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

Abstract

AbstractX‐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.