Powdered rock versus solid rock comparisons in particle‐induced X‐ray emission measurements for planetary geochemical exploration

• Published in: X-ray spectrometry Vol. 49; no. 6; pp. 651 - 662
• Main Authors: Flannigan, Erin L., Campbell, John L., Spray, John G., Thompson, Lucy M.
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Inc 01.11.2020; Wiley Subscription Services, Inc
• Subjects: Alpha particles; Alpha rays; Aluminum; Coarseness; Emission; Emission measurements; Fragments; Grain size; Interrogation; Magnesium; Mars; Pellets; Rocks; Silicon; Slabs
• ISSN: 0049-8246; 1097-4539
• DOI: 10.1002/xrs.3172

Grain size is an important consideration in the determination of the bulk chemistry of Martian rocks and unconsolidated materials in situ by the alpha particle X‐ray spectrometer (APXS), deployed on the NASA‐led Mars Science Laboratory mission. We used 2.5 MeV protons to emulate the particle‐induced X‐ray emission (PIXE) branch (5 MeV alphas) of the APXS. Seven polished rock slabs (igneous and sedimentary), ranging from fine‐ to coarse‐grained, were analyzed by PIXE in their original form, then milled to powders and pressed into pellets for further analysis. The summed area (160 mm2) over 10 interrogated regions on each slab is comparable to the area interrogated on the APXS; analysis of two pellets per rock, each using a 16 mm2 region, was found to be appropriate. The mean pellet/slab concentration ratio for Na, Mg, Al, Si, K, Ca, and Fe was close to 1.0 for fine‐grained samples, but changed by ±10% for the coarser cases. The variability among PIXE concentration values across the 10 rock regions increased monotonically with coarseness in the rock slabs. Comparison of overall PIXE concentrations with values measured by borate‐fusion WDXRF provides further quantitative support to the direct comparison of pellet and slab PIXE concentrations. This work affirms the use of the APXS on fine‐grained Martian materials but recommends larger interrogation areas (including rastering) when analyzing coarser‐grained materials. It also demonstrates that the presence of relatively large mineral grains (phenocrysts) or rock/mineral fragments within fine‐grained materials can contribute to greater error for specific elements associated with those phenocrysts/fragments.