Five-dimensional structure refinement of natural melilite, (Ca1.89Sr0.01Na0.08K0.02)(Mg0.92Al0.08)-(Si1.98Al0.02)O-7

The structure of a crystal of natural melilite from San Venanzo, Umbria (It aly) of the general formula X(2)T1(T2)(2)O-7, where X = Ca0.945Sr0.005Na0.0 4K0.01, T1 = Mg0.92Al0.08 and T2 = Si0.99Al0.01, has been solved and refine d as an incommensurate structure in five-dimensional superspace. The struct ure is tetragonal, superspace group P (4) over bar2(1)m:p4mg, cell paramete rs a = 7.860 (1), c =5.024 (1) Angstrom, modulation vectors q(1) = 0.2815 ( 3)(a* +b*), q(2) = 0.2815 (3)(-a* +b*). The data collection was performed o n a KumaCCD diffractometer. The structure was refined from 7606 reflections to final R = 0.0481. A special modification of the refinement program Jana 2000 was necessary to take into account overlapping of satellite reflection s m x n = +/- 1, which could not be properly separated in the integration p rocedure. The final model includes modulations of the atomic positions as w ell as modulations of the thermal parameters. The latter are induced by str ong differences in the neighbourhood of the actual modulated positions. The occupational modulation was neither significant for X nor for T1 sites and the sites were supposed to be occupied only by Ca and Mg, respectively. As a consequence of the Ca and O positional modulations six-, seven- and eigh tfold Ca coordination occur throughout the structure and the thermal ellips oid changes its shape correspondingly. The positional modulation of the ato ms causes variations in the interatomic distances which, however, do not af fect bond-valence sums considerably, but induce flattening and rotation in T1 and T2 tetrahedra, respectively.