The Role of Sulfate in Cation Exchange Reactions: Applications to Clay–Brine Interactions on Mars

Most aqueous fluids active near the surface of Mars, both in the geologic past and the present, were/are likely salty (Wang et al. 2006; Schwenzer et al. 2012, 2016). It is improbable that one singular brine composition is responsible for the spatial and stratigraphic variability of the mineralogy seen on Mars (Squyres et al. 2004b; Schwenzer et al. 2012, 2016). Therefore, this study employed MgSO₄, NaCl, and Na₂SO₄ brines in different concentrations and ratios to study the effects of clay reactions with brines, representing different diagenetic conditions on Mars. MgSO₄ and Na₂SO₄ brines were chosen because of their potential presence on Mars and abundant sulfates, as well as the cations involved. Cation exchange and selectivity in clays is complex and highly variable, even within a specific clay group, so the cations and concentrations used in this study have been chosen according to the conceptual model Na⁺ > Ca²⁺ > Mg²⁺ (Singh & Turner 1965; Sposito et al. 1983; Suarez & Zahow 1989; Tang & Sparks 1993; Appelo & Postma 2004), where Na⁺ is the most likely to exchange into the interlayer from solution, followed by Ca²⁺, and finally Mg²⁺, which is the least likely ion to move from solution into the interlayer, due to a combination of size, charge, and hydration sphere effects. All three cations are present on the martian surface, and it is possible that each had a role to play in the evolution of phyllosilicates on Mars (Vaniman & Chipera 2006; Osterloo et al. 2008; Thomas et al. 2019).

To better constrain the factors that influence clay–brine interactions on Mars, two sets of experiments were conducted. The first investigated MgSO₄ and Na₂SO₄ brine–smectite reactions at varying total salt concentrations (0.1M, 0.2M, and 2.0M Na₂SO₄, as well as 0.1M, 0.3M, and 3.0M MgSO₄; see Table 1). These reactions are referred to as "variable brine concentration" experiments. The second set of experiments, referred to as "variable cation/anion concentration" experiments, investigated the influence of individual cation/anion concentrations on gypsum precipitation. We analyzed the resulting solids with Raman and XRD, both of which are currently available for in situ analyses on Mars (Bristow et al. 2015; Scheller et al. 2022; Tripathi & Garg 2022).

To investigate the effects of varying individual cation and anion concentrations, we also prepared brines with independently varying Na⁺ and SO₄ ²⁻ concentrations, by adding chloride salts (Table 2). The substitution of Na₂SO₄ with NaCl was used to manipulate Na⁺ concentrations independent of SO₄ ²⁻ concentrations. NaCl was used because, for the purposes of this study, Cl⁻ can be considered conservative. Of the ten separate brines prepared, Brines 1–5 had constant SO₄ ²⁻ concentrations and linearly increasing Na⁺ concentrations, while Brines 6–10 had constant Na⁺ concentrations and linearly increasing SO₄ ²⁻ concentrations; all had similar water activities (Table 2).

All brines used in this study were prepared by adding high-purity salts to 18.2 MΩ-H₂O in appropriate quantities, to yield the desired concentrations. The brines were allowed to equilibrate for >24 hr, to ensure complete dissolution, before we filtered the brines with a 0.45 μm filter and subsequently mixed the filtered, near-saturated brines with smectite.

We used Ca-montmorillonite "SAz-1" (Apache County, Arizona, USA), obtained from the Clay Minerals Society (Chipera & Bish 2001; Mermut 2001a, 2001b), as our model smectite phase. While other smectites, such as nontronite and saponite, are observed in greater abundances than montmorillonite on Mars (Bibring et al. 2005), SAz-1 was chosen for several reasons. First, SAz-1 is relatively pure compared to other widely available smectite clay standards, and second, it will allow direct comparisons to previous studies of cation exchange–induced gypsum formation (Garrels & Christ 1965; Wilson & Bish 2011, 2012; Lafuente et al. 2015). Last, SAz-1 is similar in structure and layer charge to nontronite and saponite (Grim 1968; Essington 2015). While both nontronite and montmorillonite are dioctahedral smectites, saponite is trioctahedral; however, cation exchange behavior should be similar across all three mineral species, since it is primarily controlled by the magnitude of the layer charge and the ion concentration of the fluid involved, not the source of the layer charge. In addition, SAz-1 exhibits some of the highest cation exchange capacity (CEC) observed in clays, 110–123 cmol•kg⁻¹ (Mermut 2001b; Essington 2015), whereas other smectites such as nontronite have slightly lower CEC values, between 75–96 cmol•kg⁻¹ (Bischoff 1972).

