Na-jarosite dissolution rates: The effect of mineral composition on jarosite lifetimes

Icarus 223 (2013) 438–443

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Na-jarosite dissolution rates: The effect of mineral composition on jarosite lifetimes Shayda K. Zahrai a, Megan E. Elwood Madden a,⇑, Andrew S. Madden a, J. Donald Rimstidt b a b

University of Oklahoma, School of Geology and Geophysics, 100 E. Boyd, Suite 710, Norman, OK 73072, United States Virginia Tech, Department of Geosciences, 4044 Derring Hall, Blacksburg, VA 24061, United States

a r t i c l e

i n f o

Article history: Received 8 June 2012 Revised 12 November 2012 Accepted 21 December 2012 Available online 9 January 2013 Keywords: Mars Aqueous alteration Sulfates Chemical weathering

a b s t r a c t Na-jarosite dissolution rates measured at temperatures 277 K, 295 K, and 323 K and at pH  1, 2, 8, and 10 in dilute solutions yield the following rate equation:

r ¼ 0:0946e

26;921 RT

ðaHþ Þ0:830 þ 4:36e

52;272 RT

ðaOH Þ0:317

Dissolution rates are minimized between pH 3 and 4 and increase with increasing activity of both H+ and OH. Overall, Na-jarosite dissolution rates are comparable, but slightly faster than K-jarosite dissolution rates, suggesting that K–Na substitution in the 8-coordinated A site has a minimal effect on dissolution rate. Slightly faster Na-jarosite dissolution rates yielded shorter estimated lifetimes in aqueous systems, but similar activation energies compared to K-jarosite. Calculated Na-jarosite lifetimes for 1 mm jarosite particles (maximum potential jarosite particle size observed at Meridiani Planum) range from less than 1 year at pH 10 and 295 K to 200 years at 277 K at pH 3–4. These calculated lifetimes represent a maximum far-from equilibrium fluid-Na-jarosite contact time for Meridiani Planum. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Jarosite is a ferric hydroxy sulfate mineral that forms on Earth in acidic, oxidizing, and iron-rich environments, such as acid mine drainages (AMDs), fumaroles, and acidic soils. The presence of jarosite in outcrops throughout Meridiani Planum, as observed by the Mars Exploration Rover (MER) Opportunity (Christensen et al., 2004; Klingelhofer et al., 2004; McLennan et al., 2005; Squyres et al., 2006), and other localities on Mars based on spectral data from orbiting spacecraft (Farrand et al., 2009; Glotch and Bandfield, 2006; Roach et al., 2010) indicates that ephemeral, acidic fluids likely existed on Mars (Bibring et al., 2006, 2007; Elwood Madden et al., 2004; King and McSween, 2005; Tosca et al., 2008). Several studies have suggested that hematite, including the blueberries observed in outcrops and in lag deposits at Meridiani Planum, likely formed as jarosite dissolved in diagenetic fluids (Squyres and Knoll, 2005; Golden et al., 2008; Madden et al., 2010). Since jarosite is found in the same outcrops as its dissolution products, dissolution was not complete and we can use this dis-equilibrium assemblage to constrain the duration of post-jarosite aqueous diagenesis at Meridiani Planum (Elwood Madden et al., 2009). While jarosite may have formed, dissolved, and then reformed in more than one geochemical event, there is no evidence for jarosite precipitation following blueberry formation in the rocks at Meridiani Planum (McLennan et al., 2005). Therefore the ⇑ Corresponding author. E-mail address: [email protected] (M.E. Elwood Madden). 0019-1035/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2012.12.020

jarosite particle lifetimes, and hence the duration of post-jarosite aqueous diagenesis can be determined by measuring jarosite dissolution rates of under Mars-relevant conditions (Elwood Madden et al., 2009). Previous studies of jarosite mineralogy, thermodynamic stability, and dissolution kinetics have focused primarily on K-endmember jarosite, which is a common phase in terrestrial AMD and acidic soils (Baron and Palmer, 1996; Smith et al., 2006; Welch et al., 2007, 2008; Elwood Madden et al., 2012). Synthetic endmember K-jarosite was also used in previous batch reaction dissolution experiments to constrain the duration of aqueous diagenesis on Mars (Elwood Madden et al., 2009, 2012). While jarosite can incorporate many ionic substitutions into the general structure AB3(TO4)2(OH)6, including the common substitution of Na+ or H3O+ for K+ in the A site, Al3+ for Fe3+ in the B site and As5+ or P5+ for S6+ in the T site (Papike et al., 2006), the effects of these substitutions on jarosite dissolution rates have not been investigated. However, when both K and Na are present, K-jarosite is the more prevalent phase due to kinetic effects which favor K-jarosite nucleation (Oborn and Berggren, 1995). In fact, K-jarosite is the most common phase in K-enriched terrestrial continental environments, including AMD sediments and acidic soil systems. However, terrestrial Na-jarosite has been observed in soils (Oborn and Berggren, 1995) as well as hydrothermal deposits (Desborough et al., 2010). Indeed, APXS K and Na concentrations plotted as a function of Mossbauer-based jarosite concentrations in outcrops suggest that Na-jarosite ± hydronium jarosite is a better fit with the combined chemical/mineralogical observations at Meridiani Planum

