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Deriving Amorphous Component Abundance and Composition of Rocks and Sediments on Earth and Mars

Abstract : Plain Language Summary X-ray amorphous materials have been detected in all samples measured by the CheMin X-ray diffractometer (XRD) on board the Mars Science Laboratory rover in Gale Crater, Mars. The origin(s) of these materials are poorly understood, and there are significant uncertainties on their estimated abundances and compositions. Three methods are used to estimate the bulk amorphous component abundance and composition of Martian samples using XRD and bulk chemical data: (1) Rietveld refinements, (2) FULLPAT analyses, and (3) mass balance calculations. We tested these methods against a quantitative XRD (internal standard) method commonly used in terrestrial laboratories. Additionally, we tested for instrumentation effects by measuring our samples on a laboratory XRD instrument (PANalytical X'Pert Pro) and the CheMin test bed instrument (CheMin IV). We used three natural samples known to contain amorphous materials: glacial sediment, Hawaiian soil, and a paleosol. Our methods resulted in nine amorphous abundances and four amorphous compositions for each sample. For a single sample, amorphous abundance estimates and amorphous compositions are relatively similar across all estimation methods. CheMin analog measurements perform well in our tests, with amorphous abundances and compositions comparable to laboratory quantitative XRD measurements, though slightly underestimated. This suggests that previous amorphous component estimates for Martian samples are relatively accurate. This study highlights the usefulness of the mass balance calculation method for characterizing amorphous materials in terrestrial samples, providing important supplemental information to destructive and time consuming size separation and dissolution procedures. Natural soil and sediment samples on Earth and Mars are commonly mixtures of crystalline and noncrystalline materials. X-ray diffraction techniques are frequently used to quantify the abundance and composition of crystalline materials, and relatively recent developments in X-ray diffraction data analysis methods allow noncrystalline materials to also be characterized. Noncrystalline materials are studied using the following methods: (1) noncrystalline X-ray diffraction peaks modeled with broad peaks, (2) noncrystalline peaks modeled with diffraction patterns of measured noncrystalline materials, and (3) a mass balance calculation that combines chemical and X-ray diffraction data. However, it is uncertain how accurate these methods are. Here we systematically test these three data analysis methods on complex natural samples. We find that both modeling methods (1 and 2) are capable of providing relatively accurate noncrystalline component abundances. Modeling noncrystalline peaks with patterns of measured noncrystalline materials (method 2) provides the most accurate abundance results but does not necessarily provide accurate compositional information, whereas the mass balance calculation provides accurate noncrystalline material compositions, but not abundances. Our results suggest that a combination of methods (either 1 and 3 or 2 and 3) should be used to more completely characterize the abundance and composition of noncrystalline materials.
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Rebecca J. Smith, Elizabeth B. Rampe, Briony H. N. Horgan, Erwin Dehouck. Deriving Amorphous Component Abundance and Composition of Rocks and Sediments on Earth and Mars. Journal of Geophysical Research. Planets, Wiley-Blackwell, 2018, 123 (10), pp.2485--2505. ⟨10.1029/2018JE005612⟩. ⟨hal-02326508⟩



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