Characterization of ferric hydroxysulfate on Mars and implications of the geochemical environment supporting its formation

Domain: earth_science
Published: 2025-08-05
Editorial outcome: major_revision

Abstract
Sulfate minerals are significant components of the martian surface and provide clues about the martian geochemical environment. One unusual Fe-sulfate phase has been intriguing Mars scientists for over a decade due to its unique spectral bands that are distinct from any known minerals and its occurrence in layered sedimentary rocks. We describe here detection of ferric hydroxysulfate (Fe3+SO4OH) and its implications for the geochemical history of Mars. Crystalline ferric hydroxysulfate is formed by heating hydrous Fe2+ sulfates to 100 °C or above and has a strong spectral band at 2.236 µm, similar to the spectral feature observed on Mars at Aram Chaos and on the plateau above Juventae Chasma. Hydrated sulfates at these locations likely formed through evaporative processes or low-temperature alteration. In contrast, Fe3+SO4OH is more consistent with heating and oxidation of hydrated ferrous sulfates, potentially through deposition of lava, ash, or through hydrothermal processes.

Introduction
The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument in orbit at Mars has provided hyperspectral data for mapping numerous minerals that guide our understanding of the ancient geochemical history of Mars. A variety of sulfate minerals have been identified on Mars from orbital (e.g. refs. 1, 2) and landed (e.g. refs. 3–5) missions through comparison with terrestrial minerals using spectral parameters, X-ray diffraction, and elemental abundances. An unusual spectral band at 2.236 µm was discovered in CRISM spectra of Mars at the plateau bordering Juventae Chasma 6 and in Aram Chaos 7 and has provided a challenge for mineral identification for over 15 years because this spectral band is not consistent with any known minerals. Preliminary lab investigations proposed dehydrated iron sulfates as the source of this unexplained material 6–8. Recent advances in lab experiments on iron sulfates and improvements in the calibration of CRISM images are enabling identification and characterization of these intriguing martian outcrops, which we discuss in this study.

Results
Investigation of martian outcrops with unique ~2.23 µm spectral band

Characterizing these outcrops with unusual spectral signatures in CRISM images has benefitted from improved processing techniques 9 and mapping algorithms 10. These remote sensing advances have uncovered previously unknown outcrops on Mars through analyses of smaller spot sizes with clearer spectra e.g. ref. 11. Analyses of CRISM images at the Juventae Plateau and Aram Chaos using these new techniques revealed additional outcrops of units containing this unique ~2.23 µm band along with other spectral features (Figs. 1 and 2). At the Juventae Plateau region, the outcrop containing the ~2.23 µm band is observed mixed with hydration bands similar to polyhydrated sulfates, and High Resolution Imaging Science Experiment (HiRISE) images show that thin units containing the 2.23 µm band occur primarily stratigraphically above the polyhydrated sulfate unit and in some cases also below the polyhydrated sulfate unit (Fig. 1, Supplementary Fig. S1, Supplementary data 1). Because these units are only 2-3 CRISM pixels wide in many cases (~40–50 m), spectra of the 2.23 µm band unit likely include a mixture of that phase and the polyhydrated sulfate unit below it. We analyzed the CRISM spectra of this 2.23 µm band material in comparison with lab spectra of hydrated sulfates and several species having bands somewhat near 2.23 µm. These include montmorillonite (Na0.3Al2(Si4O10)(OH)2·nH2O) with a band at 2.21 µm, nontronite (Na0.3Fe2(Si4O10)(OH)2·nH2O) with a band at 2.29 µm, gypsum (CaSO4·2H2O) with a doublet at 2.22 and 2.26 µm, and jarosite (KFe3(SO4)2(OH)6) with a band at 2.26 µm and shoulders at 2.22 and ~2.3 µm. None of these minerals are consistent with the CRISM spectra of this outcrop having a single band near 2.23 µm. In addition to this enigmatic 2.23 µm band, this outcrop contains features near 1.43, 1.94, and 2.4 µm that are consistent with polyhydrated sulfates including rozenite (FeSO4·4H2O) and epsomite (MgSO4·7H2O) (Fig. 1d).

Outcrops containing a band near 2.23 µm in CRISM spectra are also observed at several locations at Aram Chaos. The units containing this unique spectral feature are mapped in red (Fig. 2, Supplementary Figs. S2, S3, and Supplementary data 1) and occur as small patches, but span several CRISM pixels, enabling improved isolation of the spectral properties of this material compared to the Juventae Plateau region. The 2.236 µm band is narrower in these spectra and is accompanied by additional sharp features at 1.48, 1.82, 2.19, and 2.37 µm that are attributed to ferric hydroxysulfate (Fig. 2). In Aram Chaos this 2.23 µm band material is most commonly observed adjacent to and beneath monohydrated sulfate outcrops mapped in green. Spectra of the monohydrated sulfate minerals szomolnokite (FeSO4·H2O) and kieserite (MgSO4·H2O) both have a band at 2.40 µm and another band near 2.1 µm that varies with composition. This band occurs at 2.09 µm in spectra of szomolnokite and at 2.13 µm in spectra of kieserite. Some of the monohydrated sulfate outcrops also include weak bands near 2.23 µm, indicating partial alteration to form a mixed phase containing some monohydrated sulfate and some Fe3+SO4OH (Fig. 3). The polyhydrated sulfate spectra contain bands near 1.44 and 1.93-1.95 µm and a drop in reflectance near 2.42 µm (Fig. 3), similar to spectra of rozenite and starkeyite (MgSO4·4H2O).

Formation of Fe3+SO4OH in the lab

The potential mineral Fe3+SO4OH is formed in the lab by heating dry powders of hydrated ferrous sulfates at temperatures from 100 to 300 °C 12,13. Several Fe3+SO4OH samples were produced in the lab (Table 1) and all include several narrow features due to vibrational bands, overtones, and combinations at 1.49, 1.83, 2.19, 2.236, 2.37, 2.61 and 2.89 µm. Additional Fe electronic absorptions produce a band near 0.94–0.96 µm that is broadened towards ~0.8 µm with a reflectance maximum near 0.7 µm, a drop in reflectance near 0.5 µm, and a narrow band at 0.43 µm, consistent with Fe3+ (Table 2, Fig. 3, Supplementary data 2). These vibrational and electronic bands are distinct and readily distinguishable from the spectral properties of rozenite and szomolnokite e.g. refs. 6, 14. Rozenite is characterized by Fe2+ bands near 0.98 and 1.18 µm with a reflectance maximum near 0.56 µm, while the Fe2+ band for szomolnokite occurs at ~0.94 µm with a broad reflectance maximum near 0.65–0.75 µm. The HOH stretching overtone occurs at 1.45 µm for rozenite and at 1.52 µm for szomolnokite, while the HOH stretch plus bend combination vibrations occur at 1.95 µm for rozenite and at 2.09 µm for szomolnokite. The H2O vibrations observed for rozenite are similar to those observed for many types of polyhydrated sulfates, in contrast to the constrained H2O sites for the monohydrated sulfates szomolnokite and kieserite that are shifted to longer wavelengths e.g. refs. 15, 16. This difference in the position of the H2O overtones and combination bands in hydrated sulfates enables clear discrimination of monohydrated versus polyhydrated sulfates on Mars 6. Additional bands are observed for szomolnokite at 2.40 and 2.62 µm and for rozenite near 2.42 and 2.53 µm. The bands near 2.4 and 2.5 µm are often broadened in the spectra of polyhydrated sulfates such that only a drop in reflectance near 2.4 µm can be detected in remote sensing data.

Reflectance spectra of Fe3+SO4OH were also measured under vacuum (Supplementary Fig. S4) and low-temperature (Supplementary Fig. S5) conditions to investigate changes in this potential new mineral under different environments more relevant to Mars. Overall, Fe3+SO4OH appears stable under these conditions and only small shifts in a couple of bands were observed (Table 2). Additionally, the spectra measured under vacuum are very similar to those measured under controlled, dry conditions, thus providing confidence that purging the sample of H2O overnight in the chamber before measurement adequately removes adsorbed water molecules.

A synthetic monohydrated sulfate with 50% Mg and 50% Fe2+ cations 17 was also heated for comparison with the Fe3+SO4OH produced from heating szomolnokite and rozenite. This dehydrated Mg0.5Fe2+0.5-MHS sample contains many bands similar to those of Fe3+SO4OH (Table 2), but shifted slightly, as well as a band near 2.09 µm due to the unreacted Mg0.5Fe2+0.5-MHS. Importantly, the OH combination band observed at 2.236 µm in spectra of Fe3+SO4OH, is shifted to 2.226 µm for Fe3+0.5Mg2+0.5SO4OH/H2O. Because the presence of Mg in this sample appear to have disrupted the deprotonation process, we characterized the reaction of szomolnokite to Fe3+SO4OH further.

