The structure of a crystal of natural melilite from San Venanzo, Umbria (Italy) of the general formula X
2
T1(T2)2O7, where X = Ca0.945Sr0.005Na0.04K0.01, T1 = Mg0.92Al0.08 and T2 = Si0.99Al0.01, has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P\bar 421
m:p4mg, cell parameters a = 7.860 (1), c = 5.024 (1) Å, modulation vectors q
1 = 0.2815 (3)(a* + b*), q
2 = 0.2815 (3)(−a* + b*). The data collection was performed on a KumaCCD diffractometer. The structure was refined from 7606 reflections to final R = 0.0481. A special modification of the refinement program Jana2000 was necessary to take into account overlapping of satellite reflections m × n = ±1, which could not be properly separated in the integration procedure. The final model includes modulations of the atomic positions as well as modulations of the thermal parameters. The latter are induced by strong 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 eightfold Ca coordination occur throughout the structure and the thermal ellipsoid changes its shape correspondingly. The positional modulation of the atoms causes variations in the interatomic distances which, however, do not affect bond-valence sums considerably, but induce flattening and rotation in T1 and T2 tetrahedra, respectively.
Structure D 2000 K 2 M(H 2 P 2 O 7 ) 2 ·2H 2 O (M: Ni, Cu, Zn): Orthorhombic Forms and Raman Spectra. -The isotypic title compounds crystallize in the orthorhombic space group Pnma with Z = 4 (single crystal XRD at 292 K). These dihydrogendiphosphates also exist in triclinic forms with very similar structural features. -(ESSEHLI, R.; EL BALI*, B.; TAHIRI, A. A.; LACHKAR, M.; MANOUN, B.; DUSEK, M.; FEJFAROVA, K.; Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 61 (2005) 12, i120-i124; Dep. Chim., Fac. Sci., Univ. Mohamed, 60000 Oujda, Morocco; Eng.) -Schramke 10-012
Page s 179 sectional groups of the two individuals, in their respective orientations, obtained across a plane which corresponds to the composition plane of the twin. The second case needs the use of lattice complexes, which are types of crystallographic orbits generated by atoms in a crystal structure under the action of the symmetry operations of the space group. When part of the atoms of an individual of a twin have a continuation across the twin interface, the individuals have a substructure in common, which corresponds to a lattice complex. If a crystal structure is composed of lattice complexes some of which have an eigensymmetry higher than the space group, these can be invariant under the action of the twin operation(s) and govern thus the formation of the twin.
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