedra; edges with four equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with four Mg(3,3,3)Mg8Cu2Si2 cuboctahedra; edges with eight Mg(1,1,1,1)Mg8Cu2Si2 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Si(1)Mg10 cuboctahedra; faces with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; faces with four Mg(3,3,3)Mg8Cu2Si2 cuboctahedra; and faces with six equivalent Cu(1)Mg10 cuboctahedra. In the seventh Mg site, Mg(1) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(2)Mg10Cu2 cuboctahedra; corners with ten Mg(1,1,1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with two equivalent Mg(1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Si(1)Mg10 cuboctahedra; edges with three equivalent Cu(1)Mg10 cuboctahedra; edges with four equivalent Mg(4)Mg10Si2 cuboctahedra; edges with four equivalent Mg(3)Mg8Cu2Si2 cuboctahedra;  a faceface with one Cu(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Si(1)Mg10 cuboctahedra; faces with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(3,3)Mg8Cu2Si2 cuboctahedra. In the eighth Mg site, Mg(1) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(2)Mg10Cu2 cuboctahedra; corners with ten Mg(1,1,1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with two equivalent Mg(1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Si(1)Mg10 cuboctahedra; edges with three equivalent Cu(1)Mg10 cuboctahedra; edges with four equivalent Mg(4)Mg10Si2 cuboctahedra; edges with four equivalent Mg(3)Mg8Cu2Si2 cuboctahedra;  a faceface with one Cu(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Si(1)Mg10 cuboctahedra; faces with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(3,3,3)Mg8Cu2Si2 cuboctahedra. In the ninth Mg site, Mg(3) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(4)Mg10Si2 cuboctahedra; corners with ten Mg(3,3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(4)Mg10Si2 cuboctahedra; edges with two equivalent Mg(3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Cu(1)Mg10 cuboctahedra; edges with three equivalent Si(1)Mg10 cuboctahedra; edges with four equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with four equivalent Mg(1)Mg8Cu2Si2 cuboctahedra;  a faceface with one Si(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Cu(1)Mg10 cuboctahedra; faces with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(1,1,1)Mg8Cu2Si2 cuboctahedra. Cu(1) is bonded to two equivalent Mg(2); four Mg(1,1,1,1); and four Mg(3,3) atoms to form distorted CuMg10 cuboctahedra that share corners with four equivalent Mg(2)Mg10Cu2 cuboctahedra; corners with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; corners with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; corners with six equivalent Cu(1)Mg10 cuboctahedra; edges with two equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with two equivalent Cu(1)Mg10 cuboctahedra; edges with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; edges with four equivalent Si(1)Mg10 cuboctahedra; edges with six Mg(1,1,1,1)Mg8Cu2Si2 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two Mg(1,1)Mg8Cu2Si2 cuboctahedra; faces with two equivalent Si(1)Mg10 cuboctahedra; faces with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; and faces with six equivalent Mg(4)Mg10Si2 cuboctahedra. Si(1) is bonded to two equivalent Mg(4); four Mg(1,1); and four Mg(3,3,3) atoms to form distorted SiMg10 cuboctahedra that share corners with four equivalent Mg(4)Mg10Si2 cuboctahedra; corners with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; corners with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; corners with six equivalent Si(1)Mg10 cuboctahedra; edges with two equivalent Mg(4)Mg10Si2 cuboctahedra; edges with two equivalent Si(1)Mg10 cuboctahedra; edges with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; edges with four equivalent Cu(1)Mg10 cuboctahedra; edges with six Mg(3,3,3)Mg8Cu2Si2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two Mg(3,3)Mg8Cu2Si2 cuboctahedra; faces with two equivalent Cu(1)Mg10 cuboctahedra; faces with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; and faces with six equivalent Mg(2)Mg10Cu2 cuboctahedra.