The Fe3+SO4OH crystal structure includes octahedrally coordinated Fe3+O6 units with tetrahedrally coordinated SO4 groups on opposite O atoms (~180 degrees apart) and bridging OH groups connecting the Fe3+ octahedra to form a linear configuration (Fig. 3c). This is similar to the structure of szomolnokite that has octahedrally coordinated Fe2+ instead of Fe3+ and two protons on bridging octahedral O atoms to form H2O molecules instead of OH. Vibrations of these H2O and OH groups in the structure of szomolnokite and Fe3+SO4OH, respectively, are responsible for the broad H2O combination band near 2.1 µm and the narrow OH combination band at 2.236 µm. The structure of Fe3+SO4OH and the strength of the bond connecting the OH group to the Fe3+ cation govern the vibrational energy of the OH group. The OH fundamental stretching vibration occurs at 3460 cm−1 or 2.89 µm (Table 2, Supplementary Fig S4). Jarosite also has OH groups bound to Fe3+ cations in an octahedral configuration, but the OH stretching vibrations occur near 3385 cm−1 or 2.95 µm for K+-jarosite and 3360 cm−1 or 2.98 µm for Na+-jarosite 18. Presumably, the additional Na+ or K+ cations in the jarosite structure are pulling some of the electron density away from the OH bond, thus reducing the vibrational energy and shifting the bands towards longer wavelengths compared to the OH vibrations in ferric hydroxysulfate. In contrast, the rozenite crystal structure 19 (Supplementary Fig. S6) forms isolated clusters of two Fe2+O6 octahedra linked to two SO4 tetrahedra via O atoms that are ~90 degrees apart in the Fe2+ octahedron. For rozenite, the O atoms not connected to sulfate groups are each bound to two protons to form four H2O molecules per Fe2+O6 octahedron. The colors of the <125 µm grain size powders of these iron sulfates are very pale green for rozenite, white for szomolnokite, and orange for Fe3+SO4OH (Supplementary Fig. S7). Sample JB1917 contains 27 wt.% Fe3+SO4OH and 73 wt.% szomolnokite and is a light tan color (Supplementary Fig. S7). Samples with larger grain sizes have deeper colors (not shown).

Experiments heating szomolnokite, rozenite, melanterite (Fe2+SO4·7H2O), and copiapite (Fe2+Fe3+4(SO4)6(OH)2·20H2O) in ambient conditions in the lab resulted in production of Fe3+SO4OH at elevated temperatures 6–8,13,20. Fe3+SO4OH was produced by heating rozenite or szomolnokite at 200 °C for 26 h (Table 1) or melanterite to 240 °C for 18 h 7. Rozenite heated at 30 min intervals at 50 °C, 100 °C, then 150 °C produced some Fe3+SO4OH and continued heating at 50 °C intervals to 300 °C resulted in strong Fe3+SO4OH spectral signatures (Supplementary Fig. S8). In contrast, interval heating of copiapite produced some Fe3+SO4OH by 200 °C and more Fe3+SO4OH by 300 °C, and interval heating of szomolnokite did not result in formation of Fe3+SO4OH until 300 °C (Supplementary Fig. S8). Reaction of polyhydrated sulfates at lower temperatures than szomolnokite suggests that H2O facilitates the transformation to Fe3+SO4OH.

Slower reaction kinetics were evident at lower temperatures in a series of experiments with rozenite and szomolnokite (see Table 1 for a summary of all Fe sulfate reactions). Szomolnokite heated at 150 °C for 56 hours produced 93 wt.% Fe3+SO4OH. Rozenite heated at 100 °C for 6 days resulted in 81 wt.% Fe3+SO4OH, but only 27 wt.% Fe3+SO4OH resulted after 8 days starting from szomolnokite. Heating of rozenite at 50 °C resulted in dehydration of ~90% of the sample to szomolnokite after 7 days and heating of the rozenite sample by the FTIR beam in wide aperture mode resulted in partial formation of szomolnokite at the surface. No evidence of Fe3+SO4OH was observed for these lower temperature reactions.

Additional heating experiments of szomolnokite in sealed vials containing nitrogen gas or air demonstrated that oxygen is required for reaction of these hydrated ferrous sulfates to form ferric hydroxysulfate 13 (Supplementary S1/Supplementary Table 1). Gas pressures were recorded at the beginning and end of each experiment. Initial gas pressures ranged from 1.29 to 1.36 bars and were decreased in the reactions where O2 was consumed and Fe3+SO4OH was formed. Characterization of the solid phase by X-ray diffraction and the gas phase by gas chromatography before and after the heating experiment indicates that the formation of Fe3+SO4OH proceeds via the reaction in Eq. (1).

4Fe2+SO4·H2O+O2 → 4Fe3+SO4OH+2H2O (1)

Rietveld refinement supports formation of C2/c monoclinic Fe3+SO4OH (Fig. 4, Supplementary Fig. S9). Comparison of the crystal structures of Fe3+SO4OH and szomolnokite 17,21 suggests that the transformation is associated with a loss of a hydrogen atom bonded to the bridging oxygen likely occurring coincidently with the oxidation of Fe2+ to Fe3+. The structure remains largely intact throughout the transformation; however, the polyhedral units rotate and distort to accommodate the changes in charge distribution (Fig. 3c), where the increased charge on the Fe atom is compensated by the negative charge on the hydroxyl group. Heating experiments with kieserite were not successful in creating a stable hydroxide phase, likely because Mg does not exist in trivalent form. This is also reflected in the mixed cation monohydrated sulfate samples heated at 300 °C that only showed incomplete transformation to the hydroxide phase.

Morphologies of ferric hydroxysulfate and associated units

Evaluating the morphologies and stratigraphies of the sulfate outcrops at the Juventae Plateau and Aram Chaos enables the determination of relationships among the different sulfate units. Spectra at the Juventae Plateau were collected from ~30 to 35 m thick light-toned layered deposits that exhibit spectral signatures consistent with polyhydrated sulfates and Fe3+SO4OH, plus pyroxene-bearing units (Figs. 1 and 5, Supplementary Fig. S1). Thin units containing spectral features consistent with polyhydrated sulfates and Fe3+SO4OH are mapped in blue and red, respectively (Figs. 1 and 5). The stratigraphy of the outcrops shows a pyroxene-bearing substrate below the light-toned layered materials termed basalt-1 (dark cyan) and a different pyroxene-bearing caprock unit covering the Fe sulfates termed basalt-2 (medium green) (Supplementary Fig. S1). CRISM spectra of the basalt-1 material include spectral properties most similar to augite (high-Ca pyroxene) with a reflectance maximum near 1.65 µm and a band centered near 2.35 µm, while the basalt-2 spectra likely contain some pigeonite (pyroxene enriched in Mg and Fe) due to shifts in the reflectance maximum and the ~2 µm band toward shorter wavelengths. Morphologies of these primary four units indicate that the pyroxene bearing substrate (dark cyan in both CRISM and HiRISE color) is flatter and appears clean (i.e., minimal dust and sand) with extensive polygonal fracturing, whereas the pyroxene-bearing caprock (medium green in CRISM and brown in HiRISE color) is partially composed of linear aeolian ripples and appears hilly and uneven in topography due to differential erosion. The textures of the polyhydrated sulfate and Fe3+SO4OH/polyhydrated sulfate units are distinct from those of the pyroxene-bearing units, but appear similar to each other with fine-scale layering that varies in brightness, color, and fracturing. The Fe3+SO4OH unit is observed overlying the polyhydrated sulfate units in the stratigraphy and in some cases it is also present along the base of the light-toned materials as well. Thus, there may have been two different times when the Fe3+SO4OH unit formed.

At Aram Chaos, monohydrated sulfates are the most abundant sulfate in the two CRISM images where Fe3+SO4OH is present and are mapped in green (Figs. 2 and 6). The monohydrated sulfate units also exhibit variability in the band near 2.1 µm and are consistent with mixtures of szomolnokite and kieserite in some locations and of pure kieserite in other locations (Fig. 6, Supplementary Figs. S2, S3). Several patches of Fe3+SO4OH are observed adjacent to and beneath the monohydrated sulfate outcrops, with some closer to kieserite-type outcrops and others next to mixed monohydrated sulfate outcrops. Variations are also observed in the polyhydrated sulfate units that are mapped in blue, with some outcrops exhibiting stronger hydration signatures. The stratigraphy indicates that polyhydrated sulfate units are observed at higher elevations than the monohydrated sulfates and Fe3+SO4OH units, and in some cases pyroxene-bearing basalt is observed below the sulfates within the chaos blocks, although it is much rarer than the basalt units at the Juventae Plateau.

