Volume 119, Issue 3-4

In order to study the nature and extent of structural disorders in kaolinite, a sample from Singhbhum, India, has been subjected to x-ray diffraction and electron-microscope examination. Electron-microscope studies revealed the distribution of particle sizes. X-ray diffraction line profiles of different reflections were corrected for geometrical factors by the Stokes deconvolution method. Line broadening and Fourier analysis methods revealed that the particles were flaky with less extension in the c direction than in the a and b directions. The average particle has been shown to have a length and breadth of 700 Å and a thickness of 219 Å. Expressions for apparent particle size and particle-size line profile for triclinic crystals with parallelepiped form have been worked out. With the help of these expressions, the pure diffraction line profiles were corrected for particle-size effects and pure-defect line profiles were obtained. Fourier analysis of these line profiles showed that the probability of a layer shift by an amount ⅓ b or ⅔ b occuring was .046. From the integral width and peak shift in the pure-defect line profiles of 00 l reflections, the fractional extent and probability of interlayer variation has been found to be .126 and .057 respectively. A possible interpretation of interlayer-distance change in metakaolin has been put forward on the basis of these data.

The dehydration of goethite and diaspore is studied in detail by single-crystal and powder x-ray methods, and also by the use of the electron microscope. By variation of the temperature of dehydration and duration of heating it has been possible to observe intermediate states in the process. These intermediate states are revealed by satellite reflections which at certain temperatures occur close to the developed hematite or corundum diffraction spots. At higher temperatures these satellites are replaced by diffuse intensity regions, which disappear on further heating. The satellites could correspond to a sinusoidal structure-amplitude modulation along the c axis of the hematite or corundum crystallites, before dehydration is complete. The wavelength of the modulation was 32 Å for the hematite formed from goethite, and 39 Å for the corundum formed from diaspore. Non-uniform broadening of the lines on powder photographs of the developed hematite, or corundum, was observed only when the original goethite or diaspore showed a particular texture. This effect is ascribed to stacking faults. A well developed system of pores was observed on heated diaspore. The activation energies involved in the dehydration have been determined (19.8 kcal/mole for goethite, and 24.1 kcal/mole for diaspore). The inhomogeneous mechanism is confirmed for this type of dehydration, and a structural explanation for the formation of the satellites is presented.

A structure is called a normal tetrahedral structure if every atom of the structure has four nearest neighbors which are located at the vertices of a surrounding tetrahedron. If not all four corners of the surrounding tetrahedron are occupied with an atom, the structure is said to be a defect tetrahedral structure. Normal-tetrahedral structures are formed only if the average valenceelectron concentration per atom is four. Defect tetrahedral structures can be made to fit the valence-electron concentration rule if the number of structure defects, or the number of unused, tetrahedral orbitals, is considered. A scheme has been developed which allows one to calculate the compositions of all theoretically possible normal and defect tetrahedral structures. Some 200 tetrahedral-structure compounds are known which all fit in the composition scheme. The system can be used as well to find the compositions of new tetrahedral-structure compounds or to test reported compositions of tetrahedralstructure minerals for their correctness. It was found that it is possible to present the composition scheme not only analytically but also graphically in a master composition diagram. A plot of a compound in a master composition diagram, regardless of the element combination, will show on first sight if the composition is right for a normal tetrahedral structure or a defect tetrahedral structure. In the latter case one can read from the diagram the number of structure defects. An average valence-electron concentration of four is a necessary but not sufficient condition for the formation of a tetrahedral structure. Other factors have to be considered, like the electronegativity difference Δx and the average principal quantum number of the valence shell of the participating atoms [unk]. The influence of those factors can be conveniently studied in a so-called “ Δx -versus-[unk]“ diagram, first introduced for normal valence compounds by M ooser and P earson . It is found that of all those compositions which have the correct average valence-electron concentration, only those which lie in a well defined region of the Δx -versus-[unk] diagram will crystallize with a tetrahedral structure.

The structure of a synthetic imidazo-purine has been determined by an x-ray analysis making use of the phase-determining properties of a heavy atom in the molecule. The molecule has been shown to be 9- β -chloroethyl-7,8-dihydro-9H-imidazo-[2,1-i]purine. The x-ray analysis was undertaken to resolve some ambiguities in the determination of the structure by traditional chemical means.