Mg6CuSi crystallizes in the orthorhombic Amm2 space group. There are nine inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(2)Mg10Cu2 cuboctahedra; corners with ten Mg(1,1,1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with two equivalent Mg(1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Si(1)Mg10 cuboctahedra; edges with three equivalent Cu(1)Mg10 cuboctahedra; edges with four equivalent Mg(4)Mg10Si2 cuboctahedra; edges with four equivalent Mg(3)Mg8Cu2Si2 cuboctahedra;  a faceface with one Cu(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Si(1)Mg10 cuboctahedra; faces with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(3,3,3)Mg8Cu2Si2 cuboctahedra. There is one shorter (2.96 Å) and one longer (3.07 Å) Mg(1)-Mg(1) bond length. There is one shorter (3.01 Å) and one longer (3.05 Å) Mg(1)-Mg(2) bond length. Both Mg(1)-Mg(3) bond lengths are 3.10 Å. Both Mg(1)-Mg(4) bond lengths are 3.03 Å. There is one shorter (3.01 Å) and one longer (3.05 Å) Mg(1)-Cu(1) bond length. Both Mg(1)-Si(1) bond lengths are 3.01 Å. In the second Mg site, Mg(1) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(2)Mg10Cu2 cuboctahedra; corners with ten Mg(1,1,1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with two equivalent Mg(1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Si(1)Mg10 cuboctahedra; edges with three equivalent Cu(1)Mg10 cuboctahedra; edges with four equivalent Mg(4)Mg10Si2 cuboctahedra; edges with four equivalent Mg(3)Mg8Cu2Si2 cuboctahedra;  a faceface with one Cu(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Si(1)Mg10 cuboctahedra; faces with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(3,3)Mg8Cu2Si2 cuboctahedra. There is one shorter (3.01 Å) and one longer (3.05 Å) Mg(1)-Mg(2) bond length. Both Mg(1)-Mg(3) bond lengths are 3.10 Å. Both Mg(1)-Mg(4) bond lengths are 3.03 Å. There is one shorter (3.01 Å) and one longer (3.05 Å) Mg(1)-Cu(1) bond length. Both Mg(1)-Si(1) bond lengths are 3.01 Å. In the third Mg site, Mg(2) is bonded to two equivalent Mg(4); four Mg(1,1,1,1); four Mg(3,3); and two equivalent Cu(1) atoms to form distorted MgMg10Cu2 cuboctahedra that share corners with four equivalent Cu(1)Mg10 cuboctahedra; corners with six equivalent Mg(2)Mg10Cu2 cuboctahedra; corners with eight Mg(1,1,1,1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Cu(1)Mg10 cuboctahedra; edges with four equivalent Mg(4)Mg10Si2 cuboctahedra; edges with four Mg(1,1,1,1)Mg8Cu2Si2 cuboctahedra; edges with eight Mg(3,3,3)Mg8Cu2Si2 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Cu(1)Mg10 cuboctahedra; faces with four Mg(1,1,1,1)Mg8Cu2Si2 cuboctahedra; faces with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; and faces with six equivalent Si(1)Mg10 cuboctahedra. Both Mg(2)-Mg(4) bond lengths are 3.03 Å. Both Mg(2)-Mg(1,1) bond lengths are 3.05 Å. All Mg(2)-Mg(3,3) bond lengths are 3.04 Å. Both Mg(2)-Cu(1) bond lengths are 3.01 Å. In the fourth Mg site, Mg(3) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(4)Mg10Si2 cuboctahedra; corners with ten Mg(3,3,3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(4)Mg10Si2 cuboctahedra; edges with two equivalent Mg(3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Cu(1)Mg10 cuboctahedra; edges with three equivalent Si(1)Mg10 cuboctahedra; edges with four equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with four equivalent Mg(1)Mg8Cu2Si2 cuboctahedra;  a faceface with one Si(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Cu(1)Mg10 cuboctahedra; faces with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(1,1,1)Mg8Cu2Si2 cuboctahedra. Both Mg(3)-Mg(1) bond lengths are 3.10 Å. There is one shorter (2.90 Å) and one longer (3.13 Å) Mg(3)-Mg(3) bond length. There is one shorter (3.00 Å) and one longer (3.06 Å) Mg(3)-Mg(4) bond length. Both Mg(3)-Cu(1) bond lengths are 2.97 Å. There is one shorter (2.94 Å) and one longer (3.12 Å) Mg(3)-Si(1) bond length. In the fifth Mg site, Mg(3) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(4)Mg10Si2 cuboctahedra; corners with ten Mg(3,3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(4)Mg10Si2 cuboctahedra; edges with two equivalent Mg(3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Cu(1)Mg10 cuboctahedra; edges with three equivalent Si(1)Mg10 cuboctahedra; edges with four equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with four equivalent Mg(1)Mg8Cu2Si2 cuboctahedra;  a faceface with one Si(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Cu(1)Mg10 cuboctahedra; faces with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(1,1,1)Mg8Cu2Si2 cuboctahedra. Both Mg(3)-Mg(1) bond lengths are 3.10 Å. There is one shorter (3.00 Å) and one longer (3.06 Å) Mg(3)-Mg(4) bond length. Both Mg(3)-Cu(1) bond lengths are 2.97 Å. There is one shorter (2.94 Å) and one longer (3.12 Å) Mg(3)-Si(1) bond length. In the sixth Mg site, Mg(4) is bonded to two equivalent Mg(2); four Mg(1,1); four Mg(3,3,3); and two equivalent Si(1) atoms to form distorted MgMg10Si2 cuboctahedra that share corners with four equivalent Si(1)Mg10 cuboctahedra; corners with six equivalent Mg(4)Mg10Si2 cuboctahedra; corners with eight Mg(3,3,3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Si(1)Mg10 cuboctahedra; edges with four equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with four Mg(3,3,3)Mg8Cu2Si2 cuboctahedra; edges with eight Mg(1,1,1,1)Mg8Cu2Si2 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Si(1)Mg10 cuboctahedra; faces with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; faces