The Fe3+SO4OH unit occurs along the floor of Aram Chaos and is stratigraphically the lowest sulfate unit relative to the monohydrated and polyhydrated sulfates. The Fe3+SO4OH unit is medium-toned in brightness and exhibits polygonal fracturing and ridges, which can be either circular or linear, compared to the much brighter and scalloped fracturing observed in the monohydrated sulfate units. The monohydrated sulfate units can also be more jagged and hilly compared to the flatter and subdued morphology of the Fe3+SO4OH unit. The only difference in morphology between the kieserite-type outcrops and the szomolnokite-kieserite outcrops is that the kieserite-type outcrops are slightly darker and appear mantled by loose debris and aeolian ripples. The polyhydrated sulfate unit is generally medium-toned in brightness relative to the other sulfates, which may be due to the widespread ripples and debris covering its surface. The unit is relatively flat, forming broad plateaus, but steeper cliffs along specific layers within the polyhydrated sulfate unit correspond to bright-toned, heavily fractured, and jagged outcrops. The morphology of the Fe3+SO4OH and polyhydrated sulfate units are much easier to distinguish at Aram Chaos relative to the same units at Juventae Chasma.

Methods
CRISM image analysis

CRISM images can be found at the Planetary Data System (PDS) Geosciences server under Mars Orbital Data Explorer: https://ode.rsl.wustl.edu/mars/. We used CRISM images FRT00005814, FRT000098B2, and HRL0000646A for this study. Mineral outcrops were investigated using advanced calibration versions 9 of these CRISM images that provide improved resolution of small outcrops and offer cleaner spectra that facilitate identifying specific minerals This algorithm includes simultaneous atmospheric correction and denoising of CRISM images in the 1.0–2.6 µm spectral range that removes most of the residual atmospheric bands and spurious noise 9.

We applied a recently developed algorithm that leverages the features learned by Generative Adversarial Networks (GAN) trained on hyperspectral data from CRISM 10. This feature set has shown the ability to discriminate among different spectral shapes. CRISM maps of pyroxene, polyhydrated sulfates, monohydrated sulfates, Fe3+SO4OH, and a mixed phase containing a ~2.23 µm band and polyhydrated sulfate-type features were used to guide acquisition of spectra for minerals in the scene. Spectra were collected without ratioing the spectra of different spot sizes. For the thin polyhydrated sulfate/Fe3+SO4OH units at the Juventae Plateau 3×3 pixel regions were used, while larger 5×5 and 10×10 pixel regions were used for the pyroxene outcrops at the Juventae Plateau and most regions at Aram Chaos. CRISM spectra collected in this way and used for this study are available in Supplementary data 1.

Blended views of CRISM, HiRISE, CTX, and HRSC imagery

CRISM mineral detections are merged with Context (CTX) and HRSC images over HRSC DTMs for regional stratigraphic views and over HiRISE images and DTMs for local stratigraphic views. ArcGIS software (ESRI) was used to overlay CRISM mineral maps over Mars Reconnaissance Orbiter (MRO) HiRISE images at ~30 cm/pixel surface resolution 37 with coordinated mosaics of MRO CTX images at ~6 m/pixel surface resolution 38 and Mars Express HRSC images at ~10 m surface resolution 39,40. The HRSC DTM has a grid size of 50 m and the HiRISE DTM has a grid size of 1 m. The HiRISE DTM was generated using methods developed by Kirk et al. 41 and McEwen et al. 42.

Lab experiments with hydrous ferrous sulfates

Synthesis of the rozenite 43 and szomolnokite 44 samples were loosely based on previous methods. Rozenite was formed by the dehydration of fine-grained FeSO4·7H2O reagent powder at room temperature (~22 °C) in a dry (i.e., 33% relative humidity) atmosphere maintained by a saturated MgCl2 solution 45 in air over 7 days. Szomolnokite was obtained by dehydrating FeSO4·7H2O reagent powder over four days at 60 °C using the same humidity-buffer conditions. These samples are described in Table 1.

The phase identity and purity of the obtained sample materials and their heating products were confirmed by X-ray diffraction (Fig. 4, Supplementary Fig. S9). X-ray diffraction patterns were acquired on a Rigaku Smartlab diffractometer in Bragg-Brentano mode using Cu-Kα1,2 radiation and D/teX Ultra 250 1D silicon strip detector. Diffraction patterns were acquired in the 5–65 °2θ range with a step size of 0.01° and a maximum scan speed of 1.5°/min. The instrument profile parameters were derived from a NIST 640c silicon standard and all Rietveld refinements were performed using GSAS-II 46. The Rietveld refinements for the heating products of rozenite and szomolnokite are shown in Fig. 4.

Heating experiments were performed at temperatures of 50, 100, 200 °C for rozenite, and 100, 150, 200 and 250 °C for szomolnokite. The compositions of these samples are listed in Table 1.

Heating experiments in closed vials containing nitrogen gas and air

1.0 g of szomolnokite, synthesized and analyzed for phase purity following the workflow detailed above, were transferred into four individual vials of a volume of 155 ml. Three vials (two containing mineral, one empty control) were sparged with nitrogen gas for 2 min, whereas the other three contained ambient air as a top gas. The vials were then closed with a rubber stopper and sealed.

Using a Keller LEO 2 manometer adapted to fit a 25G disposable needle for insertion through the rubber stopper into vials, we observed a drop in pressure inside the vial during preliminary experiments, thus 50 ml of the respective top gas were injected into the vials using a 100 ml gas-tight Hamilton syringe before heating to end up with a pressure exceeding ambient atmospheric pressure. This allowed for gas sampling and analysis of O2 composition at the end of the experiment. The vial pressures were measured before and after the heating cycle using the manometer. This was followed by the measurement of the oxygen concentration in the air vials on an SRI 8610C Gas Chromatographer (GC) equipped with a HaySep D column set to a temperature of 80 °C and a thermal conductivity detector. Nitrogen was used as the carrier gas with a flow rate of 29 ml/min. A quantification curve for oxygen was built by performing triplicate injections with 50 µl and 250 µl Hamilton gas-tight syringes fitted with removable 22s gauge needles of 10, 25, 50, 100, and 250 μl volumes of a research-grade (99.999% purity) oxygen standard stored in a 155 ml serum bottle to emulate experimental conditions. This achieved standard concentrations of 4, 10, 20, 40 and 100% O2 respectively. 250 μl of the experimental vial top gas was sampled in triplicate for analysis on the GC. The analytical error for this analysis is <1% and the experiment details are summarized in Supplementary S1/Supplementary Table 1.

The samples were heated on a hot plate by increasing the temperature by 50 °C every 10 min from 100 to 250 °C, followed by 20 min each at 280, 300 °C and 30 min at 320 °C. The same holding times and temperature steps were used when the hot plate temperature was decreased to 100 °C. Then the hot plate was turned off with the vials still on the hot plate for 10 min, after which they were removed to allow them to cool down to room temperature prior to measuring the vial pressure.

This was followed by another quantification of the oxygen-concentration in the air-filled vials using the workflow stated above. Lastly, all vials were opened, and the solid heating product was analyzed by means of X-ray diffraction on the Rigaku Smartlab diffractometer using the data acquisition parameters described in detail in the previous section.

Reflectance spectroscopy measurements of Fe3+SO4OH

SETI institute. Spectra were collected from 0.35 to 2.5 µm relative to Spectralon under ambient conditions using an Analytical Spectral Devices (ASD Inc; now part of Malvern Panalytical) FieldSpec Pro FR instrument to check progress of the reactions in our experiments. The spectral resolution is 3 nm from 0.35 to 1.0 µm and 10 nm from 1.0 to 2.5 µm. Spectra were acquired with a contact probe on particulate samples in a black Teflon dish.

Reflectance Experiment Laboratory (RELAB) at Brown University. Spectra were measured under dry air with a diffuse gold standard from ~1 to 100 µm using a Thermo Nexus 870 FTIR instrument equipped with a tungsten-halogen lamp and a CaF2 beam splitter (NIR), a globar source, a KBr beam splitter (MIR-FIR), and Deuterated TriGlycine-Sulfate (DTGS) detectors with spectral resolutions of 2 cm−1 (NIR) and 4 cm−1 (MIR-FIR) as in recent studies, e.g. ref. 47. The samples were placed in the sample chamber for ~12 h under N2 in order to remove H2O and CO2 adsorbed on the surface of the grains or in the air above them, e.g. refs. 48, 49. These data were scaled near 1.2 µm to bidirectional VNIR spectra recorded from 0.3 to 2.5 µm relative to Spectralon at 5 nm spectral sampling under ambient conditions as in previous studies, e.g. ref. 50. The spectrum of rozenite burnt by the FTIR beam was achieved using the globar source with an aperture size of 150 for 90 min. Only the upper few grains of the sample surface were affected. The change in spectral features of the rozenite sample including formation of bands consistent with szomolnokite occurred together with a darkening spot on the surface of the sample. Spectral measurements of rozenite samples with an aperture size of 55 or smaller did not change color or spectral properties. RELAB spectra of the samples discussed in this study are included in Supplementary data 2.

Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) at the University of Grenoble Alpes (UGA). Spectra were measured from ~0.4 to 4.2 µm, relative to Spectralon below 1.3 µm and Infragold above that, using the SHINE bidirectional spectro-goniometer 51. All spectra have been acquired with nadir illumination and an emission angle of 20°. Spectral sampling was 10 nm with a spectral resolution varying between 5 and 38 nm. Measurements were carried out within the CARBONIR environmental chamber under ~10−4 mbar vacuum at room temperature and under 12 mbar pure N2 gas at lower temperatures down to 189 K at 30 K intervals, as in previous studies 52, 53.

Planetary Spectroscopy Laboratory (PSL) at the German Aerospace Center (DLR) in Berlin. Spectra were measured from ~1 to 25 µm using a Bruker FTIR instrument under ~10−3 torr vacuum relative to a rough gold surface, as in previous studies 52.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper describes the detection of an absorption band at 2.236 µm in Aram Chaos and Juventae Plateau using the CRISM instrument. The authors attribute this band to ferric hydroxysulfate (Fe3+SO OH) and demonstrate it through the synthesis and spectral characterization of this species in the laboratory. Fe3+SO OH is thus detected alongside other poly- and monohydrated sulfates, forming an upper horizon in Juventae Plateau and occurring beneath the MHS/PHS in Aram Chaos. The authors interpret its presence as a result of thermal processing of sulfate-containing outcrops, dehydrating and oxidizing them through contact with an overlying lava layer (Juventae Plateau) or hydrothermal circulation (Aram Chaos). Fe3+SO OH would have formed relatively late, during the Amazonian (<3 Ga), suggesting that the Martian surface remained active longer than previously thought.\n\nThe article is well-written, and the attribution of the 2.236 µm absorption band to Fe3+SO OH is convincing, as is the proposed interpretation, considering the formation method of this species and its distribution relative to MHS and PHS.\n\nAs shown in Table ED1, Fe3+SO OH is the fully dehydrated form of ferric hydroxysulfates and can rehydrate into butlerite (Fe3+SO OH·H O), which loses the 2.236 µm band. Other species of ferric hydroxysulfates exist with different hydration levels (fibroferrite, amarantite, or coquimbite), which can form through precipitation in an oxidized, sulfate-rich acidic fluid. On Earth, such compounds are observed in industrial and mining effluents released into the environment, for example, along with schwertmannite, a poorly crystalline ferric oxyhydroxysulfate commonly found in these settings. Could a similar formation scenario be envisioned on Mars, involving deposition in an acidic environment followed by dehydration? The article proposes thermal treatment of ferrous sulfates to dehydrate and oxidize them, but could an aqueous formation followed by simple dehydration under Mars' arid atmosphere be possible? Schwertmannite lacks diagnostic near-IR signatures and would not be excluded by CRISM data.\n\nA secondary remark: the 2.236 µm band supports most of the discussion on the detection of Fe3+SO OH. It would be helpful to include a figure showing this band precisely in the CRISM data alongside candidate minerals so that the reader can be convinced of its correspondence with Fe3+SO OH. The CRISM spectra presented in this paper are systematically represented with dots, forming a very thick line that reduces the contrast of spectral signatures. Is this intended to represent the SNR of the data? Presenting the data differently would be beneficial to help the reader better assess their actual SNR and the uncertainty in band positions. For example, in spectrum 1 of Fig. 2g, the 2.236 µm band is shown with disjointed points, making it difficult to judge the shape and position of this band to confirm its correspondence with Fe3+SO OH.\n\nOther minor point, I would recommend to explicitly indicate the oxidation state of Fe in every mineral/species formula, either using the charge or the oxidation number.", "reviewer_2": "This manuscript describes the occurrence of an iron hydroxysulfate on Mars, documents its spectral characteristics, and matches it to a synthetic solid. The identification of this previously unidentified solid is the main strength of this manuscript. This study has used careful evaluation of spectral signatures from the CRISM instrument aboard the Mars Reconnaissance Orbiter, combined with lab testing of lab-grown Fe-sulfates, to determine the chemical composition of the unknown substance on Mars. Spectral character of heated lab samples may provide information about possible temperatures of formation on Mars. The figures are all helpful. I find that this study has made a scientific advancement and that the manuscript, after revision, would make a welcome addition to the Mars literature.\n\nMy main suggestion for the authors is to consider not calling this a new mineral. A mineral, by definition, is an inorganic, naturally-occurring crystalline solid with a narrowly-defined chemical composition and a characteristic set of physical processes. Are all of these things known for this substance on Mars? Can a new mineral be claimed based only on remotely-sensed spectroscopy and images taken at 101-2 m scale? I don’t think so. Although the study has demonstrated a spectral match between the a lab sample and the CRISM-obtained spectra, there is no known naturally-occurring solid on Earth with the same spectral features (or, at least, the manuscript does not describe one). Can it be confirmed that this material on Mars is crystalline? The crystalline structure for the lab samples of Fe3+SO4H2O is shown in Figure 3; can the text describe in a clearer way that the crystalline structure is reflected in the spectra? Please remember that not all readers of Nature Communications will be spectroscopy or mineralogy experts. What are the physical properties of this structure? Meridianiite was a new mineral named on Mars, but that could be done because there was naturally-occurring examples of meridianiite on Earth that could be investigated for crystalline structure and physical properties. I refer the authors to Nickel and Manderino 1987 for rules on new minerals. A philosophical question: can a new mineral be named officially if it does not occur on Earth? I would say “yes” for a solid on Mars brought to Earth where it can be described fully, but “no” for a solid on Mars for which we cannot describe closely. So, to summarize, I think that either the “new mineral” in the title must be dropped (or greatly down-played) or that the manuscript make a clearer case for why this substance on Mars can be called a new mineral.\n\nThe title of the manuscript also promises characterization of the geochemical environment. However, I find some of the text on this topic to be unclear in places.\n\nI had questions about the synthetic solids; mainly, it was difficult for me to follow the details of this lab work. I think my questions could be addressed simply with clearer explanations in the text, and perhaps both moving tables ED 1 and ED3 to the main text.\n\nHere are some suggestions for making the manuscript more accessible to the reader:\n1) Please don’t use the abbreviation PHS. Simply write it out each time you use it. It does not take up much space. Using that abbreviation makes it seem like this paper is serving only those scienstists that normally use this term as an abbreviation. I also study polyhydrated sulfates, but don’t abbreviate the term. I think the abbreviation, each time it’s used, causes a distraction for the reader.\n2) last line of abstract: change “emplacement of lava, ash, or geothermal processes” to “deposition of lava or ash or through hydrothermal processes”. Lava and ash form on a planet’s surface; “emplacement” refers to magma-cooled bodies.\n3) lines 72-74: This sentence seemed overly long and awkwardly worded to me. Please reword this sentence and/or split it into two or three sentences.\n4) Figures 2, 3, and 4use different sized fonts and scale and box line thicknesses. I suggest making them look more consistent for a more professional look. Also, the yellow font on Fig. 4 does not look good. Please put a scale on all photos.\n5) Line 115: I realize that the style of Nature Communications is to place the methods section last, but it seemed to me that this sentence “The new mineral Fe3+SO4OHis formed in the lab by heating” does not provide enough information to satisfactorily work well here. What were the temperatures? What was the pressure? Was this grown in the lab from a synthetic solution? Can you truly call it a mineral? It might be helpful to include a short table here that explains the experimental solid synthesis.\n6) Lines 116-141: make sure figures and tables are referenced in order. Currently, they do not seem to be.\n7) Lines 234, 235, and elsewhere in manuscript: “observed neighboring” and “neighboring” used as a verb is awkward. Please change to “adjacent to”.\n8) Line 266: replace “while” with “whereas”\n9) Lines 268 and 269: get rid of abbreviations MHS and PHS.\n10) Line 282: change “that” to “and were”\n11) Line 285: “highly pure” is redundant. Either something is pure or it is not.\n12) Line 286: replace “neighboring” – it is a poor verb\n13) Lines 305-306: reword “basaltic bedrock could have been derived from volcanic ash or lava”; first, of course basaltic bedrock is derived from lava; ALL basalt forms by cooling of magma. Also, how does volcanic ash become a basaltic bedrock? Volcanic ash and basalt are different kinds of rocks. As far as I know, volcanic ash cannot turn into basalt. Perhaps this is just unclear wording?