While Ca²⁺ may not be the dominant interlayer cation in smectites on Mars, in order to ensure a homogeneous starting material, natural SAz-1 interlayer cations were exchanged with Ca²⁺ to produce a uniform endmember starting composition. We saturated the SAz-1 with Ca²⁺ by allowing the clay to react with ∼3.5 M CaCl₂ for 24 hr, followed by four cycles of washing with 18.2 MΩ H₂O, then centrifugation at 10,000 RPM for 20 minutes and decanting the supernatant. After washing, the Ca-exchanged SAz-1 was freeze dried using a Labconco freeze dryer.

Clay–brine pastes were created by mixing 0.5 g of clay with 0.5 mL of brine and mixing them thoroughly, using a vortex mixer, for 30 s. We analyzed the samples immediately, using powder XRD and Raman spectroscopy, to characterize the reaction products, with analysis times of <2 hr and <5 minutes, respectively.

We used a Rigaku Ultima IV X-Ray Diffractometer (Cu tube, 40 kV, and 44 mA curved graphite monochromator) to analyze the mineralogy of the samples via powder XRD. XRD slides were prepared following the smear technique (Moore & Reynolds 1997) and analyzed immediately. The XRD patterns were interpreted using MDI Jade Pro with the ICDD PDF4+ database; mineral phase weight percentages were calculated using FULLPAT (Chipera & Bish 2002). It is worth noting that one of the key diagnostic XRD peaks for smectites, the 001 basal reflection, is sensitive to both cation saturation and water activity; however, Chipera et al. (1997) have demonstrated Ca-exchanged SAz-1 001 spacing to be stable in laboratory conditions (Chipera et al. 1997; Moore & Reynolds 1997). Thus, for the timescales involved in these analyses (<4 hr), any shift detected in the 001 reflection can be ascribed to a change in the intercalating cation, rather than hydration from atmospheric water sorption (Moore & Reynolds 1997).

We used a Renishaw InVia Raman microscope, equipped with a 500 mW 785 nm red laser and 1200 l/cm grating, centered at 900 cm⁻¹, to acquire Raman spectra from 300 to 1500 wavenumbers. The laser was set at 1% power, to minimize sample heating and possible mineral degradation. Immediately before Raman analysis, SAz-1 and the individual brines were mixed to a paint-like consistency on a glass slide. Spectra were collected for 300 s, with repeat sampling to determine if laser-induced alteration occurred.

Gypsum was observed in all the variable brine concentration NaSO₄ experiments (0.1 M, 0.2 M, and 2.0 M Na₂SO₄). However, of the MgSO₄ clay–brine mixtures, only 3.0 M MgSO₄ produced detectable gypsum (∼2 wt%), along with 58 wt% epsomite (MgSO₄7H₂O; Table 3; Figure 1).

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Raman spectra of the Ca-exchanged SAz-1 that reacted with 18.2 MΩ H₂O exhibited peaks that are typical of SAz-1, such as the SiO₄ and Al₂O₃ structural lattice modes at 435 cm⁻¹ and 700 cm⁻¹, while lacking any vibrational modes associated with SO₄ ²⁻ (Bishop & Murad 2004; Figures 1(C) and (D)). Raman spectra of the 3.0 M MgSO₄ and 2.0 M Na₂SO₄ clay–brine mixtures clearly exhibit a v₁(SO₄ ²⁻) peak at 981 cm⁻¹, as well as a characteristic gypsum peak at 1007 cm⁻¹ (Wang et al. 2006; Ben Mabrouk et al. 2013; Lafuente et al. 2015; Figure 1(D)).