The crystal structure of CdSO 4 and HgSO 4 has been determined with x-ray single-crystal data. The unit cell is orthorhombic with a = 4.704, b = 4.722, c = 6.565 Å for CdSO 4 and a = 4.785, b = 4.821, c = 6.581 Å for HgSO 4 . Z = 2, space group Pn 2 1 m . The SO 4 tetrahedron in both isotypic substances is distorted. The Cd and Hg atoms are surrounded by four O atoms in the form of a very distorted tetrahedron. The SO 4 tetrahedra share a corner with the MO 4 tetrahedra and form a three-dimensional network.

Dimethyl sulfone is orthorhombic with a = 7.36 Å, b = 8.04 Å, c = 7.34 Å. The space group is Cmcm and there are four molecules per unit cell. The molecules are distorted tetrahedra with C 2 v symmetry, a S–O bond length of 1.44 Å, a S–C bond length of 1.78 Å, a C–S–C angle of 103.0°, and an O–S–O angle of 117.9°.

The hydrogen-bonding scheme proposed by M anohar and R amaseshan for echitamine iodide, which included postulations of C – H … O and O – H … C interactions, has been re-examined. It is found that the crystal structure can be discussed in terms of N – H … I − and O – H … I − interactions, in conformity with the well-established principles governing hydrogen-bond formation, and that there is no necessity to invoke C – H … O or O – H … C hydrogen bonds. It is also pointed out that the general conformation of the echitamine ion in its iodide is remarkably similar to that found in echitamine-bromide methanol solvate, even though the hydrogen bonding schemes in the two crystals are quite different.

The design and mode of operation of a simple heating attachment for the precession camera is described, which allows diffraction patterns to be recorded at temperatures up to 1200°C without depriving the camera of its most important characteristics.

Single crystal photographs at temperatures between 1000°C and 2000°C using a Buerger precession camera can be obtained by heating the crystal with a small gas flame. The temperature of exposure may be controlled by gas pressure and gas flow (pressure regulator and needle valve). Advantages and disadvantages of the device are discussed. X-ray exposures at high temperatures of anorthite (CaAl 2 Si 2 O 8 ) demonstrate the application of the apparatus. The lattice angle α * and the intensity ratios of some reflections show a continuous change at temperatures between 300°C and 1400°C.

A first published proposal for the crystal structure of phenol has been corrected and refined with three-dimensional data. The structure has been found to have monoclinic symmetry of space group P 2 1 ; which differs only very little from orthorhombic symmetry of space group P 2 1 22 1 . For the determination of the monoclinic features a new method of refinement has been used, which allows to make use of the known configuration of the phenol molecules.

Crystals of bis -(hydrazine)manganese(II) chloride, [Mn(N 2 H 4 ) 2 ]Cl 2 are mono-clinic, space group C 2/ m , isotypic with the corresponding compounds of zinc and of cadmium. They are twinned according to either the (100), (001) or (110) law. The structure has been determined by using hk 0 and 0 kl reflections, taken with crystals twinned according to the (100) and (001) laws respectively. Three-dimensional intensity data could not have been used because the crystals are twinned and the intensities of upper layers are unreliable. Refinement has been carried out by the least-squares method. The structure is formed by chains of pseudo-octahedral complexes [Mn(N 2 H 4 ) 4/2 ]Cl 2 held together by bridges of hydrazine. Main distances within each complex are: Mn–N = 2.266 ± 0.021 Å and Mn–Cl = 2.570 ± 0.006 Å. The hydrazine molecule is in the “staggered” form. The distance between nitrogen atoms in hydrazine is N–N = 1.459 ± 0.031 Å. Chains of complexes are not bound to one another by hydrogen bonds and the formation of twins is therefore easy.

The crystal structure of six biotite specimens has been studied by means of one-dimensional Fourier projections on c *. The mean tetrahedral rotation is about 10° for biotites. The K + ion shows an almost regular octahedral coordination with an average K–O bond distance of 2.92 Å. Layer separation around K + has a mean value of 3.37 Å. The Si–O tetrahedra are rather regular. The structural model for the biotites is very close to the recently described structures of muscovite and phlogopite.

The trigonal crystals of trioxane belong to the non-centrosymmetric space group R 3 c = C 3 v 6 . Atomic and thermal parameters have been refined by three-dimensional F o and ΔF synthesis. The results of the analysis are discussed with particular regard to the molecular packing and its influence on the polymerization mechanism by which twinned crystals of polyoxymethylene are formed-

Manganite crystals heated to 300°C developed countless submicroscopic pyrolusite crystals on their surfaces which grew with their (010) planes parallel to those of the manganite. A contraction occurs in the Y -direction of the manganite crystals.