¹Marina Martínez,^1,2Charles K. Shearer,¹Adrian J. Brearley
American Mineralogist 108, 2024-2042 Link to Article [http://www.minsocam.org/msa/ammin/toc/2023/Abstracts/AM108P2024.pdf]
¹Department of Earth & Planetary Sciences, MSC03-2040
²University of New Mexico, Albuquerque, New Mexico 87131, U.
Copyright: The Mineralogical Society of America

The microstructures of selected F-, Cl-, and OH-bearing martian apatite grains, two in Northwest
Africa (NWA) 998 (cumulus apatites, embedded in pyroxene) and a set of four in Nakhla (intercumulus
apatites), were studied by focused ion beam–transmission electron microscopy (FIB-TEM) techniques.
Our results show that the nanostructure of martian apatite is characterized by a domain structure at
the 5–10 nm scale defined by undulous lattice fringes and slight differences in contrast, indicative
of localized elastic strain within the lattices and misorientations in the crystal. The domain structure
records a primary post-magmatic signature formed during initial subsolidus cooling (T <800 °C), in which halogens clustered by phase separation (exsolution), but overall preserved continuity in the crystalline structure. Northwest Africa 998 apatites, with average Cl/F ratios of 1.26 and 2.11, show higher undulosity of the lattice fringes and more differences in contrast than Nakhla apatites (average Cl/F = 4.23), suggesting that when Cl/F is close to 1, there is more strain in the structure. Vacancies likely played a key role stabilizing these ternary apatites that otherwise would be immiscible. Apatites in Nakhla show larger variations in halogen and rare-earth element (REE) contents within and between grains that are only a few micrometers apart, consistent with growth under disequilibrium conditions and crystallization in open systems. Nakhla apatite preserves chemical zonation, where F, REEs, Si, and Fe are higher in the core and Cl increases toward the outer layers of the crystal. There is no evidence of subsolidus ionic diffusion or post-magmatic fluid interactions that affected bulk apatite compositions in NWA 998 or Nakhla. The observed zonation is consistent with crystallization from a late-stage melt that became Cl-enriched, and assimilation of volatile-rich crustal sediments is the most plausible mechanism for the observed zonation. This work has broader implications for interpreting the chemistry of apatite in other planetary systems.

¹Kaushik Mitra¹,^2,3Jeffrey G. Catalano,¹Yatharth Bahl,¹Joel A. Hurowitz
Earth and Planetary Science 624, 118464 Link to Article [https://doi.org/10.1016/j.epsl.2023.118464]
¹Department of Geosciences, Stony Brook University, Stony Brook, NY 11727 USA
²Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130 USA
³McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130 USA
Copyright Elsevier

Sulfate salts are abundant, widespread, and temporally distributed on the surface of Mars. Several processes, including sulfide weathering, have been proposed to explain the formation and accumulation of oxidized iron(III) hydroxysulfate (e.g., jarosite) in Martian sediments. Oxidative weathering of iron sulfide minerals (like pyrite and pyrrhotite) could explain the presence of sulfate as well as the relative absence of sulfide minerals in Martian sediments. However, the effectiveness of oxyhalogen compounds, plausible oxidants present on Mars, to weather iron sulfides remain unknown. Here we investigate the oxidative weathering of iron sulfide minerals, pyrite and pyrrhotite, by chlorate and bromate in Mars-relevant fluids. Our results demonstrate that both oxyhalogen species readily oxidize iron sulfides and produce an alteration assemblage comprised of elemental sulfur, Fe(III) (oxyhydr)oxide (magnetite, goethite, and lepidocrocite), and Fe(III) hydroxysulfates (jarosite and schwertmannite). The mineral products depend strongly on the type of sulfide, oxidant, and initial solution pH. Owing to their abundance and highly reactive nature, oxyhalogen brines could be important Fe(III) hydroxysulfate-forming reagents on early and modern Mars and substantially impact the survivability of sulfide minerals at the Martian surface. Additionally, sulfide bearing units might serve as indicators of minimal post-depositional alteration. Oxyhalogens may be responsible for the loss of magmatic sulfides in surface materials given the prevalence of oxyhalogen brines and the reactivity of the sulfides.