Abstract Li 8MgSi 6 is the compound with the highest silicon content in the ternary system Li/Mg/Si. The gray compound forms columnlike crystals with metallic lustre and is very sensitive to moisture. It reacts spontaneously with water to silanes and amorphous silicon. Li 8MgSi 6 is a diamagnetic semiconductor with E g = 0.72 eV, ϱ(292 K) = 1.3 × 10 3 Ω cm −. The compound is monoclinic and crystallizes in space group P2 1 m , a = 12.701, b = 4.347, c = 10.507 Å, β = 107.58°, Z = 2. The structure of Li 8MgSi 6 contains isolated silicon atoms and planar five-membered Si 5 rings which form 1 ∞[LiSi 5] sandwich stacks. Semiempirical SCF calculations are in accordance with the physical properties and support a description of the five-membered silicon rings as quasi-aromatic 26 electron systems. A generalization of the electron counting rules of Zintl and Klemm is proposed. A remark on the ambiguous Li 11Ge 6 is given.