Investigation of Martian Analog Basalt from Terrestrial Craters Using XRF and XRD

Terrestrial analogs to Martian geologic conditions are being employed with data gathered during the numerous Martian rover and satellite expeditions that have occurred since the 1960s, to characterize Mars' physiological and chemical characteristics. These materials are now being used to examine a broad selection of topics, ranging from general geochemistry to the potential for human exploration of Mars.

Moreover, in the realm of material analysis, X-ray diffraction (XRD) and X-ray fluorescence (XRF) are understood to be gold standard methods. These two synergistic approaches equip the geologic domain, which ranges from research to mining, with diverse uses and applications. Contemporary research methodologies necessitate quick and precise instruments with minimal downtime, and these refinements permit core laboratories to make data acquisition and investigation more efficient.

Instruments

The Thermo Scientific™ ARL™ EQUINOX 100 X-ray diffractometer (Figure 1) utilizes a bespoke Cu (50 W) or Co (15 W) micro-focus tube supported by mirror optics. The minimal wattage required by the unit permits it to be totally portable, as it does not require an external water chiller. This portability also enables inter-laboratory transportation, and negates the requirement for specific infrastructure.

ARL EQUINOX 100 X-ray diffractometer

Figure 1. ARL EQUINOX 100 X-ray diffractometer

The ARL EQUINOX 100 instrument offers extremely rapid data acquisition rates in comparison to competing diffractometers, as a result of its distinctive curved position sensitive detector (CPS). This calculates, in real time, all diffraction peaks synchronously, and is consequently of great utility when it comes to both reflection and transmission calculations.

The Thermo Scientific™ ARL™ QUANT’X Energy Dispersive X-ray fluorescence (EDXRF) spectrometer (Figure 2) undertakes primary filtered radiation for sample excitation, which in turn causes fluorescence of constituent elements. The ARL QUANT’X is supported by a series of eight filters, as well as unfiltered tube excitation, which is uniquely calibrated to maximize the peak-to-background ratio for elements from Na to U.

ARL QUANT'X EDXRF spectrometer

Figure 2. ARL QUANT'X EDXRF spectrometer

The instrument therefore provides versatile research-grade apparatus, that can be effortlessly modified to every application or range of elements. Utilizing a synthesis of voltage and filter (kV), which is known as the “excitation condition”, results in a unique spectrum that represents the sample.

A multi-element assessment ordinarily executes a number of excitation conditions, offering optimal excitation efficiency for a suite of elements. In EDXRF terminology, the total set of conditions constitutes the foundation of the analytical “method” for a specific sample matrix.

Experimental

XRD

A sample of basalt gathered from craters located at the Moon National Monument, Idaho, USA was examined by granulating the bulk material and inserting it in a reflection sample holder. The sample, which revolved during the analysis to minimize the effects of preferred orientation, was interpreted from 0-115° 2θ under Co Kα (1.78897 Å) radiation for 30 minutes.

Raw data examination was undertaken with I_MAD. Data processing, comprising whole pattern fitting Rietveld refinement (WPF), was carried out with the use of MDI JADE 2010 furnished with the Crystallographic Open Database (COD) and AMCSD.

XRF

The same sample was scrutinized using the UniQuant standardless fundamental parameters (FP) software package. The spectrometer engaged a 50 W Rh source, which operates at voltages up to 50 kV, and is fitted with an 8 mm beam collimator. Data is gathered using a digital pulse processor on an electronically cooled SDD with 140 eV resolution and 30 mm2 x 0.45 mm active volume.

Results

The XRF data exhibited in Table 1 implies a high P basalt principally composed of plagioclase and potassic feldspars. Alongside a reasonably low silica content, SiO2 = 43.25%, the sample is enhanced in both Fe, Fe2O3 = 19.30%, and P, P2O5 = 2.21%, more or less 2x and 10x terrestrial normal respectively (Adcock et al., 2018). The XRD raw data (Figure 3) was examined with the use of MDI JADE 2010.  The data set was subjected to a WPF Rietveld refinement to attain a quantitative phase analysis (QPA) of the sample.

Table 1. XRF results

XRF Results
Compound m/m % Std Err
SiO2 43.25 0.24
Fe2O3 19.30 0.44
Al2O3 15.75 0.19
CaO 7.16 0.13
TiO2 3.00 0.17
MgO 3.00 0.09
Na2O 2.68 0.08
K2O 2.25 0.03
P2O5 2.21 0.10
ZrO2 0.371 0.019
MnO 0.281 0.021
SO3 0.281 0.017
Co3O4 0.174 0.045
BaO 0.139 0.007
SrO 0.0486 0.0024
ZnO 0.0286 0.0012
Y2O3 0.0243 0.0012
Cl 0.0203 0.0031
Nb2O5 0.0148 0.0007

30 minute raw dataset

Figure 3. 30 minute raw dataset

The terminal refinement (Figure 4) had an Rwp = 6.21, with GooF = 1.25, and demonstrated the following phases: plagioclase feldspars (anorthite, albite), k-spars (orthoclase, microcline and sanidine), hematite, augite, apatite, pyroxene, and olivine (forsterite). Table 2 exhibits the phase assemblage results for the whole range of scans.

Table 2. XRD phase assemblage

Phase Assemblage
Phase Formula WT% ESD
Anorthite (Ca0.533,Na0.467)(Si2.501,Al1.499)O8 25.2 1.7
Hematite (Fe1.86,Ti0.14)O3 21.9 0.7
Albite (Na0.685,Ca0.315)(Si2.54,Al1.46)O8 15.7 1.6
Orthoclase K(Si2.98,Al1.02)O8 11.5 1.1
Augite Ca(Mg0.75,Fe0.25)Si2O6 10.0 0.6
Apatite Ca5(PO4)3(F,OH,Cl) 7.9 0.4
Sanidine KAlSi3O8 3.7 0.7
Microcline KAlSi3O8 1.9 0.5
Forsterite Mg2(SiO4) 1.4 0.4
Pyroxene MgSiO3 0.7 0.3

Refined data from 4-114° 2θ using MDI JADE 2010

Figure 4. Refined data from 4-114° 2θ using MDI JADE 2010

The trend of the phases corresponds more than adequately with the results deriving from the XRF. Plagioclase feldspars comprise 40.9% of the sample, while potassic feldspars account for a further 17.1%.  The rest of the sample is constituted by hematite = 21.9%, pyroxenes = 10.7% and olivine = 1.4%.

Conclusion

The resolution and speed of the ARL EQUINOX 100 diffractometer supports its capacity to comprehensively analyze geologic materials, ranging from qualitative phase assemblages to full QPA. A 30 minute calculation period was selected to optimize minor phase intensities. Moreover, when supplemented with the ARL QUANT'X EDXRF spectrometer elemental date, a complete synergistic analysis of the alkaline basalt sample can be acquired.

Consequently, the ARL EQUINOX 100 benchtop diffractometer and ARL QUANT’X benchtop EDXRF spectrometer are a perfect synthesis of process applications and analytical instruments for geologic research.

References

Adcock et al, 2018. Craters of the Moon National Monument Basalts as Unshocked Compositional and Weathering Analogs for Martian Rocks and Meteorites. American Mineralogist. In Press.

Acknowledgments

The author would like to express their thanks to Red Rock Community College for supplying the sample this paper used. The data was collected as part of a broader study collaboration between Thermo Fisher Scientific and Red Rocks Community College.

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

For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers.

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