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.
Morphology and alteration of asbestiform grunerite and anthophyllite
Abstract Sections perpendicular to [001] of ion-thinned specimens of amosite (fibrous grunerite) from Penge, Transvaal and anthophyllite from Paakila, Finland and Söndeled, Norway, have been examined by high-resolution transmission electron microscopy. Observations on the nature of grain boundaries and alteration are compared with those of other workers on other fibrous amphiboles. Fibrous crystals grow with their fibre (c) axes approximately parallel to one another but they have considerable rotational disorder about that axis. Grain boundaries are generally irregular in shape and only follow low-index planes for short sections. In all specimens the amphibole has undergone some alteration to sheet silicates along grain boundaries, fractures, cleavages, and multiple-chain lamellae. Anthophyllite alteration products are talc, serpentine and chlorite, and amosite alteration products approximate to iron analogues of talc and serpentine. Talc layers are generally planar and their orientation is strongly controlled by the amphibole structure, whereas serpentine and chlorite layers often curve and their orientations are less frequently related to that of the amphibole. Comparison of specimens which appear finely fibrous in hand specimen with those which appear coarser, acicular or massive suggests that the nature of fibres produced by crushing is mainly controlled by the grain boundaries in the former type, but other factors such as fractures, cleavages, and defects are more important for the latter types.
The fragmentation of small asteroids in the atmosphere
The fragmentation of a small asteroid in the atmosphere greatly increases its cross-sections for aerodynamic braking and energy dissipation. The differential pressure across a meteoroid disperses its fragments at a velocity that increases with atmospheric density and impact velocity and decreases with meteoroid density. At a typical impact velocity of 22 km/s, the atmosphere absorbs more than half the kinetic energy of stony meteoroids with diameters, D M < 220 M AND IRON METEOROIDS WITH D M < 80 m. The corresponding diameter for comets with impact velocity 50 km/s is D M < 1600 m. Most of this energy dissipation occurs in a fraction of a scale height, which causes large meteoroids to appear to »explode« or »flare« at the end of their visible paths