with four Mg(3,3,3)Mg8Cu2Si2 cuboctahedra; and faces with six equivalent Cu(1)Mg10 cuboctahedra. The Mg(4)-Mg(3) bond length is 3.06 Å. Both Mg(4)-Si(1) bond lengths are 3.01 Å. In the seventh Mg site, Mg(1) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(2)Mg10Cu2 cuboctahedra; corners with ten Mg(1,1,1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with two equivalent Mg(1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Si(1)Mg10 cuboctahedra; edges with three equivalent Cu(1)Mg10 cuboctahedra; edges with four equivalent Mg(4)Mg10Si2 cuboctahedra; edges with four equivalent Mg(3)Mg8Cu2Si2 cuboctahedra;  a faceface with one Cu(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Si(1)Mg10 cuboctahedra; faces with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(3,3)Mg8Cu2Si2 cuboctahedra. There is one shorter (2.96 Å) and one longer (3.07 Å) Mg(1)-Mg(1) bond length. Both Mg(1)-Mg(4) bond lengths are 3.03 Å. There is one shorter (3.01 Å) and one longer (3.05 Å) Mg(1)-Cu(1) bond length. Both Mg(1)-Si(1) bond lengths are 3.01 Å. In the eighth Mg site, Mg(1) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(2)Mg10Cu2 cuboctahedra; corners with ten Mg(1,1,1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with two equivalent Mg(1)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Si(1)Mg10 cuboctahedra; edges with three equivalent Cu(1)Mg10 cuboctahedra; edges with four equivalent Mg(4)Mg10Si2 cuboctahedra; edges with four equivalent Mg(3)Mg8Cu2Si2 cuboctahedra;  a faceface with one Cu(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Si(1)Mg10 cuboctahedra; faces with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(3,3,3)Mg8Cu2Si2 cuboctahedra. Both Mg(1)-Mg(4) bond lengths are 3.03 Å. There is one shorter (3.01 Å) and one longer (3.05 Å) Mg(1)-Cu(1) bond length. Both Mg(1)-Si(1) bond lengths are 3.01 Å. In the ninth Mg site, Mg(3) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Cu(1), and two equivalent Si(1) atoms to form distorted MgMg8Cu2Si2 cuboctahedra that share corners with two equivalent Cu(1)Mg10 cuboctahedra; corners with two equivalent Si(1)Mg10 cuboctahedra; corners with four equivalent Mg(4)Mg10Si2 cuboctahedra; corners with ten Mg(3,3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Mg(4)Mg10Si2 cuboctahedra; edges with two equivalent Mg(3)Mg8Cu2Si2 cuboctahedra; edges with two equivalent Cu(1)Mg10 cuboctahedra; edges with three equivalent Si(1)Mg10 cuboctahedra; edges with four equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with four equivalent Mg(1)Mg8Cu2Si2 cuboctahedra;  a faceface with one Si(1)Mg10 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two equivalent Cu(1)Mg10 cuboctahedra; faces with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; and faces with eight Mg(1,1,1)Mg8Cu2Si2 cuboctahedra. Both Mg(3)-Mg(2) bond lengths are 3.04 Å. There is one shorter (2.90 Å) and one longer (3.13 Å) Mg(3)-Mg(3) bond length. Both Mg(3)-Cu(1) bond lengths are 2.97 Å. There is one shorter (2.94 Å) and one longer (3.12 Å) Mg(3)-Si(1) bond length. Cu(1) is bonded to two equivalent Mg(2); four Mg(1,1,1,1); and four Mg(3,3) atoms to form distorted CuMg10 cuboctahedra that share corners with four equivalent Mg(2)Mg10Cu2 cuboctahedra; corners with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; corners with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; corners with six equivalent Cu(1)Mg10 cuboctahedra; edges with two equivalent Mg(2)Mg10Cu2 cuboctahedra; edges with two equivalent Cu(1)Mg10 cuboctahedra; edges with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; edges with four equivalent Si(1)Mg10 cuboctahedra; edges with six Mg(1,1,1,1)Mg8Cu2Si2 cuboctahedra; faces with two equivalent Mg(2)Mg10Cu2 cuboctahedra; faces with two Mg(1,1)Mg8Cu2Si2 cuboctahedra; faces with two equivalent Si(1)Mg10 cuboctahedra; faces with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; and faces with six equivalent Mg(4)Mg10Si2 cuboctahedra. Si(1) is bonded to two equivalent Mg(4); four Mg(1,1); and four Mg(3,3,3) atoms to form distorted SiMg10 cuboctahedra that share corners with four equivalent Mg(4)Mg10Si2 cuboctahedra; corners with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; corners with four Mg(3,3)Mg8Cu2Si2 cuboctahedra; corners with six equivalent Si(1)Mg10 cuboctahedra; edges with two equivalent Mg(4)Mg10Si2 cuboctahedra; edges with two equivalent Si(1)Mg10 cuboctahedra; edges with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; edges with four equivalent Cu(1)Mg10 cuboctahedra; edges with six Mg(3,3,3)Mg8Cu2Si2 cuboctahedra; faces with two equivalent Mg(4)Mg10Si2 cuboctahedra; faces with two Mg(3,3)Mg8Cu2Si2 cuboctahedra; faces with two equivalent Cu(1)Mg10 cuboctahedra; faces with four Mg(1,1)Mg8Cu2Si2 cuboctahedra; and faces with six equivalent Mg(2)Mg10Cu2 cuboctahedra.