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Akaganeite and jarosite were detected in two mudstone drill samples from Vera Rubin ridge (VRR), Gale crater by Chemistry & Mineralogy X‐Ray Diffraction (CheMin). The co‐occurrence of these two minerals is quite rare in both terrestrial and Martian aqueous environments. In order to confine the chemical conditions of paragenetic akaganeite and jarosite, and provide insight into late‐stage diagenetic alterations at VRR, we synthesized akaganeite and jarosite with varying SO42− concentrations and initial pH levels. Synthetic samples were characterized using Field Emission Scanning Electron Microscopy, X‐ray powder diffraction and Raman spectroscopy. Our study reveals that akaganeite and jarosite exist in equilibrium in the solution with 0.011–0.028 M SO42− with respect to 0.6 M Cl− and an initial pH of 1.3–2.2. In combination with the CheMin detection results, the chemistry and pH values of the fluids at VRR can be further constrained. Considering the absence of goethite and the relative higher portion of akaganeite than jarosite in the drill samples, the pH values should be 1.4–2 and the S/Cl molar ratio should be within the range of 0.018–0.042. Based on our laboratory results, we hypothesize that the presence of akaganeite and jarosite at VRR represents an individual episode of acidic groundwater activity. During the late‐stage diagenetic process at VRR, upwelled acidic groundwater dissolved the local chlorides to form the Cl−‐dominated fluids. Subsequent evaporation further concentrated the acid saline fluids and therefore resulted in an extremely acidic environment (1.4 ≤ pH＜2 with S/Cl molar ratio of 0.018–0.042), which produced akaganeite and jarosite. Plain Language Summary: Akaganeite and jarosite have been detected in drill samples of Gale crater. But their co‐occurrence has only been found in two drill samples Stoer and Rock Hall at VRR. The formation conditions of akaganeite and jarosite at VRR remain unknown. To solve this issue, we conducted synthesis experiments under different SO42− concentrations and initial pH levels. We found that akaganeite and jarosite coprecipitate in the solution with 0.011–0.028 M SO42− with respect to 0.6 M Cl− and an initial pH of 1.3–2.2. In conjunction with the CheMin data and our experimental results, the chemistry and pH values of the fluids at VRR can be further constrained. The formation of akaganeite and jarosite at VRR likely occurred in a Cl−‐dominating and SO42−‐bearing solution (S/Cl molar ratio of 0.018–0.042) under extremely acidic conditions (pH range of 1.4–2). We hypothesize that the coprecipitation of akaganeite and jarosite represents an individual episode of acidic groundwater activity at VRR. The upwelling acidic groundwater possibly dissolved the local chlorides to form a Cl‐dominated fluid with trace amounts of SO42−. Evaporation further concentrated the fluid to extreme acidic and thus formed akaganeite and jarosite. Key Points: Akaganeite and jarosite at Vera Rubin ridge (VRR) formed in a Cl‐dominated solution with trace amounts of SO42− under extremely acidic conditionThe formation of akaganeite and jarosite at VRR represents an individual episode of acidic groundwater activityThe abundant Cl in the fluids is possibly of local origin while the extreme acidity is accomplished by evaporation [ABSTRACT FROM AUTHOR]

Crystalline ferric sulfates (e.g., ferricopiapite and Fe(OH)SO4) have been proposed at multiple locations on Mars by the orbiter. While at Meridiani Planum and Gale Crater by rover missions, amorphous ferric sulfates were also suggested to exist in soils and sedimentary rocks. Ferric sulfates with different hydration degrees may play a key role in the water cycle on Mars. In order to understand the role of the hydrated ferric sulfates in the water cycle and their exact hydration states on Mars, five crystalline ferric sulfates and seven amorphous ferric sulfates with different hydration states were synthesized in the laboratory. The water content (number of H2O molecules per ferric sulfate molecule) was quantified by Raman spectroscopy and Laser‐induced breakdown spectroscopy (LIBS), respectively. It was found that the amorphous ferric sulfates water content has a good relationship with the SO4 tetrahedron main Raman feature position around 1,000 cm−1, shown as the intensity and area of water feature around 3,500 cm−1 over SO4 tetrahedron main Raman feature around 1,000 cm−1, respectively. Twelve ferric sulfates' Hα emission line area at 656.7 nm was normalized by the O emission line area at 778 nm in LIBS spectra. The crystalline and amorphous ferric sulfates all showed a good relationship between the values of normalized results and the water content. These results will aid us in precisely constraining the exact phases of hydrated ferric sulfates, provide a better reference for ChemCam, SuperCam, and Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals data interpretation and their use to quantify the water content in detected targets. Plain Language Summary: Hydrated ferric sulfates have been suggested to occur on Mars at different sites (e.g., Meridiani Planum and Gale Crater) during orbital and rover missions. The hydration states of crystalline or amorphous ferric sulfates are greatly affected by environmental conditions, and the amorphous ferric sulfates are metastable phases, which are considered to be produced by rapid dehydration and rehydration processes. The presence of ferric sulfates indicates the occurrence of water‐related processes, while the water content within these compounds can provide valuable insights into Mars' water history. However, the exact phases of ferric sulfates cannot be precisely constrained except for the jarosite that limited our knowledge about the water‐related geochemical process on Mars. In the present study, 12 hydrated ferric sulfates were synthesized in the laboratory and measured by Raman and Laser‐induced breakdown spectroscopy (LIBS) spectroscopy. The unique Raman features of those ferric sulfates will help to identify them on Mars by Raman spectroscopy. Besides, the methods of water content quantification were developed based on their Raman and LIBS spectra, respectively. It will help us to better understand the water cycle and provide a better reference for payloads such as ChemCam, SuperCam, and SHERLOC to determine the ferric sulfates on Mars. Key Points: Seven amorphous ferric sulfates with specific hydration degrees (6w–12w) were synthesizedWater quantification of amorphous and crystalline ferric sulfates is achieved using Raman and Laser‐induced breakdown spectroscopy spectroscopic methodsA new pathway of amorphous ferric sulfates formed by unsaturated solution is suggested to occur on Mars [ABSTRACT FROM AUTHOR]

Ordinary γ‐CaSO4 is a metastable calcium sulfate, while γ‐CaSO4 from the hyperarid region on Earth and from Mars has been found with abnormally high stability. In this study, we used multiple microanalyses to characterize the chemical and structural properties of two such γ‐CaSO4: one from Atacama soil (#10‐d30) and the other from Martian meteorite MIL03346,168. Silicon was determined to be quasi‐homogeneously distributed in Atacama γ‐CaSO4, while both silicon and phosphorus were detected in Martian γ‐CaSO4. We found the abnormally high stability of those γ‐CaSO4 from hyperarid environments was due to the chemical impurities which filled their structural tunnels and blocked the entrance of atmospheric H2O, with non‐detectable structural distortion. We propose that the γ‐CaSO4 with Si or Si and P impurities could have igneous origin or evaporative origin. Due to the extreme similarity in the structures of bassanite and γ‐CaSO4, their XRD patterns are almost non‐distinguishable; thus some martian "bassanite" minerals identified by Curiosity's CheMin instrument at Gale crater can actually be γ‐CaSO4. The structural tunnels in γ‐CaSO4 would allow ions and ionic groups to fill, thus providing meaningful insights about the geological and geochemical processes experienced by it during the formation and transformation. The Raman spectrometer carried by the Perseverance and by ExoMars rovers will help the selection of samples enriched in γ‐CaSO4 at Jezero Crater and Oxia Planum, which should be sampled for in‐depth analysis on Mars and back to Earth. Plain Language Summary: In the laboratory, ordinary γ‐CaSO4 would adsorb atmospheric H2O in minutes and transform to bassanite. However, the γ‐CaSO4 from the hyperarid region on Earth and from Mars has been found maintaining abnormally high stability for years. We used multiple microanalyses to characterize the chemical and structural properties of γ‐CaSO4 from Atacama soil and Martian meteorite MIL03346,168. We found that silicon is quasi‐homogeneously distributed in Atacama γ‐CaSO4, while both silicon and phosphorus were determined in Martian γ‐CaSO4. Our study demonstrated that abnormally high stability of γ‐CaSO4 was due to chemical impurities, which filled the structural tunnels that blocked the entrance of atmospheric H2O. Due to extreme similarity in the structures of bassanite and γ‐CaSO4, their XRD patterns are almost non‐distinguishable. We anticipate that some "bassanite" identified