Epsomite formation was only observed in the 3.0 M MgSO₄ XRD analysis (Figure 1(A)). However, in the Raman spectra, the vibrational modes that are commonly associated with hydrated SO₄ ²⁻ in epsomite (v₂(SO₄ ²⁻ )) at 450 cm⁻¹ and v₃(SO₄ ²⁻) at 1100 cm⁻¹ (Wang et al. 2006)) were not observed (Figures 1(C) and (D)). Although a peak at ∼983 cm⁻¹ (Figure 1(D)) is commonly assigned as v₁(SO₄ ²⁻ ) for epsomite, the same frequency has been observed in sulfate salt hydrates, regardless of the cation (Ben Mabrouk et al. 2013; Mason & Madden 2022). Therefore, we interpret the ∼980 cm⁻¹ Raman peak as residual aqueous sulfate remaining in the sample, since the secondary peaks, v₂(450 cm⁻¹) and v₃(1100 cm⁻¹), indicative of epsomite, are missing. The absence of epsomite in the Raman analysis of the 3.0 M MgSO₄ clay–brine mixture is confirmed by examining the relative intensity ratio of the 980 cm⁻¹ to 450 cm⁻¹ peaks, as the experimental sample has a lower relative intensity ratio (60%) as compared to the RRUFF epsomite reference spectrum.

Gypsum formation was observed in all experiments where a brine containing Na⁺ and SO₄ ²⁻ reacted with Ca-exchanged SAz-1. In the experiments investigating variable Na⁺ concentrations (SO₄ ²⁻ concentrations being held constant), the volume of gypsum produced was relatively constant, ∼7 wt% (Figure 2(C)). However, in the experiments investigating varying SO₄ ²⁻ concentrations (Na⁺ concentrations being held constant), gypsum increased nonlinearly with increasing sulfate concentration (Figure 2(D)). A closer inspection of the 10–25 Å d-spacings reveals that varying the Na⁺ and SO₄ ²⁻ brine concentrations also influenced the 001 basal reflection (Figure 3). The swelling observed in SAz-1 reacted with 0.6 M Na⁺ brine to ∼19.5 Å can be explained by the presence of an added water layer. Slade et al. (1991) found that in lower-concentration ionic solutions, a transition from two to three water layers will occur when the hydration enthalpy of the interlayer cations substantially overwhelms the layer charge deficit (Slade et al. 1991).

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The results from these experiments demonstrate that gypsum formation is a function of both the concentration and the composition of the aqueous solution. Gypsum formed in less than 1 minute, as observed in our Raman analyses, corroborating the rapid gypsum formation rates that have been observed by Wilson & Bish (2011, 2012). The amount of gypsum that was formed in montmorillonite reactions with all three Na₂SO₄ brines was uniform at 7–9 wt%; however, gypsum formation was not observed in reactions involving the lower concentrations of MgSO₄ brines.

Considering the rapid nature of the reactions, as well as the preponderance of gypsum formation from Na₂SO₄ clay–brine reactions, cation exchange is likely the dominant mechanism leading to gypsum formation in our experiments. Modeling of the solution chemistry produced by brine-induced cation exchange in Antarctic sediments has also predicted the precipitation of gypsum from freezing brines (Toner & Sletten 2013). Montmorillonite ion selectivity (retention in the interlayer) generally follows Mg²⁺ > Ca²⁺ > Na⁺, but replacement is also dependent on ion activities, both in solution and at the exchanging sites (Garrels & Christ 1965; Grim 1968; Sumner & Miller 1996; Essington 2015). To complicate matters, it is not only the cation activity that must be considered, but also the anion(s); as shown in Figures 2(C) and (D), gypsum precipitation increased with increasing SO₄ ²⁻ concentrations.