\n14) Line 325-326: “Subsequent aqueous activity produced the layered sedimentary material later identified as sulfates”. This sentence could have several different meanings. What kind of aqueous activity? Water flow at the planet’s surface that transported grains? Chemical precipitation of salt minerals from a surface water? Aqueous alteration of a layered rock already there? How can any of these three possibilities be distinguished from one another, based on the images from Mars’s surface at 101-2 m scale? On Earth, one way that chemical precipitates form is by crystal growth at bottom off, or within, water body. But the same minerals can also act as clastic grains (after being weathered and eroded from elsewhere). In order to tell the difference, you’d need to see texture at a mm- or cm-scale, something not available here. Can you please clarify your thoughts on the type(s) of aqueous activity you are interpreting?\n15) Lines 383-384: “based on the methods reported by 37 and 38” is awkward and could be changed to “basin on previous methods 37, 38”\n16) Line 388: replace “setup” with “conditions”", "reviewer_3": "Review of “Identification and Characterization of the New Mineral Ferric Hydroxysulfate (Fe3+SO4OH) on Mars at Aram Chaos and the Juventae Chasma Plateau\" for Nature Communications”\n\nThe manuscript describes and interprets a unique spectral signature observed in two locations on Mars as a potential new ferric hydroxysulfate mineral that forms when mono and poly-hydrated ferrous or ferric sulfate salts are heated in an oxidizing environment. The manuscript also develops two possible alteration pathways that resulted in the formation of the unique spectral phase within these two locations- one model suggests thermal metamorphism by lava flows or ash falls, while the other mechanism involves geothermal alteration from a heat source bellow the outcrops.\n\nWhile I am not an expert in CRISM data or collecting/interpreting NIR spectra, the manuscript makes a convincing case the unique spectral signature observed on Mars is a good match for Fe3+ SO4OH produced by heating mono- or poly-hydrated salts. The experiments used to demonstrate that the Fe3+ SO4OH phase can be formed from hydrated ferrous and ferric sulfates that are also observed in the region are also convincing. In addition, the thermal alteration/stratigraphic models presented to explain the occurrences of the unique spectral feature seem well-developed and plausible.\n\nThe one area of critique that I will note is that the paper assumes that oxidative heating is the only way that the Fe3+ SO4OH could have formed from the hydrated sulfate minerals. While oxidative heating may be one potential formation mechanism, it does not appear that they authors tested other mechanisms beyond anaerobic heating (which did not produce the phase). In the gypsum-bassanite-anhydrite system, heating is a common mechanism for dehydration, but another mechanism also exists-- reaction with low activity of water fluids (high salinity brines). Since this other mechanism exists for dehydrating poly and monohydrated sulfates, I find it surprising that I did not see it clearly discussed or tested as an alternative formation mechanism. I am not suggesting that additional experiments need to be conducted in order for this paper to be sound, but a discussion of other potential mechanism(s) would be warranted.\n\nOther specific comments:\nLine 1: I do not see a reference to a new mineral designation from the IMA for the Ferric hydroxysulfate phase. If it hasn’t been designated a new mineral, I don’t think it is appropriate to call it a new mineral in the title or in the body of the text. Perhaps “potential new mineral”?\nLine 36-37: Consider revising to “Other hydrated salts at these locations” for clarity.\nLine 46 consider revising to “through comparison of spectra” to be more precise and clear.\nLine 51 The new material is not certain- it has been proposed in this study. I would recommend replacing this with “source of the unique spectral features” to be more accurate.\nFigure 2. It is very difficult to see the blues and reds on the dark background images, especially in panels c and e. I also do not see any of the yellow spectra indicated in any of the maps- is it there, but not evident? If so, some guidance with arrows where to look would be helpful.\nFigure 4. In looking at these images, what are the circles that are clearly evident in panels b and c?\nFigure 5. The shadows on the text for Fe3+ SO4OH and PHS make them difficult to read.\nLines 265-268 and lines 303-304. It took me a couple of minutes to figure out that the 2 different forms that are referred to in line 304 are the two different locations of the occurrences references in 265-268. I think it was the reference to “two different forms” that got me confused, since I think as forms as different mineral species rather than outcropping styles. I think adding “taking place at the two different locations on Mars” would help clarify.\nLine 308. Only hot ash? or could it also be a lava flow?\nLine 311 I think you mean unit here? I suggest removing ”during emplacement” as it is confusing the sentence structure and isn’t needed.\nFigure 6- Difficult to read PHS on the textured background.\nReferences- overall, the paper appears to be well-referenced, however 6/48 references are abstracts that have received less intensive peer-review."}, "author_response": "Responses to reviewer comments and suggestions on Nature Communications manuscript NCOMMS-25-06188-T\n\nReviewer #1 (Remarks to the Author):\nThis paper describes the detection of an absorption band at 2.236 µm in Aram Chaos and Juventae Plateau using the CRISM instrument. The authors attribute this band to ferric hydroxysulfate (Fe3+SO4OH) and demonstrate it through the synthesis and spectral characterization of this species in the laboratory. Fe3+SO4OH is thus detected alongside other poly- and monohydrated sulfates, forming an upper horizon in Juventae Plateau and occurring beneath the MHS/PHS in Aram Chaos. The authors interpret its presence as a result of thermal processing of sulfate-containing outcrops, dehydrating and oxidizing them through contact with an overlying lava layer (Juventae Plateau) or hydrothermal circulation (Aram Chaos). Fe3+SO4OH would have formed relatively late, during the Amazonian (<3 Ga), suggesting that the Martian surface remained active longer than previously thought. The article is well-written, and the attribution of the 2.236 µm absorption band to Fe3+SO4OH is convincing, as is the proposed interpretation, considering the formation method of this species and its distribution relative to MHS and PHS.\n\nThank you! We are grateful for your interest in our study and the time and effort taken to review this paper.\n\nAs shown in Table ED1, Fe3+SO4OH is the fully dehydrated form of ferric hydroxysulfates and can rehydrate into butlerite (Fe3+SO4OH·H2O), which loses the 2.236 µm band. Other species of ferric hydroxysulfates exist with different hydration levels (fibroferrite, amarantite, or coquimbite), which can form through precipitation in an oxidized, sulfate-rich acidic fluid. On Earth, such compounds are observed in industrial and mining effluents released into the environment, for example, along with schwertmannite, a poorly crystalline ferric oxyhydroxysulfate commonly found in these settings. Could a similar formation scenario be envisioned on Mars, involving deposition in an acidic environment followed by dehydration? The article proposes thermal treatment of ferrous sulfates to dehydrate and oxidize them, but could an aqueous formation followed by simple dehydration under Mars' arid atmosphere be possible? Schwertmannite lacks diagnostic near-IR signatures and would not be excluded by CRISM data.\n\nRegarding acid formation:\nIt is an interesting suggestion that Fe3+SO4OH might be able to form through precipitation in an oxidized, sulfate-rich acidic fluid. Several studies have indeed observed formation of many hydrated ferric hydroxysulfates this way as suggested here. In fact, (Lu & Wang, 2012) observed slow precipitation of Fe3+SO4OH at 120 °C from a saturated aqueous iron sulfate solution after 1-2 months, suggesting that even in acidic solution elevated temperatures are required for formation of Fe3+SO4OH. However, hydrothermal alteration of basalt under extremely low pH can result in Fe(III) cations in solution rather than formation of ferric oxides/hydroxides/hydroxysulfates and further mineral formation from aqueous solution is not necessarily dependent on temperature (e.g., Bischoff & Dickson, 1975; Ding & Seyfried, 1992; Al-Samir et al., 2017). Thus, additional experiments are warranted to further test formation of Fe3+SO4OH under extremely acidic conditions and lower temperatures.\n\nDespite this, there is a general trend in formation of hydrated minerals that highly hydrated phases form at low-temperatures, whereas the lower hydrates form at higher temperatures (e.g. MgSO4·11H2O forms at temperatures below freezing, while MgSO4·H2O forms above 50 °C; NaCl·2H2O forms below freezing, while NaCl above freezing,…). We therefore conclude that it is possible that Fe3+SO4OH could form from acidic fluid, but likely only at elevated temperatures, exceeding those prevailing in terrestrial acid mine drainage systems. The largest support for our formation models which are based on the burial and subsequent exhumation of Fe2+ sulfate hydrates is the stratigraphic context of the Fe3+SO4OH phase observed from orbit.\n\nRegarding dehydration/oxidation on Mars:\nWe would like to point out that our heating experiments in the very oxygen-rich terrestrial atmosphere yielded extremely sluggish Fe3+SO4OH formation even at 100 °C; at 50 °C the reaction has not been observed at all. In addition, Fe2+SO4·H2O crystals synthesized more than 35 years ago for the Wildner & Giester (1991) study and stored at room temperature are still preserved and did not oxidize to form Fe3+SO4OH. The present-day Mars atmosphere is much colder (thus reducing reaction rates) and oxygen-poor, rendering oxidation of Fe2+SO4·H2O induced by the atmosphere unlikely.