Analyses of Crystalline and X-Ray Amorphous Materials in Gale Crater Rocks and Soils

Abstract

The Mars Science Laboratory (MSL) rover, Curiosity, is exploring the layered sediments of Gale crater with the primary goal the primary goal of finding and assessing environments that are, or once were, favorable habitats for life. Curiosity’s scientific payload was designed to analyze the mineralogical, geochemical, and textural properties of rocks and soils encountered by the rover. Among this payload is the CheMin X-ray diffractometer, an instrument that provides data to determine the identity of crystalline phases present at >1 wt%, the crystal chemistry of major minerals, and the distribution of crystalline, clay mineral, and amorphous phases in each analyzed sample. Mineralogical assessments are important to understanding sediment sources, weathering histories, depositional environments, and diagenetic processes that influenced the formation of analyzed soils and rocks. This dissertation discusses the mineralogy and crystal chemistry of an active eolian dune in Gale crater and compares the results to the basaltic soils analyzed by the Mars Exploration Rovers and to the predicted mineralogy of the dune field based on orbital spectral measurements. Quantitative mineralogical analyses of an eolian dune sample, named Gobabeb, revealed that the sand is dominated by basaltic minerals and X-ray amorphous phases. Plagioclase, olivine, and two Ca-Mg-Fe pyroxenes account for the majority of crystalline phases detected. Minor phases include magnetite, quartz, hematite, and anhydrite. The X-ray amorphous fraction, ~42 wt% of the Gobabeb sample, is composed primarily of SiO2, FeOT, Al2O3 and SO3. Proposed amorphous phases include basaltic glass, maskelynite, amorphous silica, nanophase iron oxides, and amorphous sulfates. The predicted mineralogy of the Bagnold Dune Field based on VNIR (visible-near-infrared) and MIR (mid-infrared) orbital spectral measurements shows high fidelity to in situ CheMin mineralogical analyses. Feldspar and olivine abundances are consistent with each other, and the CheMin-derived composition of the olivine, Fo56(3), is nearly identical to the value calculated from orbital analyses, Fo55(5). In addition to Gale crater eolian materials, this dissertation describes the mineralogy of four mudstones from an ancient lacustrine environment and discusses how mineralogical trends were used to identify lake and groundwater fluid conditions that interacted with detrital sediments over time. The crystalline, clay mineral, and amorphous phase distributions for the Oudam, Marimba, Quela, and Sebina drill samples were estimated from CheMin diffraction analyses. The mineralogies of these four samples are dominated by plagioclase, Ca-sulfates, hematite, and phyllosilicates. The mineral phase assemblages suggest mafic detrital sources for all drill samples and contribution from a silicic source in the Oudam drill sample. The presence of hematite and Ca-, Fe- and Mg-rich sulfates indicate that all four drill sites were exposed to oxidizing conditions, dominantly diagenetic. X-ray amorphous components are SiO2-rich (~50 wt%) suggesting the presence of amorphous silica, aluminosilicates, and/or Fe-silicates. Overall, mineralogy of the analyzed lacustrine rocks suggests a depositional environment where sediments were exposed to open-system aqueous alteration processes and subsequent diagenetic events resulting in the formation of matrix Ca-sulfates and hematite. Lastly, a crystal structure redetermination of lead manganese hydroxide mineral, quenselite, is discussed. Single crystal diffraction methods were utilized to determine quenselite unit-cell parameters, a = 9.1618(6), b = 5.6927(3), c = 5.6191(3) Å, β = 92.979(3)°, and V = 292.67 Å3. Although quenselite is not a martian-relevant mineral, a fundamental understanding of atomic bonding, structure determination, and publication of crystallographic information files (CIF) are essential to the analysis of powder diffraction data. The results presented in this dissertation illustrate the significance of mineralogical analyses to the characterization of eolian and fluvio-lacustrine sediments on Mars.