Band 121, Heft 2-4

Crystals of arsenic triiodide, AsI 3 , are rhombohedral, a Π = 7.208 Å, c H = 21.436 Å, Z = 6; a R = 8.269 Å, α = 51°41′, Z = 2; space group R 3. The structure was determined from Fourier and difference projections along the crystallographic axes, the final discrepancy factors being R ( h 0· l ) = 0.11, R ( hk ·0) = 0.16. The iodine atoms are in an approximately hexagonal closepacked array, but the compound does not have the bismuth iodide structure, as reported previously. The arsenic atoms are significantly displaced from the centres of iodine octahedra, and have only three near-neighbour iodine atoms, so that the structure may be considered as built up from discrete AsI 3 molecules. The dimensions of the molecules are As–I = 2.556 ± 0.004 Å, I–As–I = = 102.0° ± 0.1°, not significantly different from the values in the vapour state. The intermolecular As … I distances are 3.50 Å, indicative of some intermolecular attractive forces.

Turquois is triclinic, space group P [unk], with cell dimensions a = 7.424 Å, b = 7.629 Å, c = 9.910 Å, α = 68.61°, β = 79.71°, γ = 65.08°. The cell contains one formula of CuAl 6 (PO 4 ) 4 (OH) 8 · 4H 2 O, so the Cu atom is fixed in an inversion center. Three-dimensional intensity data were collected on a singlecrystal diffractometer using a proportional counter as detector, and were corrected for Lorentz-polarization factors and absorption. The interpretation of a three-dimensional Patterson function and of a three-dimensional electrondensity function based on signs due to the Cu contribution only, gave a trial

Crystals of Na 2 S · 9H 2 O and Na 2 Se · 9H 2 O have the lattice parameters a = 9.33 1 , c = 12.85 0 Å, and a = 9.49 6 , c = 13.05 7 Å, space group P 4 3 22– D 4 7 ( P 4 1 22– D 4 3 ), Z = 4. The structures have been determined by two-dimensional Fourier and ( F o – F c ) syntheses along [001] and [100]. Each Na ion is surrounded by six water molecules. These Na–H 2 O octahedra form two different types of chains [Na(H 2 O) 5 ] and [Na(H 2 O) 4 ], both along [001]. The chains are linked to each other by hydrogen bonds, O–H…O and O–H…S and O–H…Se, respectively.

Crystals of Na 2 S · 5H 2 O, Na 2 Se · 5H 2 O and Na 2 Te · 5H 2 O have the lattice parameters a = 6.47 5 Å, a = 6.56 9 Å, a = 6.73 5 Å, b = 12.54 5 Å, b = 12.87 3 Å, b = 13.46 9 Å, c = 8.65 5 Å, c = 8.91 2 Å, c = 9.21 9 Å, space group Cmcm – D 2 h 17 , Z = 4. The structure has been determined by two-dimensional Fourier and ( F o – F c ) syntheses along [001], [010] and [100]. One Na I ; ion is surrounded octahedrally by six water molecules and one Na II ion is surrounded by four water molecules and one S ion forming a quadratic pyramid. The Na I –H 2 O octahedra form chains which are connected to layers by the pyramidal coordinated Na II ions. These layers are linked to each other by hydrogen bonds, O–H … S and O–H … Se and O–H … Te, respectively.

The co-operation of the Si and O atoms in quartz during the α - β transition was determined by means of diffuse x-ray scattering. Above 400°C a strong interaction between the Si–O–Si–O chains occurs. They move in the <100> chain direction, co-operating statistically in {100} planes. A distinction between static disorder and lattice vibrations could not be made by means of the diffuse x-ray patterns alone. For the displacements of the atoms are small compared with their interatomic distances.

The oxygen parameters in one representative of the scheelite-type structure: lead molybdenum oxide, wulfenite, were determined by neutron diffraction using a single crystal. The following coordinates of the oxygen atom were found: x o = 0.2352, y o = 0.1134, z o = 0.0439. The refinement was carried out by a least-squares procedure on an electronic computer. The structure consists of MoO 4 tetrahedra. The Mo–O distance within this tetrahedron is 1.772 Å indicating a covalent link. Each lead atom is surrounded by 8 oxygen atoms belonging to 8 different MoO 4 units. The Pb–O distances are: 2.610 Å, four times, and 2.630 Å, four times.