\n\nRegarding fibroferrite/amarantite/schwertmannite precursors:\nIt would be interesting to test dehydration of fibroferrite (Fe3+SO4OH·5H2O) and amarantite (Fe3+SO4OH·3H2O) in future studies. In fact, I did attempt that with a “fibroferrite” sample back in 2008, but unfortunately the sample contained a number of admixtures in addition to fibroferrite and the products were also complex. For the current study at two martian sites, Fe3+SO4OH was observed next to szomolnokite/kieserite or mixed with polyhydrated sulfate (presumably rozenite). Neither fibroferrite (~1.96, 2.25, 2.29 µm) nor amarantite (~1.98, 2.21, 2.27 µm) spectral features were observed, so it is less likely that they were involved in the formation of Fe3+SO4OH at the Juventae Plateau or at Aram Chaos.\n\nSchwertmannite is actually a ferric oxyhydroxide similar to akaganéite, but has sulfate groups in the tunnels in place of Cl- ions (e.g. Bishop & Murad, 1996; Bishop et al., 2014). Thus, heating schwertmannite would not be expected to produce a ferric hydroxide similar to jarosite or the Fe3+SO4OH discussed in this study. Also, the VNIR spectrum of schwertmannite includes broad and slightly variable hydration bands centered near 1.43-1.45 and 1.94-1.95 µm due to a distribution of H2O sites in the mineral structure and likely some variations among samples (e.g., Bishop et al., 2004, 2014; Bishop, 2019), with vibrational energies somewhat similar to the features observed in spectra of natural and synthetic ferrihydrites near 1.41-1.43 and 1.92-1.94 µm (e.g. Bishop & Murad, 2001). Both schwertmannite and ferrihydrite could be consistent with the hydration bands observed in CRISM spectra at multiple locations on Mars where hydrated materials are observed, as suggested by this reviewer for schwertmannite.\n\nVNIR reflectance spectra of the schwertmannite and ferrihydrite samples measured in our lab exhibit a range of hydration band positions, depending on the exact form of each particular sample. This is in contrast with the spectra of rozenite, jarosite, butlerite, fibroferrite, amarantite, copiapite, coquimbite, and Fe3+SO4OH in our lab that appear to have unique and highly repeatable spectral band positions.\n\nWe have revised the text to clarify this.\n“Determining a temperature for these hydrothermal sulfate reactions is difficult given the potential for long geologic timescales. Lab heating experiments at 100 °C observed slow formation of Fe3+SO4OH from both rozenite and szomolnokite, although the reactions proceeded faster from rozenite. This is presumably because of the additional H2O present in the rozenite structure that facilitates the reaction. Lab experiments heating rozenite at 50 °C did not produce Fe3+SO4OH after several days. Additionally, szomolnokite crystals synthesized over 35 years ago (Wildner and Giester, 1991) and stored at room temperature in Earth’s oxygen-rich atmosphere remain well preserved, suggesting that the transformation to Fe3+SO4OH does not occur under present-day Martian atmospheric conditions, which are both oxygen-poor and low in temperature. In contrast to other sulfates such as kieserite and anhydrite, water activity does not appear to play a role in the formation temperature of Fe3+SO4OH, since Talla and Wildner (2019) did not find any indications for Fe3+SO4OH formation even in highly concentrated sulfuric acid solutions and 210 °C. The reason for this is that Fe3+SO4OH is not a dehydrated form of szomolnokite, but rather formed via deprotonation and oxidation. This suggests that the hydrothermal event at Aram Chaos that formed Fe3+SO4OH reached temperatures above 50 °C. Thus, aqueous precipitation of Fe3+SO4OH is not expected at low temperatures and is only sluggish at elevated temperatures. Further, it would be difficult to explain the observed stratigraphy of sulfates at Aram Chaos if Fe3+SO4OH had formed via aqueous precipitation. Together, this reasoning suggests that the hydrothermal event at Aram Chaos that formed Fe3+SO4OH reached temperatures above 50 °C. The largest support for our formation model at Aram Chaos, based on the burial and subsequent exhumation of Fe2+ sulfate hydrates, is the stratigraphic context of the Fe3+SO4OH phase observed from orbit. Potentially, long-term heating of rozenite on Mars could have occurred at lower temperatures to form the szomolnokite and then shorter-term hydrothermal events produced the Fe3+SO4OH phase.”\n\nA secondary remark: the 2.236 µm band supports most of the discussion on the detection of Fe3+SO4OH. It would be helpful to include a figure showing this band precisely in the CRISM data alongside candidate minerals so that the reader can be convinced of its correspondence with Fe3+SO4OH. The CRISM spectra presented in this paper are systematically represented with dots, forming a very thick line that reduces the contrast of spectral signatures. Is this intended to represent the SNR of the data? Presenting the data differently would be beneficial to help the reader better assess their actual SNR and the uncertainty in band positions. For example, in spectrum 1 of Fig. 2g, the 2.236 µm band is shown with disjointed points, making it difficult to judge the shape and position of this band to confirm its correspondence with Fe3+SO4OH\n\nWe agree that the spectral features near 2.236 µm are difficult to see and have added an additional spectral plot to Figure 3 to highlight these bands.\n\nThe dots in the CRISM spectra represent actual CRISM datapoints and thin lines connect these dots. This can be seen better in the new panel of Figure 3. The reason for showing the CRISM spectra as dots rather than lines is to provide the reader with a feel for the actual spectral resolution and noise in these spectra. The noise in these spectra is remarkably low compared to traditional CRISM spectra due to the new calibration techniques used in processing these images. Despite that, there is still variance in many places from one spectral channel to another that is due to factors other than surface mineralogy.\n\nOther minor point, I would recommend to explicitly indicate the oxidation state of Fe in every mineral/species formula, either using the charge or the oxidation number.\n\nRequested change completed.\nWe think it is a bit overkill to list this throughout as it is obvious after the first couple of mentions, but we’ll let the editors decide on this.\n\nReviewer #2 (Remarks to the Author):\nThis manuscript describes the occurrence of an iron hydroxysulfate on Mars, documents its spectral characteristics, and matches it to a synthetic solid. The identification of this previously unidentified solid is the main strength of this manuscript. This study has used careful evaluation of spectral signatures from the CRISM instrument aboard the Mars Reconnaissance Orbiter, combined with lab testing of lab-grown Fe-sulfates, to determine the chemical composition of the unknown substance on Mars. Spectral character of heated lab samples may provide information about possible temperatures of formation on Mars. The figures are all helpful. I find that this study has made a scientific advancement and that the manuscript, after revision, would make a welcome addition to the Mars literature.\n\nThank you! We are grateful for your interest in our study and the time and effort taken to review this paper.\n\nMy main suggestion for the authors is to consider not calling this a new mineral. A mineral, by definition, is an inorganic, naturally-occurring crystalline solid with a narrowly-defined chemical composition and a characteristic set of physical processes. Are all of these things known for this substance on Mars? Can a new mineral be claimed based only on remotely-sensed spectroscopy and images taken at 101-2 m scale? I don’t think so.\n\nWe believe this is a new mineral; however, it has not yet been identified in nature on Earth. We have changed the text accordingly.\n\nAlthough the study has demonstrated a spectral match between the a lab sample and the CRISM-obtained spectra, there is no known naturally-occurring solid on Earth with the same spectral features (or, at least, the manuscript does not describe one). Can it be confirmed that this material on Mars is crystalline? The crystalline structure for the lab samples of Fe3+SO4H2O is shown in Figure 3; can the text describe in a clearer way that the crystalline structure is reflected in the spectra? Please remember that not all readers of Nature Communications will be spectroscopy or mineralogy experts. What are the physical properties of this structure?\n\nGood suggestions. The word count for this paper was initially limited but we have now expanded the physical description of Fe3+SO4OH in the revised text due to the more generous word count allowed.\n\nAdditional text was added to explain the relationship between the crystal structure and the spectral features:\n“Vibrations of these H2O and OH groups in the structure of szomolnokite and Fe3+SO4OH, respectively, are responsible for the broad H2O combination band near 2.1 µm and the narrow OH combination band near 2.2 µm. The structure of Fe3+SO4OH and the strength of the bond connecting the OH group to the Fe3+ cation govern the vibrational energy of the OH group. The OH fundamental stretching vibration occurs at 3460 cm-1 or 2.89 µm (Table 2, Figure S4). Jarosite also has OH groups bound to Fe3+ cations in an octahedral configuration, but the OH stretching vibrations occur near 3385 cm-1 or 2.95 µm for K+-jarosite and 3360 cm-1 or 2.98 µm for Na+-jarosite (Bishop & Murad, 2005). Presumably, the additional Na+ or K+ cations in the jarosite structure are pulling some of the electron density away from the OH bond, thus reducing the vibrational energy and shifting the bands towards longer wavelengths compared to the OH vibrations in ferric hydroxysulfate.”\n\nAdditionally, a new Supplementary Figure (Fig. S7) was added to illustrate the colors of the iron sulfates discussed.