High-Resolution Electron Microscopy of Hematite Formed by External Oxidation of Iron-Bearing Olivine
- PDF / 2,374,253 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 39 Downloads / 199 Views
HIGH-RESOLUTION ELECTRON MICROSCOPY OF HEMATITE FORMED BY EXTERNAL OXIDATION OF IRON-BEARING OLIVINE. Stuart McKernan, D. Reni Rasmussen, and C. Barry Carter, Department of Materials Science and Engineering, Bard Hall, Cornell University, Ithaca, NY 14853
ABSTRACT The preparation of interfaces for TEM investigation is inherently prone to the production of artifacts. In some cases it is possible to avoid potential hazards in the interpretation of highresolution images by creating the interfaces in pre-existing, thin, TEM specimens so that no further preparation is required. As an example of this approach, results from a study of interfaces produced by the heat-treatment of iron-bearing olivine TEM samples are presented. Image simulations are presented which show that even in these favorable situations, the greatest care must be taken in interpreting image contrast at interfaces in high-resolution electron micrographs. INTRODUCTION One of the problems in transmission electron microscopy (TEM), and particularly in the application of high-resolution electron microscopy (HREM), is that of recognizing the extent to which damage and other artifacts have been produced in the specimen by the preparation process. Grain boundaries and heterojunctions in thin TEM foils are prone to preferential attack, whether during chemical polishing or ion-thinning, forming grooves or steps at the interface, and thus making the matching of experimental and simulated images difficult. In an effort to provide a better understanding of these effects the problem has been approached in two ways. Firstly, by creating boundaries without the use of the more damaging preparation techniques it may be possible to preserve the structure at the boundary and, secondly, by performing image-simulation studies it should become more feasible to interpret the information contained in the contrast of grain boundary images. The annealing of thin TEM samples of iron-bearing olivine in air, at temperatures below -1000°C, produces small hematite islands on the surface [1]. A thin film of silica is also produced in this reaction and the remaining olivine is left depleted in iron. After suitable annealing, the thin hematite particles may grow large enough to impinge and form grain boundaries in electron transparent material. This procedure eliminates the possible production of artifacts in the specimens due to the preparation of TEM foils from the bulk material. Of particular interest are twin boundaries in the hematite which occur because these hematite islands are formed epitactically on the olivine in two different, twin-related orientations. High-resolution lattice images were obtained from these annealed olivine specimens using a JEOL 4000EX electron microscope. The images thus obtained were simulated using the TEMPAS program [2]. Image processing was performed on the experimental images using the SEMPER image-processing suite of programs [3]. RESULTS The orientation relationship between the olivine and the hematite is such that the olivine (200) planes are pa
Data Loading...
