Deriving amorphous component abundance and composition of rocks and sediments on Earth and Mars

1Rebecca J. Smith, 2Elizabeth B. Rampe, 1Briony H. N. Horgan, 3Erwin Dehouck
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005612]
1Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN
2NASA/Johnson Space Center, Houston, TX
3Laboratoire de Géologie de Lyon – Terre, Planètes, Environnement, UMR 5276, CNRS, Université Lyon 1, ENS Lyon, Villeurbanne, France
Published by arrangement with John Wiley & Sons

X‐ray amorphous materials have been detected in all samples measured by the CheMin X‐ray diffractometer (XRD) onboard 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 (MBCs). 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 QXRD measurements, though slightly underestimated. This suggests that previous amorphous component estimates for martian samples are relatively accurate. This study highlights the usefulness of the MBC method for characterizing amorphous materials in terrestrial samples, providing important supplemental information to destructive and time‐consuming size‐separation and dissolution procedures.

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