XRD Investigation of Geologic Materials

X-ray diffraction (XRD) is deemed a gold standard method for mineralogical analysis. From mining to research purposes, XRD has discovered several uses and applications from routine QA/QC of mine run to resolving the structure of clays from powder. Due to the need to aid research and process efforts, the demand on the versatility of X-ray diffractometers is equally rising.  

Not only do modern applications in process geology need a precise instrument with minimal downtime, they also need one which can accumulate data quickly. Central laboratories are regularly working to streamline data collection and analysis.

Instrument

The Thermo Scientific™ ARL™ EQUINOX 1000 employs a standard 3 kW source for which chilled water cooling to the tube is required. The ARL EQUINOX 1000 is the only bench-top XRD that has high power and standard XRD tubes ensuring peak performance of both resolution and sensitivity.

Either or both a Ge (high resolution) or graphite monochromator (high intensity)  can equip the system in a distinctive optional twin monochromator system (SIAM X), which enables the ARL EQUINOX 1000 to offer performance equivalent to larger and more expensive instruments.

The ARL EQUINOX 1000 offers rapid data collection times contrasted to other diffractometers because of the unique curved position sensitive detector (CPS) unique to the model that records all diffraction peaks concurrently and in real time and is therefore ideal for research and process applications (Figure 1).

Figure 1. ARL EQUINOX 1000 X-ray diffractometer. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Experimental

By detaching crystals from a cluster, grinding and putting it on a reflection sample holder, a sample of peridotite collected from Trudy’s Mine, San Carlos, Arizona, USA (Figure 2) was examined.

Figure 2. Peridotite sample from Trudy’s Mine, San Carlos, AZ, USA. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Under Co Kα (1.78897 Å) radiation this sample was analyzed for four different time periods: 15 minutes, 5 minutes, 1 minute and 30 seconds, with the sample rotating throughout the analysis. Raw data evaluation was conducted with I_MAD (Figures 3 through 6). Data processing, consisting of whole pattern fitting Rietveld refinement (WPF) or RIR, for lower intensity data, was performed using MDI JADE 2010 equipped with the Crystallographic Open Database (COD) and AMCSD for quantitative and qualitative analysis phase analysis.

Figure 3. 15-minute raw data. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Figure 4. 5-minute raw data. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Figure 5. 1-minute raw data. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Figure 6. 30-second raw data. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

The crystal structure of olivine is comprised of two distinct metal sites (M1 and M2), three different oxygen sites (O1, O2 and O3) and one silicon site (Si). The position of the sample along the solid solution series may be calculated by refining the site occupancies of the M1 and M2 site JADE 2010 was used to analyze the raw data from the four scans. 

The 15-minute and five-minute datasets (Figures 7 and 8) experienced a WPF analysis. There was a full Rietveld refinement of the structures, refining the site occupancies of the M1 and M2 sites of forsterite (Table 1). The final refinement for the 15-minute dataset had Rwp = 6.12 with S = 1.65. The five-minute dataset had Rwp – 7.83 with S= 1.23.

Table 1. Refined site occupancy values. Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers 

Figure 7. 15-minute refined WPF data using JADE 2010. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Figure 8. 5-minute refined WPF data using JADE 2010. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

The sum of the Mg and Fe site occupancy values did not add to 100% occupied and as such, the remaining value was allocated to Ca to be added to the ternary series diagram given in Figure 9.  It can be determined from the diagram that the optimal fit is to the mineral forsterite with a determined chemistry of (Mg_1.24,Fe_0.43,Ca_0.33) SiO₄ for the 15 minute scan and (Mg_1.21,Fe_0.45,Ca_0.34)SiO₄ for the five-minute scan.

Figure 9. Ternary diagram showing the positions of the 15-minute (red) and 5-minute (blue) refined site occupancy values. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Results

In the crystalline clusters the primary mineral is olivine ((Mg,Fe)₂ SiO₄). Olivine creates a solid solution series between the Mg and Fe endmembers forsterite (Mg₂ SiO₄) and fayalite (Fe₂ SiO₄). Additionally, it creates a ternary series with a Ca endmember (Ca₂ SiO₄).

Because of the lesser intensities for the 1 minute and 30 second datasets (Figures 10 and 11), a full Rietveld analysis was not ideal, therefore a more semi-quantitative RIR approach, utilizing individual peak profile fitting, was executed to analyze the phase assemblage.

Figure 10. 1-minute fit RIR data using JADE 2010. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Figure 11. 30-second fit RIR data using JADE 2010. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

In Table 2, phase assemblage results for all scans can be seen. It is clear that each scan time provides an assemblage that agrees well with the others. Because the standard deviation values are mainly low, it could be suggested that for an ultra-fast scan, a phase assemblage can be quantified with extreme accuracy.

Table 2. Phase assemblage results. Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Conclusion

Due to the speed and resolution, the ARL EQUINOX 1000 has the capacity to completely analyze geologic materials from phase assemblages to full QPA and structure refinements. To perform WPF quantitative phase analysis, a five minute measurement time is adequate and for a semiquantitative RIR based analysis only 30 seconds is necessary.

It is possible for structure refinements of the crystalline phases to be conducted with accuracy  needing only a few minutes of scan time. Thus, the ARL EQUINOX 1000 benchtop diffractometer is a tool that is straightforward to use for geologic research and process applications.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers.

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