Obscuration of Crystalline Ca‐Sulfate in XRD and Raman Data When Coated by Amorphous Ferric Sulfate: Implications for the Amorphous Components at Gale Crater

Abstract Throughout Gale crater on Mars, the Curiosity rover has found high abundances (15–73 wt%) of X‐ray amorphous materials in the rocks and sediments. The composition of this amorphous fraction is primarily calculated by subtracting crystalline abundances measured by the Chemistry & Mineralogy X‐Ray Diffractometer (CheMin) from bulk elemental abundances from the Alpha Particle X‐Ray Spectrometer (APXS). If any crystalline phase was underrepresented in the CheMin data, the elemental components of that phase would be overestimated in the amorphous fraction. This study examines the possibility of underrepresented crystalline phases in X‐ray diffraction data using mixtures of amorphous ferric sulfate (AFS) and crystalline Ca‐sulfate. Two compositionally equivalent mixtures were made with different morphologies: first, a simple mixture of AFS and Ca‐sulfate grains and second, Ca‐sulfate grains with AFS coatings. Each mixture was characterized with Raman spectroscopy, and two X‐ray diffractometers (XRDs): a Bragg‐Brentano instrument with CuKα radiation and a Debye‐Scherrer instrument with CoKα radiation. Raman peaks from Ca‐sulfate dominate both mixtures, but are dampened when AFS coats the Ca‐sulfate grains. In XRD data, AFS coatings cause an overestimation of the amorphous percentage, with a difference between the known and refined amorphous abundances of 29–34 wt%, compared to 2–2.8 wt% for the uncoated mixtures. The effects of the coatings were slightly amplified with the Debye‐Scherrer XRD, primarily due to the increased scattering and absorption of the CoKα radiation. This has implications for interpreting the XRD data of mixed amorphous‐crystalline samples on Mars, as any Fe‐rich amorphous coating may cause an overestimation of the amorphous abundance.