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.

中文翻译：

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行星地球化学勘探中的粒子诱发X射线发射测量中的粉状岩石与固体岩石比较

在通过NASA领导的火星科学实验室任务部署的α粒子X射线光谱仪（APXS）确定原位火星岩石和未固结材料的化学性质时，晶粒大小是一个重要的考虑因素。我们使用了2.5 MeV质子来模拟APXS的粒子诱导X射线发射（PIXE）分支（5 MeV alpha）。用细颗粒到粗颗粒的七个抛光岩石板（火成岩和沉积岩）以原始形式通过PIXE分析，然后研磨成粉末并压成颗粒以进行进一步分析。每个平板上10个询问区域上的总面积（160 mm ²）与APXS上询问的区域相当；每个岩石分析两个颗粒，每个颗粒使用16 mm ²地区，被认为是适当的。细颗粒样品中Na，Mg，Al，Si，K，Ca和Fe的平均颗粒/平板浓度比接近1.0，而较粗糙的样品则变化了±10％。在10个岩石区域中，PIXE浓度值之间的变化随岩石平板中的粗糙度而单调增加。将总PIXE浓度与通过硼酸盐熔融WDXRF测量的值进行比较，为直接比较丸粒和平板PIXE浓度提供了进一步的定量支持。这项工作肯定了在细粒度火星材料上使用APXS，但在分析粗粒度材料时建议使用更大的询问区域（包括栅格化）。