The crystal structures of telyushenkoite, ideally Cs Na 6 (Be2 Al3 Si15 O39 F2), trigonal, space group P¯ 3m1, a 14.3770(8), c 4.8786(3) Å, V 873.2(1) Å3, Z = 1, and leifite, ideally Na Na 6 (Be2 Al3 Si15 O39 F2), trigonal, space group P¯ 3m1, a 14.361(1), c 4.8570(5) Å, V 867.5(2) Å3, Z = 1, have been refined to R indices of 2.4(2.9)% based on 874(837) observed (4 � F) reflections measured with MoKX-radiation. Electron-microprobe analysis of the crystals used to collect the X-ray intensity data gave the following unit formulae: telyushenkoite: (Cs 0.74 Na0.31 K0.14 Rb0.02) Na6 (Be2 Al2.06 Zn0.30 Si15.46 O39 F2); leifite: (Cs 0.05 Na0.57 K0.10 Rb0.11) Na6 (Be2 Al1.96 Zn0.17 Si15.74 O39 F2). There are four tetrahedrally coordinated T sites with the following site- occupancies: T(1) = (Si,Al,Zn), T(2) = T(3) = Si, T(4) = Be, and the Be coordination is O 3F with a Be-F distance of 1.576 Å. There is one Na site occupied by Na and coordinated by seven anions in an augmented trigonal-prismatic arrangement with a < Na-O,F> distance of ~2.545 Å. There is one A site occupied by large alkali cations with Cs dominant in telyushenkoite and Na dominant in leifite; the A site is coordinated by six O atoms in telyushenkoite in an octahedral arrangement, and by six O atoms and two (H2O) groups in leifite. The B site is unoccupied in telyushenkoite and is partly occupied by (H 2O) groups in leifite. The T(1) tetrahedra link to form six-membered rings parallel to {001} that are linked together in both the {001} plane and along (001) b y four-membered rings of T(2) and T(3) tetrahedra. The six-membered rings of T(1) tetrahedra stack along the c direction to form channels that accommodate the A and B sites. The Na polyhedra share edges with the T(4) (= Be) tetrahedron. The end-member formulae of telyushenkoite and leifite may be written as A Na6 (Be2 Al3 Si15 O39 F2), where A = Cs and Na, respectively. Consideration of the possible end-members of the leifite group led to the following compositions: