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For the first time on Mars, the crystalline magnesium‐sulfate mineral starkeyite (MgSO4‧4H2O) was definitively identified using the CheMin X‐ray diffraction instrument at Gale crater. At the Canaima drill site, starkeyite along with amorphous MgSO4‧nH2O are among the "polyhydrated Mg‐sulfates" interpreted in orbital reflectance spectra. Mg‐sulfates are good climate indicators as they are very responsive to changes in temperature and relative humidity. We hypothesize that, through evaporation, Mg‐sulfates formed at the end of brine evolution when ion concentrations became saturated and precipitated on the surface or near sub‐surface as either epsomite or meridianiite. These minerals were subsequently dehydrated later to starkeyite and amorphous MgSO4‧nH2O in response to a drier Mars. At Canaima, starkeyite is stable and would form during the warmer Mars summers. Due to very slow kinetics at the low Mars winter temperatures, starkeyite and amorphous MgSO4‧nH2O would be resistant to recrystallize to more hydrous forms and thus likely persist year‐round. During the course of analyses, starkeyite transforms into amorphous MgSO4‧nH2O inside the rover body due to the elevated temperature and greatly reduced relative humidity compared to the martian surface at the Canaima drill site. It is possible that crystalline sulfate minerals existed in earlier samples measured by CheMin but altered inside the rover before they could be analyzed. Starkeyite is most likely prevalent in the subsurface, whereas both starkeyite and amorphous MgSO4‧nH2O are likely present on the surface as starkeyite could partially transform into amorphous MgSO4‧nH2O due to direct solar heating. Plain Language Summary: For the first time on Mars, starkeyite (MgSO4‧4H2O), one of the many possible naturally occurring Mg‐sulfate minerals, has been identified using instruments that determine the mineralogy and chemistry of samples on the Curiosity rover in Gale crater. Minerals form in response to conditions they are subjected to and can be used to infer temperature, pressure, water availability, and other factors that will influence their formation. As such, their presence can be used to help define current and past climate conditions on Mars. When Mg‐sulfate minerals first precipitate from brine that is evaporating on the surface, they form more hydrous varieties. In response to drier conditions, they will recrystallize to Mg‐sulfate forms with less water in them. When Mg‐sulfate minerals are desiccated under extremely dry conditions, they will lose their crystal structure and form an amorphous phase. This occurred inside the rover body during the analysis of the Canaima sample. It is likely that earlier samples may also have contained crystalline Mg‐sulfates but altered to an amorphous phase while in the rover body before analyses could take place. Mg‐sulfate in the subsurface is likely starkeyite but could partially transform on the surface to amorphous MgSO4‧nH2O due to direct solar heating. Key Points: For the first time on Mars, starkeyite (MgSO4‧4H2O) was positively identifiedStarkeyite along with amorphous MgSO4‧nH2O are the "polyhydrated Mg‐sulfates" interpreted in orbital reflectance spectraMg‐sulfate mineralogy serves as an indicator of transition to a drier climate and helps outline the climatic and depositional changes on Mars [ABSTRACT FROM AUTHOR]

Hydrothermal high sanidine and specular hematite are found within ferric‐rich and gray‐colored cemented basaltic breccia occurring within horizontal, weathering‐resistant strata exposed in an erosional gully of the Pu'u Poliahu cinder cone in the summit region of Maunakea volcano (Hawai'i). The cone was extensively altered by hydrothermal, acid‐sulfate fluids at temperatures up to ~400 °C, and, within strata, plagioclase was removed by dissolution from progenitor Hawaiitic basalt, and sanidine and hematite were precipitated. Fe2O3T concentration and Fe3+/∑Fe redox state are ~12 wt.% and ~0.4 for progenitor basalt and 46–60 wt.% and ~1.0 for cemented breccias, respectively, implying open‐system alteration and oxic precipitation. Hydrothermal high sanidine (adularia) is characterized by full Al,Si structural disorder and monoclinic unit‐cell (Rietveld refinement): a = 8.563(19) Å, b = 13.040(6) Å, c = 7.169(4) Å, β = 116.02(10)°, and V = 719.4(19) Å3. Hematite (structure confirmed by Rietveld refinement) is the predominant Fe‐bearing phase detected. Coarse size fractions of powdered hematite‐rich breccia (500–1000 μm) are dark and spectrally neutral at visible wavelengths, confirming specular hematite, and SEM images show platy to polyhedral hematite morphologies with longest dimensions >10 μm. Smectite and 10‐Å phyllosilicate, both chemically dominated by Mg as octahedral cation, are additional diagenetic hydrothermal alteration products. By analogy and as a working hypothesis, high sanidine (Kimberly formation) and specular hematite (Mt. Sharp group at Hartmann's Valley and Vera Rubin ridge) at Gale crater are interpreted as diagenetic alteration products of Martian basaltic material by hydrothermal processes. Plain Language Summary: A layer of weathering‐resistant material is located within the walls of an erosional gully of the Pu'u Poliahu cinder cone in the summit region of Maunakea volcano (Hawai'i). The volcanic cone, initially composed of unaltered basaltic material (tephra), was extensively altered throughout by hot, sulfuric‐acid solutions. The layer is a location where the alteration by hot water was particularly aggressive, cementing the volcanic sediment and causing extensive chemical and mineralogical changes. Instead of basaltic chemical and mineralogical compositions, altered tephra was enriched in iron from aqueous precipitation of the mineral hematite (Fe2O3) and was characterized by high sanidine with full structural disorder as the feldspar (instead of plagioclase, which was removed by dissolution) and by Mg‐rich phyllosilicates as additional precipitation products. Hematite, often present as a red pigment in geologic materials, was precipitated from the hot water as specular (i.e., gray) hematite. By analogy, high sanidine and specular hematite at Gale crater (Mars) can be interpreted as alteration products of preexisting Martian basaltic sediment by hot‐water solutions. Key Points: High sanidine (full Al,Si structural disorder) and specular hematite hydrothermally precipitated on basaltic Maunakea volcano, Hawai'iSmectite and 10‐Å phyllosilicate (both Mg‐rich) also hydrothermally precipitated as diagenetic phasesHigh sanidine and specular hematite at Gale crater can result from diagenetic alteration of basaltic compositions by hydrothermal fluids [ABSTRACT FROM AUTHOR]

Vera Rubin ridge (VRR) is an erosion‐resistant feature on the northwestern slope of Mount Sharp in Gale crater, Mars, and orbital visible/shortwave infrared measurements indicate it contains red hematite. The Mars Science Laboratory Curiosity rover performed an extensive campaign on VRR to study its mineralogy, geochemistry, and sedimentology to determine the depositional and diagenetic history of the ridge and constrain the processes by which the hematite could have formed. X‐ray diffraction (XRD) data from the CheMin instrument of four samples drilled on and below VRR demonstrate differences in iron, phyllosilicate, and sulfate mineralogy and hematite grain size. Hematite is common across the ridge, and its detection in a gray outcrop suggest localized regions with coarse‐grained hematite, which commonly forms from warm fluids. Broad XRD peaks for hematite in one sample below VRR and the abundance of FeOT in the amorphous component suggest the presence of nanocrystalline hematite and amorphous Fe oxides/oxyhydroxides. Well crystalline akaganeite and jarosite are present in two samples drilled from VRR, indicating at least limited alteration by acid‐saline fluids. Collapsed nontronite is present below VRR, but samples from VRR contain phyllosilicate with d(001) = 9.6 Å, possibly from ferripyrophyllite or an acid‐altered smectite. The most likely cementing agents creating the ridge are hematite and opaline silica. We hypothesize late diagenesis can explain much of the mineralogical variation on the ridge, where multiple fluid episodes with variable pH, salinity, and temperature altered the rocks, causing the precipitation and crystallization of phases that are not otherwise in equilibrium. Plain Language Summary: Vera Rubin ridge (VRR) is an erosion‐resistant feature on the northwestern slope of Mount Sharp in Gale crater, Mars, and it contains the Fe (III) oxide mineral hematite. Hematite is an especially important mineral to study on the Martian surface because it commonly forms in the presence of liquid water. Identifying the processes that formed hematite on VRR can help constrain the history of liquid water in Gale crater. The Mars Science Laboratory Curiosity rover performed an extensive campaign on VRR to determine the geologic history of the ridge. Mineralogical data collected by the CheMin instrument of four rock samples drilled on and below the ridge demonstrate changes in the mineralogy and provide clues about the aqueous history of VRR. Red hematite is common across the ridge, but detection of gray hematite in one sample indicates the presence of localized coarse‐grained hematite, which commonly forms under warm temperatures. Minerals that form in acidic, saline solutions were found in two samples, indicating localized alteration in acidic and relatively salty waters. We hypothesize that multiple episodes of groundwater with variable pH, salinity, and temperature altered the sediments after they lithified. Key Points: Data from the CheMin X‐ray diffractometer demonstrate a variety of secondary alteration products on Vera Rubin ridgeHematite particle size changes across and below the ridge, and this change may be a marker of diagenetic reactionThe aqueous history of Vera Rubin ridge was complex, with several fluid episodes that varied in temperature, salinity, and pH [ABSTRACT FROM AUTHOR]