Some physical properties of chondritic meteorites.

Meteorites appear to be the most primitive objects in the solar system and, therefore, contain valuable information on the early history of solar system material. The most abundant meteorites, the chondrites, are characterised by the presence of small spherical silicate inclusions, chondrules. Although the chemical, mineralogical and petrological properties of chondrules have been investigated their purely physical properties have received little attention. The present thesis therefore examined the size, shape and orientation of long axes of chondrules and tested the fit of chondrule mass distribution to known statistical functions in order to obtain constraints on the origin of chondrules and the accumulation and early history of meteorites. The size range of chondrule diameters was found to be rather limited from a pronounced lower limit cut-off around 0.2mm up to a few millimeters. A preponderance of particles of smaller diameter (~0.3 - 0.7mm) was observed. Although there appeared to be no marked difference in chondrule size distributions in the ordinary chondrites, those in the carbonaceous chondrites are more restricted with a greater preponderance of smaller particles. Departures from sphericity of chondrules were found to be small and random. Chondrule mass-frequency distributions could be fitted to the Weibull statistical function (except for Nikolskoe) but were found not to obey Rosin's law suggesting, by analogy, that chondrules were not produced by crushing phenomena. Most chondrites were found to exhibit some degree of preferential orientation of chondrule long axes which was less marked in relatively friable meteorites and appeared unrelated to degree of metamorphism. It was concluded that the observations suggest that chondrules are most probably secondary objects formed by a high-energy event acting on material with a restricted mass range. Accumulation of meteoritic material was a gentle process with the petrofabric most likely being imposed during this period.