Advanced Phase Identification for Mineralogical Samples

Atomic contrast image acquired with ARGUS imaging detector on the measured area. The presence of at least 5 different phases can be inferred from the different grey levels.

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EBSD is a very powerful technique in Geosciences studies for analysing microstructures and phase identification. By measuring the lattice orientation of the grains and providing the phase distribution, EBSD helps determining the crystallographic preferred orientations (CPO) of mineral fabrics and understand deformation and phase transformation mechanisms.

One of the challenges with mineralogical samples is the numerous phases and the considerable difference in grain size when considering accessory phases which can be easily missed when setting up the measurement. However, phases with similar chemical compositions but different crystallographic structures can be distinguished by EBSD, while EDS discriminates phases with similar crystallographic structures but of different chemical compositions. Using a specimen which contains both phases demonstrates that simultaneous measurement and combined processing of EDS and EBSD data can overcome these limitations.

In this example, a sample of oceanic gabbro (ODP 304/305 U1309D) was analysed by simultaneous EBSD and EDS measurement. 10 phases of 5 different symmetries were identified and successfully indexed. Note that phase identification and distinction is performed semi-automatically, either online with the SEM or offline with saved data. This allows the user to correct or complete an analysis, without the need for re-measurement. The EBSD phase and orientation distribution maps (figures 3 and 4) illustrate the ability of the software to simultaneously index numerous low and high crystal symmetry phases.

Orientation contrast image acquired with ARGUS imaging detector. It reveals the microstructure and despite some curtaining from the ion milling, the sample is well prepared: internal strain in the gold bond is clearly visible by color-coded orientation contrast.

Unprocessed EBSD Phase distribution map revealing the presence of 10 phases indexed at a high indexing rate of 95%. In black are the produce of the erosion, porosity and cracks. The map illustrates the ability of the software to simultaneously index low and high crystal symmetry phases. Note that fine exsolutions of clinohypersthene in the large diopside grains are resolved: despite having similar crystallography, the robustness of the QUANTAX EBSD indexing algorithm has produced no misindexing between these phases.

Pattern Quality Map overlaid by the orientation distribution (along tilt axis) for illustration of the performant indexing of 10 phases.

Sample courtesy: Dr. A. Halfpenny*, Dr. M. Verrall, CSIRO, Perth, Australia. *now at Standford University