X-ray diffraction can be used to identify and quantify the structural phases existing in a sample in a unique manner, unlike identification of the constituent elements carried out with other techniques. Sensitivity to phase structures is important during the investigation of geological samples, as several phases can possess the same elemental building blocks, which may vary only in their arrangement manner. It is often beneficial to maintain the sample in a collected condition, instead of powdering it in a ball mill, to allow onward petrographic analysis. Two-dimensional (2D) detectors are ideal for such analysis, as the large diffraction space coverage can absorb reflections arising from crystallographic phases with preferred orientations and large-sized grains, which are likely to be missed.
The D8 suite of diffraction instruments along with the PILATUS3 R 100K-A hybrid photon counting (HPC) pixel detector is a novel X-ray diffraction (XRD²) technique, which is well-suited for research characterization of multipurpose modern materials. This article presents the features of this system in a 2D diffraction configuration for the investigation of large-sized geological samples for usage such as quantitative phase analysis and phase identification (Phase ID).
A sample of quartz monzonite, which is a large-sized granular igneous rock (Figure 1), was mounted so that the flat section of the sample was aligned to the center of the instrument. Measurements were carried out in reflection geometry through a D8 DISCOVER, provided with a PILATUS3 detector, and a spot-beam source. Two coupled scans from 10° to 70° in 2θ with an increase of 5° and 2 minutes per step were carried out. With a sample-to-detector distance of 33 cm, the XRD² data was obtained. A 300 µm primary beam collimator was utilized to measurethe incident beam diameter. Figure 2a shows the scan that was collected when the sample was statically positioned at the X in Figure 1. Figure 2b shows the scan that was collected with Phi rotation of 3 rpm and X and Y oscillation of +/-2 mm, equivalent to the circle in Figure 1.
Figure 1. Quartz monzonite rock sample. The yellow X indicates the position where Figure 2a was collected, while the circle indicates the region rastered over resulting in image 2b.
Figure 2. Resulting images from a coupled scan without (a) and with (b) X, Y and Phi motion. Red circles indicate some single crystal reflections.
The two dimensional scattering was integrated to a one dimensional diffractogram with DIFFRAC.EVA. A quantitative Rietveld analysis and Phase ID were conducted on the resulting one dimensional pattern. Figure 2a shows the strong single crystal reflections. Movement of the sample eliminates the coarse grain phenomena that are commonly seen in plutonic rocks, yielding Debye rings found usually in powdered samples (Figure 2b). Figure 3 shows the integrated diffractogram of this pattern.
Figure 3. Phase Identification in DIFFRAC.EVA of an integration of the image collected in Figure 2b.
The crystalline phases that exist in the sample can be easily identified. The data base entries match the relative intensities and peak positions. Other than the relative intensities in the 1D diffractogram formed from the obtained XRD² data, the static 2D image can be utilized to suppliment the Phase ID process. Subsets of the reflections are identifiable and searchable by identification of diffraction rings possessing identical uniformity. The 1D diffractogram was further examined for quantification of phase by using DIFFRAC.TOPAS. Various phenomena, such as axian divergence and preferred orientation, are reduced or removed by the big gamma coverage of a 2D detector.
Figure 4. Quantitative Rietveld Refinement in DIFFRAC.TOPAS of an integration of the image collected in Figure 2b.
Figure 4 shows the analysis result on recognition of the rock sample as quartz monzonite.
This information has been sourced, reviewed and adapted from materials provided by Bruker AXS Inc.
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