The crystal chemistry of telyushenkoite and leifite, A Na (sub 6) [Be (sub 2) Al (sub 3) Si (sub 15) O (sub 39) F (sub 2) ], A = Cs, Na

The crystal structures of telyushenkoite, ideally Cs Na (sub 6) [Be (sub 2) Al (sub 3) Si (sub 15) O (sub 39) F (sub 2) ], trigonal, space group P3ml, a 14.3770(8), c 4.8786(3) Aa, V873.2(1) Aa (super 3) , Z = 1 and leifite, ideally Na Na (sub 6) [Be (sub 2) Al (sub 3) Si (sub 15) O (sub 39) F (sub 2) ], trigonal, space group P3ml, a 14.361(1), c 4.8570(5) Aa, V 867.5(2) Aa (super 3) , Z = 1, have been refined to R indices of 2.4(2.9)% based on 874(837) observed (4sigma F) reflections measured with MoKalpha X-radiation. Electron-microprobe analysis of the crystals used to collect the X-ray intensity data gave the following unit formulae: telyushenkoite: (Cs (sub 0.74) Na (sub 0.31) K (sub 0.14) Rb (sub 0.02) ) Na6[Be (sub 2) Al (sub 2.06) Zn (sub 0.30) Si (sub 15.46) O (sub 39) F (sub 2) ]; leifite: (Cs (sub 0.05) Na (sub 0.57) K (sub 0.10) Rb (sub 0.11) ) Na (sub 6) [Be (sub 2) Al (sub 1.96) Zn (sub 0.17) Si (sub 15.74) O (sub 39) F (sub 2) ]. 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 (sub 3) F with a Be-F distance of 1.576 Aa. 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 approximately 2.545 Aa. 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 (H (sub 2) O) groups in leifite. The B site is unoccupied in telyushenkoite and is partly occupied by (H (sub 2) O) 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] by 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 Na (sub 6) [Be (sub 2) Al (sub 3) Si (sub 15) O (sub 39) F (sub 2) ], where A = Cs and Na, respectively.