\n\nMeridianiite was a new mineral named on Mars, but that could be done because there was naturally-occurring examples of meridianiite on Earth that could be investigated for crystalline structure and physical properties. I refer the authors to Nickel and Manderino 1987 for rules on new minerals. A philosophical question: can a new mineral be named officially if it does not occur on Earth? I would say “yes” for a solid on Mars brought to Earth where it can be described fully, but “no” for a solid on Mars for which we cannot describe closely. So, to summarize, I think that either the “new mineral” in the title must be dropped (or greatly down-played) or that the manuscript make a clearer case for why this substance on Mars can be called a new mineral.\n\nWe agree that looking for Fe3+SO4OH in martian samples once they are returned to Earth is an excellent idea. For now, we have removed “mineral” from the title and changed this usage in many places in the text.\n\nThe title of the manuscript also promises characterization of the geochemical environment. However, I find some of the text on this topic to be unclear in places.\n\nThe title of the paper needed to be reduced due to new constraints for Nature Communications. The title has been changed to “Characterization of Ferric Hydroxysulfate on Mars and Implications of the Geochemical Environment Supporting its Formation”. We have also attempted to clarify the potential geochemical environments supportive of Fe3+SO4OH formation on Mars in the text describing its likely formation based on our observations at Aram Chaos and the Juventae Plateau.\n\nI had questions about the synthetic solids; mainly, it was difficult for me to follow the details of this lab work. I think my questions could be addressed simply with clearer explanations in the text, and perhaps both moving tables ED 1 and ED3 to the main text.\n\nHere are some suggestions for making the manuscript more accessible to the reader:\n1) Please don’t use the abbreviation PHS. Simply write it out each time you use it. It does not take up much space. Using that abbreviation makes it seem like this paper is serving only those scienstists that normally use this term as an abbreviation. I also study polyhydrated sulfates, but don’t abbreviate the term. I think the abbreviation, each time it’s used, causes a distraction for the reader.\n\nRequested change completed in most cases – MHS and PHS were left in some figure captions where these terms are used. The text contains 40-50 occurrences each of PHS and MHS, so this change actually makes the manuscript quite a bit longer. We will let the editor contribute to the decision on which format would be best to use in this case.\n\n2) last line of abstract: change “emplacement of lava, ash, or geothermal processes” to “deposition of lava or ash or through hydrothermal processes”. Lava and ash form on a planet’s surface; “emplacement” refers to magma- cooled bodies.\n\nRequested change completed\n\n3) lines 72-74: This sentence seemed overly long and awkwardly worded to me. Please reword this sentence and/or split it into two or three sentences.\n\nText reworded as 2 sentences to read:\n“None of these minerals are consistent with the CRISM spectra of this outcrop having a single band near 2.23 µm. In addition to this enigmatic 2.23 µm band, this outcrop contains features near 1.43, 1.94, and 2.4 µm that are consistent with polyhydrated sulfates including rozenite (FeSO4·4H2O) and epsomite (MgSO4·7H2O) (Fig. 1d).”\n\n4) Figures 2, 3, and 4 use different sized fonts and scale and box line thicknesses. I suggest making them look more consistent for a more professional look. Also, the yellow font on Fig. 4 does not look good. Please put a scale on all photos.\n\nFigures 2 and 4 (now 5) were revised to use only white labels on colored figures and label sizes were increased in some cases to make figures look more uniform and improve clarity. (Note that Figure 3 has spectral plots and not images; Figure 5 (now 6) uses yellow scale bars to better differentiate from white in the images.)\n\n5) Line 115: I realize that the style of Nature Communications is to place the methods section last, but it seemed to me that this sentence “The new mineral Fe3+SO4OHis formed in the lab by heating” does not provide enough information to satisfactorily work well here. What were the temperatures? What was the pressure? Was this grown in the lab from a synthetic solution? Can you truly call it a mineral? It might be helpful to include a short table here that explains the experimental solid synthesis.\n\nThe temperature range was added in the first sentence of this paragraph (old line 115) and Tables 1 and 2 were added to the text in this section to provide more details about the samples and spectral features.\n\nMost samples were heated under normal lab air. One set of experiments was tested under N2 as well as air where we measured the P in bars and the O2 consumed during the experiments. These experiments were summarized in a table (formerly ED Table 3, now SI Table 1). Additional text was added to the description of Table 3 to provide additional information about these experiments.\n\n6) Lines 116-141: make sure figures and tables are referenced in order. Currently, they do not seem to be.\n\nMany figures were renumbered and re-arranged due to the formatting allowed for Nature Communications and the Extended Data tables and figures were removed, which formerly appeared to be out of sequence in some cases due to the ED labeling.\n\n7) Lines 234, 235, and elsewhere in manuscript: “observed neighboring” and “neighboring” used as a verb is awkward. Please change to “adjacent to”.\n\nRequested change completed\n\n8) Line 266: replace “while” with “whereas”\n\nRequested change completed\n\n9) Lines 268 and 269: get rid of abbreviations MHS and PHS.\n\nRequested change completed\n\n10) Line 282: change “that” to “and were”\n\nText changed here to “that were”\n\n11) Line 285: “highly pure” is redundant. Either something is pure or it is not.\n\nRequested change completed\n\n12) Line 286: replace “neighboring” – it is a poor verb\n\nRequested change completed\n\n13) Lines 305-306: reword “basaltic bedrock could have been derived from volcanic ash or lava”; first, of course basaltic bedrock is derived from lava; ALL basalt forms by cooling of magma. Also, how does volcanic ash become a basaltic bedrock? Volcanic ash and basalt are different kinds of rocks. As far as I know, volcanic ash cannot turn into basalt. Perhaps this is just unclear wording?\n\nWe changed bedrock to material to improve clarity. Because Mars is a basaltic planet, both ash and lava could have basaltic composition. Indurated ash could become hardened to form rock-like material over time but that is not the focus of this paper and we prefer not to add a discussion of this to the text.\n\n14) Line 325-326: “Subsequent aqueous activity produced the layered sedimentary material later identified as sulfates”. This sentence could have several different meanings. What kind of aqueous activity? Water flow at the planet’s surface that transported grains? Chemical precipitation of salt minerals from a surface water? Aqueous alteration of a layered rock already there? How can any of these three possibilities be distinguished from one another, based on the images from Mars’s surface at 101-2 m scale? On Earth, one way that chemical precipitates form is by crystal growth at bottom off, or within, water body. But the same minerals can also act as clastic grains (after being weathered and eroded from elsewhere). In order to tell the difference, you’d need to see texture at a mm- or cm-scale, something not available here. Can you please clarify your thoughts on the type(s) of aqueous activity you are interpreting?\n\nWe agree that determining the types of aqueous activity that occurred billions of years ago on a planetary surface from orbital data would be quite challenging. Our paper did not investigate or attempt to interpret the formation of the polyhydrated sulfates as this was previously studied by (Glotch & Christensen, 2005) and (Lichtenberg et al., 2010). Our study is focused on the transformation of the polyhydrated sulfates to form monohydrated sulfates and ferric hydroxysulfate.\n\nWe did expand the text here to improve clarity.\n“Subsequent aqueous activity produced the layered sedimentary material (Glotch & Christensen, 2005), later identified as sulfates (Lichtenberg et al., 2010). These hydrated sulfates are proposed to have formed through multiple wetting events including groundwater recharge and evaporation (Lichtenberg et al., 2010).”\n\n15) Lines 383-384: “based on the methods reported by 37 and 38” is awkward and could be changed to “basin on previous methods 37, 38”\n\nRequested change completed\n\n16) Line 388: replace “setup” with “conditions”\n\nRequested change completed\n\nReviewer #3 (Remarks to the Author):\nReview of “Identification and Characterization of the New Mineral Ferric Hydroxysulfate (Fe3+SO4OH) on Mars at Aram Chaos and the Juventae Chasma Plateau\" for Nature Communications”\n\nThe manuscript describes and interprets a unique spectral signature observed in two locations on Mars as a potential new ferric hydroxysulfate mineral that forms when mono and poly-hydrated ferrous or ferric sulfate salts are heated in an oxidizing environment. The manuscript also develops two possible alteration pathways that resulted in the formation of the unique spectral phase within these two locations- one model suggests thermal metamorphism by lava flows or ash falls, while the other mechanism involves geothermal alteration from a heat source bellow the outcrops.\n\nWhile I am not an expert in CRISM data or collecting/interpreting NIR spectra, the manuscript makes a convincing case the unique spectral signature observed on Mars is a good match for Fe3+ SO4OH produced by heating mono- or poly-hydrated salts. The experiments used to demonstrate that the Fe3+ SO4OH phase can be formed from hydrated ferrous and ferric sulfates that are also observed in the region are also convincing. In addition, the thermal alteration/stratigraphic models presented to explain the occurrences of the unique spectral feature seem well-developed and plausible.\n\nThank you! We are grateful for your interest in our study and the time and effort taken to review this paper.