TEM
analysis of stress induced defects in baddeleyite xenocrysts from the Phalaborwa complex
Authors: Lee Mike , O'Connell Jacques , Janse van Vuuren Arno 
Published: December 2016

Large deposits of baddeleyite (natural zirconia) were found in the Phalaborwa complex and mainly in the foskorite ore zone and to a lesser extent in the carbonatite ore body [1]. Baddeleyite has a monoclinic zirconia (ZrO
2
) structure at room temperature and undergoes a martensitic transformation to a tetragonal structure at approximately 1100
0
C depending on the ambient pressure. Transformation of the zirconia, on cooling, results in a 5% volume change in the unit cell as well as 8‐14% increase in strain energy [2]. This volume change and strain energy during the transformation could result in crystal twinning and/or cracking during catastrophic changes in temperature and pressure especially during the emplacement process. Lumpkin [3] has commented on the lack of a detailed microstructural analysis for natural zirconia which would possibly provide information on the nature of primary and secondary physical alteration mechanisms, and hence the geothermal history of the material. Recent reports on the microanalysis for Phalaborwa zirconia produced data on the origin of secondary inclusions [4] as a result of physical alteration during the emplacement process. Considering the thermal history of the material it is expected that there should be evidence for stress induced damage. In this paper, the nature of stress induced defects in baddeleyite xenocrysts will be presented and discussed.

Sections, approximately 2 mm in thickness, were cut from a Phalaborwa baddeleyite xenocryst using a diamond wire saw and polished to a 0.25 μm finish. TEM sections were prepared in an FEI Helios NanoLab as well as a Gatan precision ion polishing system. The TEM sections were imaged and analysed using JEOL ARM 200F and JEM 2100 TEMs operated at 200 kV.

Both {001} and {011} twin domains were identified using SAD patterns. TEM bright field (BF) images for the {011} twin domains are shown in figure 1. Simulation of the diffraction pattern for the central twin domain shows a 3% elongation in the [001] direction indicating the possibility of strained material in this domain. The twin domains analysed in this work for the Phalaborwa baddeleyite are orders of magnitude larger and do not “pinch out” as described by Kerschofer et al [5]. Figure 2 presents evidence for small loop‐like defects formed along what is assumed to be an individual ion track from a fission product recoil (indicated by the dashed lines). The entire field of view is covered by randomly distributed small defect structures which have not yet been identified. Figure 3 shows a few large extended defects which are believed to be oxygen interstitial complexes. These defects are similar in nature to those observed in synthetic stabilized zirconia which has been irradiated with high fluences of swift heavy ions followed by thermal annealing [6]. These high fluences are improbable for natural material and hence the damage is believed to be stress induced followed by thermal annealing. These results have thus shown that stress induced damage is in fact present in the baddeleyite xenocryst and further analysis will be required to determine the exact characteristics of these defects. The results could possibly provide valuable information on the geochronological thermal history of the material