Volume 137, Issue 2-3

The mineral wenkite Ba 4 (Ca 0.9 □ 0.1 ) 6 (Al 0.4, Si 0.6 ) 20 O 39 (OH) 2 (SO 4 ) 3 · n H 2 O 2 was found by P apageorgakis (1962) in metamorphic marbles of the Ivrea zone (Alps). The structure determination indicated that the basic symmetry is P 31 m , however, by perfect submicroscopical twinning on (0001) or positional disorder, physical properties and diffraction effects show P [unk]2 m symmetry ( a = 13.515 Å, c = 7.465 Å) This suggests that wenkite might possess a domain structure. Wenkite has an interrupted Al/Si framework with the same hexagonal building units which are found in cancrinite and gmelinite. They are linked through an additional tetrahedron which is only bonded with three corners to the framework. In between these hexagonal cavities, which contain Ba(1), larger highly distorted cavities are formed which contain Ca and Ba(2)SO 4 . A chain of BaSO 4 molecules extends parallel to the z axis in the center of these cavities which are defined by eight-membered TO 4 rings. The framework contains systems of channels parallel to the z axis with 6- and 8-membered rings and a system of kinked channels parallel to the x axis with 10-membered rings.

Die Wärmeausdehnung von Diopsid wurde im Bereich bis 825°C mit einer Guinier-Lenné-Kammer gemessen. Das Ergebnis: α 1 = 7,7, α 2 = 17,3, α 3 = 7,0 · 10 −6 °C −1 stimmt mit den von K ôzu berichteten Werten gut überein. Das feste Strukturgerüst aus M(1)- und M(2)-Polyedern ließ nicht einen größeren Beitrag zur Ausdehnung von Schwingungen parallel zu den Ketten erwarten. Als Grund für die Höhe des α 2- Werts sind daher senkrecht zu den Ketten schwingende Transversal- und Longitudinalwellen anzunehmen.

The x-ray structure analysis of C 15 H 19 N 2 P revealed a monoclinic unit cell, space groupe P 2 1 / n , with lattice constants a = 9.237, b = 12.507, c = 13.119 Å, β = 92.36°. In the initial stages of the structure analysis Patterson and difference-Fourier syntheses were used as well as Fourier syntheses phased from Σ 2 relations. Anisotropic temperature refinement of the P, C, N atoms and isotropic of the H atoms yielded a final reliability index of R = 5.0%. The bond lengths of C 15 H 19 N 2 P are compared with those of related molecules and discussed. The partial double-bond length P–C is 1.743 Å.

High-temperature Ba 2 [Si 4 O 10 ] is monoclinic with a = 23.202(5) Å, b = 4.661(1) Å, c = 13.613(4) Å, β = 97.54(2)°, C 2/ c , Z = 6. Using 635 independent intensities of h 0 l , h 1 l and h 2 l reflections its structure was determined and refined to a residual R of 7.3%. High-temperature Ba 2 [Si 4 O 10 ] contains [Si 4 O 10 ] Zweierschichten similar to those in sanbornite, low-temperature Ba 2 [Si 4 O 10 ], but with different topology. The two crystallographically non-equivalent barium ions in high-temperature Ba 2 [Si 4 O 10 ] have 8 + 2 and 9+1 near oxygen neighbours. The slightly lower degree of convolution of the [Si 4 O 10 ] layers in comparison with those of sanbornite is explained by the increased ionic size of barium due to larger thermal motions.

Three-dimensional x-ray diffraction data have been taken from ovalene, C 32 H 14 . Both unconstrained and constrained refinements have been done. From the latter we find the assumption that the molecules move as rigid units to be very good when compared with the results of other molecular crystals. The surprising feature has been the poorness of a model which assumes the molecules retain their free state symmetry. A simple model for molecular distortion was then unsuccessfully tried, leaving open the question of the significance of the molecular distortion in the crystal.

A single-crystal neutron-diffraction study has been made of dipotassium tetrahydrogen trifumarate 2 (K + COO − · C 2 H 2 · COOH) · (C 4 H 4 O 4 ) using zonal and partial three-dimensional neutron-diffraction data. The structure has been refined to an R value of 0.136. The location of the hydrogen atoms has conclusively shown the correct composition to be 2KC 4 H 3 O 4 + C 4 H 4 O 4 rather than K 2 C 4 H 2 O 4 + 2C 4 H 4 O 4 . The crystal structure is basically the one determined earlier by x-ray analysis but the hydrogen bonds between carboxyl groups (2.54 Å and 2.56 Å) are bent, the angle O–H…O being 167° and 168° respectively.

The crystal structure of a new rhombohedral polytype, 36 R 2 , of cadmium iodide has been determined. The structure is found to be (22212111) 3 in Zhdanov notation with space group R 3 m . Its formation is discussed in terms of a mixed dislocation consisting of a screw dislocation along the c axis and a partial edge dislocation in the basal plane.

Cadmium iodide polytypes undergo phase transformation when annealed under vacuum (10 −4 Torr) at temperatures ranging between 250 and 280°C. A curious type of transformation in which identity period of the polytypes remains the same but stacking sequence of atomic layers changes, was found when the crystals were annealed at relatively lower temperatures (≈240°C). Such transformations have been discussed in detail and analysed in the light of the experimental evidences obtained. A possible mechanism for this type of transformation has been suggested in terms of stacking faults.

Rösslerite MgHAsO 4 · 7H 2 O crystallizes in the monoclinic system with cell dimensions a = 6,687 ± 0,003, b = 25,73 ± 0.01, c = 11,531 ± 0,005 Å, β = 95° 7′ ± 5′. The space group is C 2/ c and there are eight molecules per unit cell. The structure has been solved from a Patterson synthesis and partially phased Fourier syntheses and least-squares refinement has been completed on three-dimensional data (2065 structure factors within the CuKα sphere). The final residual is 11.2%. The structure consists of AsO 2 (OH ½ ) 2 tetrahedra, Mg · 6H 2 O octahedra in two independent positions and isolated water molecules held together by hydrogen bonds. The position of the hydrogen bonds have been inferred from stereochemical and crystallographic evidence. The AsO 2 (OH ½ ) 2 tetrahedron is distorted, with a mean (uncorrected) arsenic–oxygen bond length of 1,691 ± 0,003 Å and the Mg · 6H 2 O octahedra are also distorted, with a mean (uncorrected) magnesium–water oxygen of 2,077 ± 0,003 Å. However in this case if the water oxygen is a receptor atom for a hydrogen bond the magnesium–water oxygen bond is lengthened. The thermal vibrations have been analyzed and the mean corrected bond lengths are arsenic–oxygen 1,695, magnesium–water oxygen 2,086 (assuming riding motion in each case).

For identification and structure determination of CdI 2 polytypes, a tilted a *-axis oscillating-crystal photograph has been found more informative in comparison with an a -axis oscillation or Weissenberg photograph.

The crystallographic data of Mg 2 V 2 O 7 , Ca 2 V 2 O 7 and Cu 2 V 2 O 7 have been determined from x-ray powder diagrams. The results are briefly discussed, in comparison with related substances.