Occurrence of Olivine in a Type I Carbonaceous Meteorite

Abstract

A REPORT of the discovery of olivine in the Orgueil carbonaceous meteorite¹ aroused considerable scepticism^2,3. The identification was based on selected area electron diffraction patterns obtained from two mineral fragments observed during an electron microscope examination of a powdered meteorite specimen. Because, with the electron microscope in use at that time, it was only possible to examine particles in the orientation which they had arbitrarily taken up on the specimen support film, each diffraction pattern gave only limited information about the unit cell parameters and space group of any individual unknown mineral. The possibility of using such restricted data to identify mineral species rests on the fact that many minerals develop pronounced and specific faces, either by growth or cleavage, so that different fragments of the same mineral tend to lie in a common orientation, and thus to exhibit the same type of single crystal diffraction pattern in the electron microscope. The match between patterns from terrestrial olivine samples and those from olivine in Orgueil was good, but it was clearly desirable to put the identification on a firmer basis, if possible, by obtaining for a single mineral particle the full complement of unit cell parameters and the space group.