Mg2CuSi3 crystallizes in the trigonal R-3m space group. Mg(1) is bonded in a 16-coordinate geometry to four equivalent Mg(1), three equivalent Cu(1), and nine equivalent Si(1) atoms. Cu(1) is bonded to six equivalent Mg(1) and six equivalent Si(1) atoms to form CuMg6Si6 cuboctahedra that share corners with six equivalent Cu(1)Mg6Si6 cuboctahedra, corners with twelve equivalent Si(1)Mg6Cu2Si4 cuboctahedra, edges with six equivalent Cu(1)Mg6Si6 cuboctahedra, and faces with eighteen equivalent Si(1)Mg6Cu2Si4 cuboctahedra. Si(1) is bonded to six equivalent Mg(1), two equivalent Cu(1), and four equivalent Si(1) atoms to form SiMg6Cu2Si4 cuboctahedra that share corners with four equivalent Cu(1)Mg6Si6 cuboctahedra, corners with fourteen equivalent Si(1)Mg6Cu2Si4 cuboctahedra, edges with six equivalent Si(1)Mg6Cu2Si4 cuboctahedra, faces with six equivalent Cu(1)Mg6Si6 cuboctahedra, and faces with twelve equivalent Si(1)Mg6Cu2Si4 cuboctahedra.
Mg2CuSi3 crystallizes in the trigonal R-3m space group. Mg(1) is bonded in a 16-coordinate geometry to four equivalent Mg(1), three equivalent Cu(1), and nine equivalent Si(1) atoms. There is one shorter (2.95 Å) and three longer (3.21 Å) Mg(1)-Mg(1) bond lengths. All Mg(1)-Cu(1) bond lengths are 3.11 Å. There are three shorter (2.84 Å) and six longer (3.05 Å) Mg(1)-Si(1) bond lengths. Cu(1) is bonded to six equivalent Mg(1) and six equivalent Si(1) atoms to form CuMg6Si6 cuboctahedra that share corners with six equivalent Cu(1)Mg6Si6 cuboctahedra, corners with twelve equivalent Si(1)Mg6Cu2Si4 cuboctahedra, edges with six equivalent Cu(1)Mg6Si6 cuboctahedra, and faces with eighteen equivalent Si(1)Mg6Cu2Si4 cuboctahedra. All Cu(1)-Si(1) bond lengths are 2.47 Å. Si(1) is bonded to six equivalent Mg(1), two equivalent Cu(1), and four equivalent Si(1) atoms to form SiMg6Cu2Si4 cuboctahedra that share corners with four equivalent Cu(1)Mg6Si6 cuboctahedra, corners with fourteen equivalent Si(1)Mg6Cu2Si4 cuboctahedra, edges with six equivalent Si(1)Mg6Cu2Si4 cuboctahedra, faces with six equivalent Cu(1)Mg6Si6 cuboctahedra, and faces with twelve equivalent Si(1)Mg6Cu2Si4 cuboctahedra. All Si(1)-Si(1) bond lengths are 2.67 Å.