A carbosulfide of hafnium of the formula Hf 2 FeC 2 S has been prepared. Its structure has been determined by means of single-crystal and powder photographs. The space group is D 6 h 4 – P 6 3 / mmc containing the three metal atoms in 4( f ) with z = 0.099, one S atom in 2( d ) and two C atoms in 2( a ). The lattice constants of this remarkably stable phase are a = 3.35 8 kX, c = 11.97 kX, c / a = 3.56 5 .

Terramycin hydrochloride is orthorhombic, space group is P 2 1 2 1 2 1 , with cell constants a = 11.19 Å, b = 12.49 Å, c = 15.68 Å, with four formulae per unit cell. The crystal structure proposed by Takeuchi and Buerger in 1960 proved to be essentially correct and has been refined by Fourier and least-squares methods. The original R factor of 36% was reduced to 17.6% for 2100 reflections, and 14% if 350 non-observed reflections are excluded. Even though the structure of terramycin hydrochloride bears a close general resemblance to the structure of aureomycin hydrochloride reported by H irokawa et al . and by D onohue et al ., there are appreciable differences between them. Some chemical predictions about the terramycin structure made by D onohue on the bases of this recent investigation on aureomycin do not agree with the results reported here.

The crystal structure of 2,2-dicloro-3-phenylcyclobutenone, [unk] has been determined directly by the application of the symbolic-addition phase-determination procedure. The material crystallizes in the monoclinic system in space group P 2 1 / c . The unit-cell dimensions are: a = 9.47, b = 9.68, c = 10.94, β = 103°04′ and Z = 4. The almost planar molecules, except for the two Cl atoms, lie nearly parallel to the (101) planes. Several of the bond distances deviate considerably from single- and double-bond values, thus suggesting the existence of a considerable degree of conjugation between the benzene ring and the cyclobutenone group.

A least-squares refinement of the structure of celestite has been carried out using three-dimensional counter-diffractometer data. The results agree, in general, with those for the isotypic minerals barite (BaSO 4 ) and anglesite (PbSO 4 ).

The crystal structure of the synthetic zeolite ZK-5, Na 30 Al 30 Si 66 O 192 · 98 H 2 O, was determined from powder data both at room temperature and at 150°C. The zeolite is cubic with space group Im 3 m and a = 18.7 Å. The aluminosilicate framework is composed of large cages representing truncated cubooctahedra which are linked to each other through the six-membered rings. The main channels are parallel to the cubic axes and are formed by near-planar eight-membered rings of about 3.8 Å free diameter. The structure is related to Linde Molecular Sieve A. The positional parameters of the framework atoms and the cations change significantly on heating.

Vaterite ( μ CaCO 3 ) is obtained when CaCO 3 is precipitated under favourable conditions of nucleation (high supersaturation and additional components in solution) and the transformation into calcite or aragonite proceeds sufficiently slowly (mainly because of relatively slight lattice perturbations). Several methods of synthesis are given. A large variability in the “somatoid” morphology of vaterite as a function of precipitation conditions is demonstrated. A critical discussion follows of the “somatoid” concept, based upon the work of the K ohlschütter school.

