Crystal Chemistry of Martian Minerals

Abstract

The NASA Mars Science Laboratory (MSL) rover, Curiosity, began exploring Gale crater, Mars in August, 2012 with the primary goal of assessing the past and present habitability of the martian surface. To meet this goal, Curiosity is equipped with an advanced suite of scientific instruments capable of investigating the geology, geochemistry, and atmospheric conditions on Mars. Among these instruments is the CheMin (Chemistry and Mineralogy) X-ray diffractometer whose function is to identify mineral phases present in sediments and rocks by means of X-ray diffraction (XRD). Characterizing the mineralogical make-up of a rock unit is an important step in determining its geologic history. Primary igneous minerals, such as feldspar, olivine, and pyroxene, give information about parental magmas - their composition, temperature, depth and so on. Secondary alteration minerals, like jarosite or akaganeite, point to distinct weathering or diagenetic processes. As such, understanding the mineral occurrence and abundance in Gale crater provides the MSL team with a robust foundation from which to make geologic interpretations. This dissertation details the methods used to determine the chemical composition of selected mineral phases based solely on XRD patterns from CheMin. Curiosity is equipped with instruments capable of measuring bulk composition of a sample [e.g., APXS (Alpha Particle X-ray Spectrometer)] but has no instrument capable of measuring the composition of a single phase in a multi-phase sample. Therefore, we developed crystal chemical algorithms and calibrations based on refined unit-cell parameters in order to predict mineral phase compositions. We have calculated algorithms for plagioclase, alkali feldspar, Mg-Fe-Ca clinopyroxene, Mg-Fe orthopyroxene, Mg-Fe olivine, Fe-oxide spinel, and alunite-jarosite group minerals. Furthermore, we use the estimated compositions of crystalline material in conjunction with bulk sample chemistry from APXS to estimate of the composition of the X-ray amorphous material present in each of the samples analyzed by CheMin in Gale crater.