MAGNETIC ORIENTATION OF AMPHIBOLE FIBRES
Amphibole fibres have been exposed to a unitbrm magnetic field while suspended in a dilute solution of celloidin in amyl acetate. The solvent was allowed to evaporate while the field was maintained so as to obtain a coherent film of magnetically oriented fibres bonded together by celloidin. X-ray-diffraction patterns were then recorded in transmission geometry, and the orientation of the fibrils around their fibre axis was determined. The fibres are oriented in azimuth as well as lengthwise, though those that align normal to the field show a much more restricted range of azimuthal orientations than those that align parallel to the field. Crystallographic control of the principal maguetic axes is discussed; the orientation properties are not fully explained, but their attribution to magretite present in bulk samples is rejected.
Structural imperfection and morphology of crocidolite (blue asbestos)
ELECTRON microscopic studies of fibrous materials, including a range of chain silicates1–6, synthetic polymers7 and carbon fibres8, have almost exclusively involved examination of the structural characteristics visible when the fibre axis is perpendicular to the electron beam. It is extremely difficult to prepare ultra-thin sections of fibrous samples such that this axis is parallel to the beam; and yet it is in this projection that the greatest insights into structural and morphological irregularities are likely to be achieved, a fact well illustrated by the high-resolution studies of Yada9 on chrysotile asbestos which, being less brittle than crocidolite and other amphiboles, was amenable to the normal techniques of ultra-microtomy. We present here preliminary results of a successful axial study of crocidolite (blue asbestos), the fibrous form of the amphibole mineral riebeckite (idealised formula Na2Fe5(Si4O11)2(OH)2, space group C2/m, a = 0.973, b = 1.806, c = 0.533 nm, β = 103.3°). The electron micrographs reveal several novel features.
n-Beam lattice images. I. Experimental and computed images from W4Nb26O77
A series of through-focus lattice images has been obtained with an electron microscope for single crystals of W4Nb26O77 set at an orientation to give mainly a systematic set of 00l reflexions. Dynamical n-beam computations (n = 435) have been made for a crystal in this orientation to determine the values of the amplitudes and phases of the diffracted beams. Subsequently, the fringe profiles for lattice images have been computed for various thicknesses of crystal and defects of focus. When the experimental factors, including instrument aberrations, are taken into account, good agreement between computed and observed fringes is obtained. An earlier assumption of direct correlation of features in underfocused images with structure in the crystal, is found to be valid, at least for this compound.