Departmental Seminar

X-ray tomography – 3D targeted analysis for better 2D results

Dr Jukka Kuva
Geological Survey of Finland GTK

Analysis of geological samples is commonly done with 2D surface methods applied directly to the sample surface or the surface of a carefully prepared thin section. Automated analysis tools, such as QEMSCAN (Quantitative Evaluation of Minerals by SCANning electron microscopy) with SEM-EDS (Scanning Electron Microscopy – Energy Dispersive X-ray Spectroscopy), have become quite efficient and convenient, while more manual tools, such as EPMA (Electron Probe Microanalyzer) or LA-ICP-MS (Laser Ablation – Inductively Coupled Plasma Mass Spectroscopy) provide unparalleled accuracy.

The main drawback with 2D surface methods is knowing a priori where to section a 3D sample in order to obtain the best representative analysis. Contextual knowledge of the sample material can help a lot, but the most interesting parts of a sample often nevertheless remain unexamined. We have therefore developed a workflow which takes away most of the subjectivity and chance of preparing 2D slices from 3D objects, such as rocks.  The process begins with an XCT (X-ray Computed Tomography) scan of a complete sample, which is often an uncut drill core sample.  Efficiently scanning samples at the drill core scale and larger (up to 30 cm in diameter) has only recently been made possible in Finland with the installation of GTK’s new XCT equipment. The resulting tomographic image shows the internal structure, along with the heterogeneities, and can be used to locate areas of specific interest to the analyst. Thin sections, both optical and polished, are then prepared so as to deliberately intercept the features of interest observed within the 3D volumes. The first step of the workflow, XCT, is non-destructive and can later be used as a map for all subsequent analyses. This workflow has been successfully applied to geological samples, combining XCT with QEMSCAN, and is now being steered towards other types of samples as well, such as batteries, where other types of analysis, such as trace element analysis with LA-ICP-MS, can be of use.  From geochemical analysis covering >90% of the periodic table, using LAICPMS, the economic evaluation of the sample improves exponentially from a surface to a volume.

GTK has a unique facility in Finland with XCT and advanced 2D analysis systems in the same lab, with the researchers working in closely collaboration. We present results where this workflow has been used to great effect in the discovery of minerals in exploration samples of commercial importance, as well as preliminary results from other studies.

 

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