While cation exchange adequately explains reactions between clays, brines and the resulting gypsum precipitation, it is unlikely that clays on Mars experienced a sudden shift in the concentration of a singular brine. Rather, evapoconcentration, fluid mixing, and an evolving climate likely altered the chemistry of the brines in contact with clays (Tu et al. 2021b; Bristow et al. 2021).

The experiments in this study that include varied concentrations of Na⁺ and SO₄ ²⁻ shed some light on how cation exchange reactions may have progressed as brines on Mars evolved. The correlation between gypsum precipitation and SO₄ ²⁻ concentration seen in Figure 2(D) is not unexpected, as SO₄ ²⁻ is a primary constituent of gypsum. And yet, the apparent irrelevance of Na⁺ concentrations requires some consideration. The constant amount of gypsum precipitated from the variable Na⁺ brines, in juxtaposition with the variable SO₄ ²⁻ brines, demonstrates that although cation exchange may be the driving mechanism, the anions present in solution also influence cation exchange beyond just contributing to the ionic strength of a brine. As SO₄ ²⁻ concentrations are increased, a progressive reduction in the 001 reflection d-spacings, by 11.9%, is observed (Figure 3). However, as Na⁺ concentrations increase, the shift to smaller d-spacings is less pronounced (4.0%). We interpret this reduction in d-spacings as evidence that interlayer-bound Ca²⁺ has been exchanged for Na⁺ originating from the brine, causing the collapse of the basal spacings. Progressive shifts to smaller d-spacings as a function of ion concentration are observed in both the variable SO₄ ²⁻ and Na⁺ brines, but the shift was almost three times greater for increasing concentrations of SO₄ ²⁻ than for Na⁺. Na-smectite d-spacings are typically ∼12.5 Å, while Ca-smectites typically have larger d-spacings (14.5 Å–15.5 Å; Grim 1968; Essington 2015). Since the basal spacing did not shift fully from ∼15 to ∼12.5 Å, this indicates that the exchange of Ca²⁺ for Na⁺ is incomplete, which is unsurprising, given that the time allowed for the exchange was the minimum time required for the analysis. Variable hydration layers in the interlayer that are associated with different cations could also contribute to the observed shift in d-spacings (Yotsuji et al. 2021). The substantially larger shift due to cation exchange in the variable SO₄ ²⁻ brines demonstrates that SO₄ ²⁻ concentration influences cation exchange considerably more than Na⁺ or Cl⁻, at least in the concentrations that were investigated in this study.

While it is unsurprising that an overwhelming concentration of Na⁺ will displace interlayer-bonded Ca²⁺ in smectites, the increased cation exchange as a response to SO₄ ²⁻ and the apparent irrelevance of Cl⁻ are worth noting. Early studies examining the cation selectivity of smectites record a history of inconsistent interpretations regarding the influence of Cl⁻, SO₄ ²⁻, and ClO₄ ⁻, as well as the mechanisms driving cation exchange; however, previous studies have focused on more dilute solutions, where ion concentrations are significantly lower (maximum 0.1M Na⁺/Ca²⁺/Mg²⁺) compared to the present study (Singh & Turner 1965; Singh 1982; Sposito et al. 1983; Suarez & Zahow 1989; Sposito 1991; Teppen & Miller 2006). Some of the aforementioned studies did observe increased cation exchange in the presence of SO₄ ²⁻, as compared to solutions dominated by Cl⁻, NO₃ ⁻, or ClO₄ ⁻, indicating that cation exchange is more active as SO₄ ²⁻ concentrations increase, given similar brine chemistry.

The variable brine concentration experiments demonstrate that lower concentrations of MgSO₄ brines will not induce gypsum formation. As free liquid water availability decreased on Mars, brines would have become increasingly concentrated, eventually reaching saturation. The results shown above suggest that gypsum would not have precipitated from clay–brine reactions until the clays were exposed to brines with significant concentrations of MgSO₄, and in this case there would be more epsomite present than gypsum, so the gypsum may not be discernable from orbit at Mars. However, exposure to brines containing both Na⁺ and SO₄ ²⁻ could readily produce gypsum, even at relatively low concentrations of either ion.