\n\nThe one area of critique that I will note is that the paper assumes that oxidative heating is the only way that the Fe3+ SO4OH could have formed from the hydrated sulfate minerals. While oxidative heating may be one potential formation mechanism, it does not appear that they authors tested other mechanisms beyond anaerobic heating (which did not produce the phase). In the gypsum-bassanite-anhydrite system, heating is a common mechanism for dehydration, but another mechanism also exists-- reaction with low activity of water fluids (high salinity brines). Since this other mechanism exists for dehydrating poly and monohydrated sulfates, I find it surprising that I did not see it clearly discussed or tested as an alternative formation mechanism. I am not suggesting that additional experiments need to be conducted in order for this paper to be sound, but a discussion of other potential mechanism(s) would be warranted.\n\nWe agree that it is well established for the Mg-sulfate and Ca-sulfate systems that water activity may have a pronounced influence on the formation temperature of the various phases, however, the formation of Fe3+SO4OH from szomolnokite is not a dehydration reaction but rather an oxidation and deprotonation reaction, and therefore not expected to be influenced by changes in water activity. This is also further supported by the work of Talla and Wildner 2019 (DOI: https://doi.org/10.2138/am-2019-6983) where heating of Fe-SO4 solutions with 73 wt.% sulfuric acid (extremely low water activity) at 210 °C did not yield any evidence for Fe3+SO4OH formation. We have addressed this now in the text:\n\nRevised text:\n“Determining a temperature for these hydrothermal sulfate reactions is difficult given the potential for long geologic timescales. Lab heating experiments at 100 °C observed slow formation of Fe3+SO4OH from both rozenite and szomolnokite, although the reactions proceeded faster from rozenite. This is presumably because of the additional H2O present in the rozenite structure that facilitates the reaction. Lab experiments heating rozenite at 50 °C did not produce Fe3+SO4OH after several days. Additionally, szomolnokite crystals synthesized over 35 years ago (Wildner and Giester, 1991) and stored at room temperature in Earth’s oxygen-rich atmosphere remain well preserved, suggesting that the transformation to Fe3+SO4OH does not occur under present-day Martian atmospheric conditions, which are both oxygen-poor and low in temperature. In contrast to other sulfates such as kieserite and anhydrite, water activity does not appear to play a role in the formation temperature of Fe3+SO4OH, since Talla and Wildner (2019) did not find any indications for Fe3+SO4OH formation even in highly concentrated sulfuric acid solutions and 210 °C. The reason for this is that Fe3+SO4OH is not a dehydrated form of szomolnokite, but rather formed via deprotonation and oxidation. This suggests that the hydrothermal event at Aram Chaos that formed Fe3+SO4OH reached temperatures above 50 °C. Thus, aqueous precipitation of Fe3+SO4OH is not expected at low temperatures and is only sluggish at elevated temperatures. Further, it would be difficult to explain the observed stratigraphy of sulfates at Aram Chaos if Fe3+SO4OH had formed via aqueous precipitation. Together, this reasoning suggests that the hydrothermal event at Aram Chaos that formed Fe3+SO4OH reached temperatures above 50 °C. The largest support for our formation model at Aram Chaos, based on the burial and subsequent exhumation of Fe2+ sulfate hydrates, is the stratigraphic context of the Fe3+SO4OH phase observed from orbit. Potentially, long-term heating of rozenite on Mars could have occurred at lower temperatures to form the szomolnokite and then shorter-term hydrothermal events produced the Fe3+SO4OH phase.”\n\nOther specific comments:\nLine 1: I do not see a reference to a new mineral designation from the IMA for the Ferric hydroxysulfate phase. If it hasn’t been designated a new mineral, I don’t think it is appropriate to call it a new mineral in the title or in the body of the text. Perhaps “potential new mineral”?\n\nRequested change completed. We now refer to this as a “phase” or “potential new mineral” as it has not yet been identified in natural environments on Earth.\n\nLine 36-37: Consider revising to “Other hydrated salts at these locations” for clarity.\n\nWe prefer to keep “hydrated sulfates at these locations” because we do not have evidence of other salts besides the sulfates mentioned in the text. Also, the word “other” implies that Fe3+SO4OH is hydrated, when it contains no water in its structure.\n\nLine 46 consider revising to “through comparison of spectra” to be more precise and clear.\n\nWe changed the text here to “through comparison with terrestrial minerals using spectral parameters, X-ray diffraction, and elemental abundances”. Spectra were used in the orbital studies, but XRD and major elements were used in the rover studies.\n\nLine 51 The new material is not certain- it has been proposed in this study. I would recommend replacing this with “source of the unique spectral features” to be more accurate.\n\nA new material was detected at these 2 locations on Mars in past studies and we have refined knowledge of this material in our current study. Mineral was not mentioned in this case, so we think the text is accurate as it stands.\n\nFigure 2. It is very difficult to see the blues and reds on the dark background images, especially in panels c and e. I also do not see any of the yellow spectra indicated in any of the maps- is it there, but not evident? If so, some guidance with arrows where to look would be helpful.\n\nWe have revised panels c and e of Figure 2 to make the blue and red colors more visible.\n\nThe orange color of spectrum 4 in Figure 2-g was selected to show a mixture of the MHS mapped in green and the Fe3+SO4OH mapped in red, however we were only able to map in RGB in the image figures and the material shown in spectrum 4 is mapped in green. This is now clarified in the figure caption.\n\nFigure 4. In looking at these images, what are the circles that are clearly evident in panels b and c?\n\nThe small circular forms in panels b and c are impact craters.\n\nFigure 5. The shadows on the text for Fe3+ SO4OH and PHS make them difficult to read.\n\nNow Figure 6. The text labels on panels d,e,h,I were difficult to read in either black or white alone, so we chose colors and tried to connect these with the features they represent in the colored panels. The shading for the PHS label was revised to improve clarity.\n\nLines 265-268 and lines 303-304. It took me a couple of minutes to figure out that the 2 different forms that are referred to in line 304 are the two different locations of the occurrences references in 265-268. I think it was the reference to “two different forms” that got me confused, since I think as forms as different mineral species rather than outcropping styles. I think adding “taking place at the two different locations on Mars” would help clarify.\n\nText changed to: “Models of formation mechanisms of the two different forms of the Fe3+SO4OH outcrops observed at the two different locations (Fig. 7) illustrate potential pathways that could have taken place on Mars.”\n\nLine 308. Only hot ash? or could it also be a lava flow?\n\nRequested change completed.\n\nLine 311 I think you mean unit here? I suggest removing ”during emplacement” as it is confusing the sentence structure and isn’t needed.\n\nRequested change completed.\n\nFigure 6- Difficult to read PHS on the textured background.\n\nShaded backgrounds added behind PHS and other labels to improve readability.\n\nReferences- overall, the paper appears to be well-referenced, however 6/48 references are abstracts that have received less intensive peer-review.\n\nThat is true; however, most are extended abstracts. In some cases, peer-reviewed papers are not yet available and these are the best sources to cite. We have added links for the abstracts to facilitate finding these online.\n\nReferences mentioned in this response document:\nAl-Samir M., Nabhan S., Fritz J., Winkler A., Bishop J. L., Gross C. & Jaumann R. (2017) The paleolacustrine evolution of Juventae Chasma and Maja Valles and its implications for the formation of interior layered deposits on Mars. Icarus, 292, 125-143.\nBischoff J. L. & Dickson F. W. (1975) Seawater-basalt interaction at 200°C and 500 bars: Implications for origin of sea-floor heavy-metal deposits and regulation of seawater chemistry. Earth and Planetary Science Letters, 25, 385-397.\nBishop J. L. & Murad E. (2005) The visible and infrared spectral properties of jarosite and alunite. American Mineralogist, 90, 1100-1107.\nDing K. & Seyfried W. E. (1992) Determination of Fe-Cl complexing in the low pressure supercritical region (NaCl fluid): Iron solubility constraints on pH of subseafloor hydrothermal fluids. Geochimica et Cosmochimica Acta, 56, 3681-3692.\nGlotch T. D. & Christensen P. R. (2005) Geologic and mineralogic mapping of Aram Chaos: Evidence for a water-rich history. Journal of Geophysical Research, 110, doi:10.1029/2004JE002389.\nLichtenberg K. A., Arvidson R. E., Morris R. V., Murchie S. L., Bishop J. L., Glotch T. D., Noe Dobrea E., Mustard J. F., Andrews-Hanna J. & Roach L. H. (2010) Stratigraphy of hydrated sulfates in the sedimentary deposits of Aram Chaos, Mars. Journal of Geophysical Research, 115, E00D17.\nLu Y. & Wang A. (2012) Synthesis and Spectral Characterization of OH-bearing Ferric Sulfates. In 43rd Lunar Planet Sci. Conf., pp. Abstract #2514.\nWildner, M. & Giester, G. (1991) The crystal structure of kieserite-type compounds. I. Crystal structures of Me(II)SO4•H2O (Me = Mn, Fe, Co, Ni, Zn). Neues Jahrbuch für Mineralogie - Monatshefte 1991, 296-306."}, {"round": 2, "reviewer_comments": {"reviewer_2": "I am satisfied with the revised version of this manuscript. I recommend it for publication."}, "author_response": ""}]

Decision letter