Samples of Californian taramellite permitted the determination of the crystal structure of this mineral to be brought to completion. The unit-cell parameters of Californian taramellite are: a = 13.95 Å, b = 12.21 Å, c = 7.15 Å (± 0.1%); space group Pmmn . A chemical analysis is reported. A first stage of the work was carried out on taramellite from Candoglia (Val di Toce, Italy), which was the object of preceding studies. Oscillation photographs along [010] were taken with Mo Kα radiation: four Patterson sections, calculated with the data so obtained, permitted the identification of the positions of the heavy atoms. [001] precession photographs (levels 0 to 3) as well as zero-level [010] precession photographs were successively taken on samples of Californian taramellite with Mo Kα radiation. With these experimental data, the complete determination of the crystal structure, impossible with the taramellite from Candoglia because of its peculiar habit, was carried out. The refinement with several cycles of full-matrix least squares on 275 hkl reflections, gave a final R -factor of 0.119, on 515 observed reflections. The structure contains rings of four tetrahedra SiO 4 , partially distorted and nearly parallel to (010); octahedral complexes (Fe,Ti)O 6 are also present and as pairs of them share an edge, chains of octahedra are formed along [010]. Barium is surrounded by at least six oxygens lying at distances ranging from 2.73 to 3.00 Å. Three different irregular coordination polyhedra are formed around the three non-equivalent Ba atoms. From a crystal-chemical point of view, the formula of taramellite can be written either: Ba 2 (Fe +++ ,Fe ++ ,Ti,Mg) 2 [O(Si 4 O 12 )] · H 2 O or Ba 2 (Fe +++ ,Fe ++ ,Ti,Mg) 2 O 2 [Si 4 O 10 (OH) 2 ]. Four formula units are contained in the unit cell.

For some compounds it is possible to form both a urea and a thiourea adduct (inclusion compound). When the length of channel occupied by a molecule of the compounds is measured with the use of continuous layer lines in the x-ray diffraction pattern of a single crystal of the adduct, the length measured in thiourea is appreciably shorter than that measured in urea. The channel length occupied by a molecule depends mainly upon 1. the number of atoms in the chain, 2. the degree of coiling, 3. the amount of overlapping of the ends and, 4. whether the guest is “locked into” the structural features of the host. Data are given for the channel lengths of a number of compounds, some measured in the urea system, some in the thiourea system and some in both systems. From these data it is possible to compute the amount of coiling, the amount of overlapping of ends and the channel lengths of parts of molecules.

X-ray data are presented which give information on the lengths of various squalene molecules (natural, synthetic and regenerated from squalene hexahydro-chloride) and related molecules when these are adducted with thiourea as in elusion compounds. These data further support our earlier report that natural squalene must have the all- trans configuration. The x-ray measurements also prove that when thiourea adducts are formed from mixtures of squalene isomers containing some eis double bonds as well as the all- trans isomer, the latter is preferentially adducted. However, some eis isomers can be incorporated into the adduct. Since the preference for the all- trans isomer is high, thiourea-adduct formation can be used as an effective method for removing the all- trans isomer from mixtures of stereoisomers. No cis isomers of squalene appear to be present in the natural product. The symmetry of the squalene-thiourea adduct deviates from the rhombohedral ([unk] m – D 3 d ) which is usually found for other thiourea adducts. It is monoclinic (2/ m – C 2 h ) but pseudohexagonal in such a way that the crystal structure must be very similar to the usual structure of thiourea adducts.

Optical and interferometric studies have been made of surface structures of natural Alpine quartz and some from other sources. Eccentric growth spiral on a (01[unk]1) face is illustrated and interpreted as due to unequal rate of advance of growth sheets in different directions. Triangular and almost circular growth spirals observed on such other faces are also illustrated. Closed loops of growth fronts are also reported and interpreted. Evidence is given for movement of a screw dislocation during the process of growth.

The molecular structure of asym - αβ -naphthazine, already determined by x-ray diffraction methods, has been refined by a least-squares treatment. The average standard deviations of the bond lengths and bond angles are 0.018 Å and 1.1° respectively. The molecule may be considered planar.

Spheres of NH 4 H 2 PO 4 placed from a supersaturated solution developed faces of the forms {100}, {011}, {112}, {1[unk]2} and {110}. On spheres of KH 2 PO 4 treated in a similar manner faces of {010}, {011}, {112} and {1[unk]1} were observed. These observations are more or less in accordance with the predictions of the existing morphological theories. ‘Bands’ observed on spheres of A.D.P. reduce the generally accepted pointgroup of A.D.P. from [unk]2 m to 222.

Oxygen parameters in xenotime have been refined in order to compare bond lenghts in two isostructural minerals, zircon ard xenotime. The results have shown that the different behaviour toward metamictization of these minerals could not be explained on the basis of cation–anion distances alone.

Greek alkali feldspar cryptoperthite with monoclinic Na feldspar (monalbite) exsolved has been described.