The discovery of phyllosilicates in terrains of Noachian age (>3.5 Ga) on Mars implies a period in the planet’s history that was characterized by wetter, warmer conditions that may have been more hospitable for life than the cold and dry conditions prevalent today. More specific information about the original locations and mechanisms of clay mineral formation on Mars is not as well constrained, however, in part because the origin of particular clay minerals is often non-unique. For example, Fe and Mg smectite-bearing deposits on Mars may have formed in various environments, including the weathering profiles of basic volcanic rocks, impact-induced hydrothermal sites, or in bodies of standing water. The identification of lacustrine deposits on Mars is of great interest due to their potential for the preservation of organic material, but identifying any given suite of sedimentary rocks as such is difficult when limited to mineralogy and morphology derived from orbital data. Here, the processes and conditions leading to clay mineral formation in lakes and evaporative marine basins on Earth are reviewed, with a focus on the spatial and stratigraphic distribution of clays in these settings. The goal is to provide criteria to determine if certain Martian clay deposits are consistent with such an origin, which in turn will aid in the identification of possible ancient habitable environments on Mars.

The enhanced spatial and spectral resolution provided by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the Mars Reconnaissance Orbiter (MRO) has led to the discovery of numerous hydrated silicate minerals on Mars, particularly in the ancient, cratered crust comprising the southern highlands. Phases recently identified using visible/near-infrared spectra include: smectite, chlorite, prehnite, high-charge phyllosilicates (illite or muscovite), the zeolite analcime, opaline silica, and serpentine. Some mineral assemblages represent the products of aqueous alteration at elevated temperatures. Geologic occurrences of these mineral assemblages are described using examples from west of the Isidis basin near the Nili Fossae and with reference to differences in implied temperature, fluid composition, and starting materials during alteration. The alteration minerals are not distributed homogeneously. Rather, certain craters host distinctive alteration assemblages: (1) prehnite-chloritesilica, (2) analcime-silica-Fe,Mg-smectite-chlorite, (3) chlorite-illite (muscovite), and (4) serpentine, which furthermore has been found in bedrock units. These assemblages contrast with the prevalence of solely Fe,Mg-smectites in most phyllosilicate-bearing terrains on Mars, and they represent materials altered at depth then exposed by cratering. Of the minerals found to date, prehnite provides the clearest evidence for subsurface, hydrothermal/metamorphic alteration, as it forms only under highly restricted conditions (T = 200–400°C). Multiple mechanisms exist for forming the other individual minerals; however, the most likely formation mechanisms for the characteristic mineralogic assemblages observed are, for (1) and (2), low-grade metamorphism or hydrothermal (<400°C) circulation of fluids in basalt; for (3), transformation of trioctahedral smectites to chlorite and dioctahedral smectites to illite during diagenesis; and for (4), low-grade metamorphism or hydrothermal (<400°C) circulation of fluids in ultramafic rocks. Evidence for high-grade metamorphism at elevated pressures or temperatures >400°C has not been found.

Beidellites may exist on Mars and represent intermediate alteration products; their presence would indicate different alteration environments than previously identified because montmorillonite is a low-grade alteration mineral whereas beidellite is a higher-temperature alteration mineral, and often represents a step toward illite formation. The reflectance spectra of beidellites are under study to support their orbital detection on Mars, where spectral signatures of other Al-rich phyllosilicates have been observed. Reflectance spectra of ten Al-rich smectites are presented here which include pure beidellites and Al smectites having compositions between those of beidellite and montmorillonite, and emphasis is placed here on the OH combination bands near 4545 cm−1 (2.2 μm) as these vibrational features are commonly used in the identification of phyllosilicates on Mars. Shifts were observed in the Al2OH band centers, which occur near 4590 cm−1 (2.18 μm) in reflectance spectra of beidellite and near 4525 cm−1 (2.21 μm) in reflectance spectra of montmorillonite. These are compared with the Al2OH bending vibrations observed near 941–948 cm−1 (10.5–10.6 μm) for beidellite and near 918–926 cm−1 (10.8–10.9 μm) for montmorillonite. Although the octahedral site cation composition provides the greatest influence on the vibrational energies of the M2OH groups, the tetrahedral site cation composition also influences these vibrations. Shifts were observed in the Si-O-Al bending vibrations from 552 and 480 cm−1 (18.1 and 20.8 μm) in beidellite spectra to 544 and 475 cm−1 (18.4 and 21.0 μm) in montmorillonite spectra. Gaussian modeling of the 4545 cm−1 (2.2 μm) bands led to the discrimination of four overlapping bands in each of the ten Al smectite spectra examined in this study. Shifts in the band center and area of the primary spectral band are coordinated with substitution of Al for Si in the tetrahedral sheet. This is consistent with beidellites having a greater tetrahedral layer charge than montmorillonites. The observed spectral differences were sufficiently large that these Al-rich smectites can be differentiated in orbital data of Mars. A pure beidellite-type spectrum is observed in an isolated Al phyllosilicate-bearing outcrop in Libya Montes, a region where Fe-rich smectite is common but Al-rich smectite is rare. Beidellite-type reflectance spectra were also observed in one area of the Nili Fossae region. In contrast, a variety of Al phyllosilicates were found in the ancient rocks at Mawrth Vallis, including some smaller clay-bearing regions exhibiting spectral signatures more consistent with beidellite-like than montmorillonite-like chemistry.