by Curiosity's CheMin instrument at Gale crater could actually be γ‐CaSO4. Our study shows that the structural tunnels in γ‐CaSO4 could be readily filled by ions and ionic groups, and even organic molecules as demonstrated by other studies. The Raman spectrometer carried by the Perseverance and ExoMars rovers would help the selection of samples enriched in γ‐CaSO4, for in‐depth analyses on Mars and return to Earth. Key Points: High stability of γ‐CaSO4 from hyperarid areas was found due to chemical impurities that filled tunnels and blocked entrance of H2OThe impure γ‐CaSO4 could have igneous and evaporative origins, significant for understanding geochemical processSome "bassanite" minerals identified by the CheMin instrument on Mars can actually be γ‐CaSO4 [ABSTRACT FROM AUTHOR]

Petrological and mineralogical analyses of Northwest Africa (NWA) 10720 indicate that this meteorite is one Martian nakhlite with an unbrecciated augitic igneous cumulate texture, consisting of clinopyroxene (65.4 vol.%) and olivine (19.4 vol.%) set in a fine‐grained intercumulus matrix. Texturally and compositionally, complex primary zoning in cumulus augite and the presence of oscillatory zoning in olivine phenocrysts indicate that NWA 10720 has experienced at least two‐stage magmatic activity before the eruption. The first primary parental magmas of nakhlites generated magnesian augite cores (Mg#62). A subsequent more evolved magma partly melted and resorbed augite cumulus, facilitating that olivine crystallized and augite growth resumed simultaneously and nearly at a final stage. Secondary mineral assemblages are composed of iddingsite‐like veins (~0.2 vol.%), threadlike maghemite, hematite veins, and altered pyrrhotite. Olivine alteration veins are likely to contain a higher proportion of saponite and low abundant serpentine in terms of major elements. Their compositions (Mg# = 13.5 and Fe/Si wt = 0.83) indicate that NWA 10720 appears to have encountered a weak acidic fluid (pH ~ 3.5–4.5). The lack of siderite also supports the acidic nature of the fluid. The absence of sulfates demonstrates the following: (1) The fluid permeating downwards through this nakhlite could have depleted SO42− when it reached the parental rocks; (2) the hydrothermal fluid leaching NWA 10720 parental rocks was no longer saturated in SO42− at the extremely hydrothermal system initiated by melting of buried water ice; and (3) sulfates might have been lost during terrestrial residence. These observations reveal that NWA 10720 could have experienced separate evolutionary pathways in the shallow Martian surface with extensive liquid fractionation activities, different from secondary alteration processes of the Lafayette subgroup in an open‐system environment. Various Fe‐oxide species and high abundant hematite veins demonstrate that the oxidization and dehydration might dominate late‐stage alteration once the exhaustion of parental fluid. [ABSTRACT FROM AUTHOR]

Chloride salts have been identified on the surface of Mars and Europa based on the results from the two soil samples analyzed by Phoenix's wet chemistry laboratory for Mars, as well as some interpretations of the data from the remote sensing for Mars and Europa. To help elucidate our understanding of Mars' and Europa's surface mineral composition and relevant environmental conditions, we studied a series of magnesium chlorides with all six hydration degrees that were synthesized using various laboratory conditions. The identifications of synthesized magnesium chloride hydrates were confirmed using X‐ray diffraction. We took a systematic study of the Raman, mid‐infrared (MIR), visible near‐infrared (VNIR), and near‐infrared (NIR) spectroscopies. We found various spectral features that can be linked to the structural characters of H2O molecules in the six crystal structures. On the basis of those knowledge, we found several trends of spectral feature variations that follow the increase of hydration degree of these magnesium chlorides, which can be used to distinguish them during the planetary exploration missions using the Raman, MIR, NIR, and VNIR spectroscopic techniques. Our data will not only fill the gap in the libraries of planetary spectroscopy but also contribute to the interpretation of Mars' and Europa's current and future exploration data. Specifically, identifying the types of Cl salts (hydrous or anhydrous) within geological context at the surface of Mars and Europa will constrain the current and past processes that they experienced, which can help in understanding the evolutions of these planetary bodies. [ABSTRACT FROM AUTHOR]