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Obituary for Dieter Schwarzenbach. [ABSTRACT FROM AUTHOR]

The incommensurately modulated structure of the mineral natrite has been refined for the first time. Two single-crystal grains, Lv and Kh, from two different occurrences [Mt. Karnasurt, Lovozero massif(Lv), Kola peninsula, Russia, and the pegmatite of Mt. Koashva, Khibiny massif (Kh)], have been investigated at 293 and 120 K using synchrotron X-ray data. The average structures of both minerals are identical and the basic features &the structural modulations are similar to the synthetic [gamma]-[Na.sub.2]C[O.sub.3] phase previously published. The 7 (incommensurate) [right arrow] (lock-in) phase transition reported at low temperature for the synthetic compound was not observed down to 120 K in natural natrite. Crystalchemical aspects, especially about the second coordination sphere for the carbon atoms, are examined to explain the different structural behaviors observed at low temperature. The possible role played by the minor isomorphous substitutions in natural natrite specimens is also discussed. Keywords: Incommensurate structure, natrite, superspace, synchrotron X-ray data DOI: 10.2138/am.2010.3384

Structural investigations of several minerals belonging to the calaverite group with composition [Au.sub.1-x][Ag.sub.x][Te.sub.2] (x = 0.00, 0.02, 0.05, 0.09, 0.19, and 0.33) indicate that Ag is randomly distributed on the Au sites. This suppresses the valence fluctuation of Au and, therefore, the structure modulations. The results are compared with the previously published incommensurately modulated structure of calaverite, [Au.sub.0.9][Ag.sub.0.1][Te.sub.2], which is characterized by valence fluctuations of Au reinforced by an ordered distribution of Ag. The (3+ 1)-dimensional calaverite structure type is able to reproduce both (3+1)D and 3D related structures with the general formula A[B.sub.2] (A = Au, Ag, Cu, Nb, Ta; B = Te). Keywords: Calaverite, modulated structure, valence fluctuations, krennerite

A detailed synchrotron X‐ray diffraction (XRD) study performed with a single crystal of BaVS3 (barium vanadium trisulfide) in the temperature range between 10 and 295 K is reported. Aside from the known tetragonal–orthorhombic (240 K) and orthorhombic–monoclinic (69 K) phase transitions, in the 130 < T ≤ 295 K range the overall structure can be viewed as a host–guest (H–G) composite. The BaS3 matrix is the host, while the V‐chains form the guest. The two subsystems lock in at TLOCK = 130 ± 20 K. This temperature is marked by a symmetry change from orthorhombic to monoclinic. This results in the formation of twins, implying a structural phase transition identified here for the first time. From the refined structural data, it is possible to follow, starting already at 295 K downwards, the stepwise transformation of VS6 octahedra into VS5 tetragonal pyramids as the origin of the structure evolution. The new findings will yield a better understanding of the complex electronic phase diagram of BaVS3. [ABSTRACT FROM AUTHOR]