The Al-clay-rich rock units at Mawrth Vallis, Mars, have been identified as mixtures of multiple components based on their spectral reflectance properties and the known spectral character of pure clay minerals. In particular, the spectral characteristics associated with the ~2.2 μm feature in Martian reflectance spectra indicate that mixtures of AlOH- and SiOH-bearing minerals are present. The present study investigated the spectral reflectance properties of the following binary mixtures to aid in the interpretation of remotely acquired reflectance spectra of rocks at Mawrth Vallis: kaolinite-opal-A, kaolinite-montmorillonite, montmorillonite-obsidian, montmorillonite-hydrated silica (opal), and glass-illite-smectite (where glass was hydrothermally altered to mixed-layer illite-smectite). The best spectral matches with Martian data from the present study’s laboratory experiments are mixtures of montmorillonite and obsidian having ~50% montmorillonite or mixtures of kaolinite and montmorillonite with ~30% kaolinite. For both of these mixtures the maximum inflection point on the long wavelength side of the 2.21 μm absorption feature is shifted to longer wavelengths, and in the case of the kaolinite-montmorillonite mixtures the 2.17 μm absorption found in kaolinite is of similar relative magnitude to that feature as observed in CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) data. The reflectance spectra of clay mixed with opal and of hydrothermally altered glass-illite-smectite did not represent the Martian spectra observed in this region as well. A spectral comparison of linear vs. intimate mixtures of kaolinite and montmorillonite indicated that for these sieved samples, the intimate mixtures are very similar to the linear mixtures with the exception of the altered glass-illite-smectite samples. However, the 2.17 μm kaolinite absorption is stronger in the intimate mixtures than in the equivalent linear mixture. Modified Gaussian Modeling of absorption features observed in reflectance spectra of the kaolinite-montmorillonite mixtures indicated a strong correlation between percent kaolinite in the mixture and the ratio of the area of the 2.16 μm band found in kaolinite to the area of the 2.20 μm band found in montmorillonite.

Outer main-belt asteroids are predominantly of the C-type (carbonaceous), suggesting that they are likely parent bodies of carbonaceous chondrites. Abundant phyllosilicates in some classes of carbonaceous chondrites have chemical compositions, mineral associations, and textures that preserve direct evidence of the processes by which carbonaceous chondrites and their parent asteroids originated and evolved to their present state. Serpentine is the dominant hydroxyl-bearing mineral in the most abundant (CM) group of carbonaceous chondrites. Serpentine may have formed as a direct nebular condensate during cooling of the solar nebula, or by aqueous alteration of anhydrous Mg,Fe-silicate precursors. Such alteration of anhydrous precursors may have occurred in the solar nebula prior to assembly of the meteorites’ parent bodies or on the parent bodies. The relative proportions of Fe and Mg in fine-grained CM2 serpentines have been used to compare the degree of aqueous alteration of different CM2 chondrites with one another. The Mg content of serpentine increases with increasing overall degree of aqueous alteration, so CM2 chondrites with Mg-rich serpentines experienced a more advanced degree of aqueous alteration than CM2 chondrites with Fe-rich serpentines. Attempts to quantify aqueous alteration of CM chondrites by interpreting electron microprobe analyses in terms of charge-balance and site-occupancy constraints from serpentine stoichiometry have met with mixed success. Despite its imperfections, one widely used alteration index based on serpentine stoichiometry is strongly correlated with the elapsed time since the fall and recovery of witnessed CM chondrite falls. Additionally, volatile organic contaminants introduced during sample processing in the laboratory are associated with serpentine and other matrix phyllosilicates. Together, these post-recovery changes in scientifically important sample attributes imply that oxidation-reduction and other types of weathering and contamination affect these meteorites even during curatorial storage and laboratory processing. The same phyllosilicates that make their carbonaceous-chondritic host rocks scientifically important research targets also render those same rocks extraordinarily vulnerable to terrestrial contamination of some of their most scientifically important attributes. This has possible implications for reconstructing pre-terrestrial (parent body) aqueous alteration phenomena from carbonaceous chondritic meteorites and eventually from samples returned by future missions to asteroids with spectral reflectance properties similar to carbonaceous chondrites.