Terskite Na₄ZrSi₆O₁₆·2H₂O is a zeolite-like zirconosilicate common in the hyperagpaitic pegmatites of the Lovozero alkaline massif (Kola Peninsula, Russia). The base of its crystal structure is the heteropolyhedral framework formed by branched infinite chains of Si tetrahedra and isolated Zr octahedra (Pudovkina and Chernitsova, 1991). Extra-framework Na atoms and H₂O molecules are located in the zeolite channels. The framework density is 18.0 Si and Zr atoms per 1000 ų. It allowed to suppose that the mineral is a potential ion-exchanger.