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than K-jarosite (Clark et al., 2005). Therefore, Na-jarosite (also referred to as natrojarosite in the literature), is a likely jarosite phase in K-deficient mafic-sourced sediments on Mars (McSween et al., 2003; Reider et al., 2004; Taylor and McLennan, 2009). Complete solid solution between Na and K-jarosite has been observed in laboratory synthesized materials (Drouet and Navrotsky, 2003; Drouet et al., 2004), but limited solid solution was observed in natural hydrothermal and low-temperature jarosite samples (Papike et al., 2006; Desborough et al., 2010). Similarly, hydronium jarosite has been synthesized in laboratory studies, but is largely absent in natural samples, suggesting that under terrestrial conditions Na- and K-jarosite are the more stable phases that may be preserved over geologic time scales (Desborough et al., 2010). Hence, Na- and K-jarosite are expected to be the predominant stable low-temperature jarosite phases on both Earth and Mars, while Na-jarosite will likely be more abundant on Mars than on Earth due to the relatively high Na/K ratios observed in both rocks and sediments. While the thermodynamics of Na-jarosite and mixed cation jarosite phases have been investigated for application to hydrometallurgy and toxic metal sequestration (Drouet and Navrotsky, 2003; Drouet et al., 2004; Dutrizac, 1999; Casas et al., 2007; Asta et al., 2009; Snyder and Vinals, 2011), studies examining the dissolution behavior of Na-jarosite in natural environments or dilute solutions (e.g., Oborn and Berggren, 1995; Elwood Madden et al., 2009) are limited. In this study, the rate of Na-jarosite dissolution was determined and compared to previous K-jarosite results (Elwood Madden et al., 2012) to understand the effect of jarosite cation chemistry on dissolution rates and particle lifetimes under Mars-analog conditions. 2. Methods The Na-jarosite used in this study was synthesized using the method described by Driscoll and Leinz (2005). Na-jarosite powder was cavity-mounted and analyzed by powder X-ray diffraction (XRD) with a Rigaku Ultima IV unit equipped with a Cu tube at 40 kV and 44 mA. A diffracted beam monochromator and scintillation detector were used. XRD results confirmed Na-jarosite was the dominant synthesis product (Fig. 1) with minor amounts of thenardite (Na2SO4) and ferrinatrite (Na3Fe(SO4)33H2O). Electron microprobe analysis of the bulk powder yielded the following atomic proportions based on 8 oxygen and 6 OH expected in the Na-jarosite structure: Na1.16Fe2.89(SO4)2.03(OH)6. However, Na analyses using EMPA are prone to error due to rapid diffusion from the analysis spot. In addition, the thenardite and ferrinatrite

Fig. 1. Powder XRD pattern of Na-jarosite synthesis product. Peak labels correspond to N = natrojarosite (Powder Diffraction File Card 00-036-0425), T = thenardite (PDF Card 00-037-1465), and F = ferrinatrite (PDF 00-041-0609).

439

(Na3Fe(SO4)33H2O) lead to additional Na content in the bulk analysis relative to S and Fe. Therefore, we cannot quantitatively constrain the hydronium content of the jarosite. The surface area of the synthesized Na-jarosite (2.6 m2 g1) was measured using nitrogen adsorption BET surface area analysis following overnight degassing at 303 K. Dissolution experiments were conducted at 277 K, 295 K, and 323 K in ultrapure water, as well as buffered room temperature experiments at pH 1 (H2SO4), pH 2 (H2SO4), pH 8 (0.5 M Tris buffer (HOCH2)3CNH2 adjusted with 6 N HCl), and pH 10 (0.5 M Tris buffer). Solutions were mixed on a stir plate with a Teflon-coated magnetic stir bar constantly stirring the solution. This method effectively kept the jarosite suspended without disintegrating the jarosite (based on TEM observations from similar experiments reported in Elwood Madden et al. (2012)). Since the jarosite remained suspended during the experiments, we also did not have to correct the concentrations measured for solution removed since the system was fully mixed when sampled and the filtered solids were not returned to the experiment. The 277 K, 295 K, and 323 K experiments initially contained 0.25 g Na-jarosite and 240 g of solution in a 600 mL beaker, while the pH 1, pH 8, and pH 10 experiments were conducted in a 150 mL beaker initially containing 0.15 g jarosite and 150 g of solution. To prevent contamination of the experiments with ions from the pH electrode, an aliquot of the initial solution was removed and the pH measured prior to jarosite addition. The initial slurry pH was determined by measuring the pH of a replicate batch reactor at 1–10 min intervals, then calculating the y-intercept of a linear fit of the time versus pH data (Elwood Madden et al., 2012). The 277 K experiments were conducted in an ice bath on top of a stir plate within a standard refrigerator, while heated stir plates with a thermocouple feedback temperature control system were used for the 323 K experiments. In each experiment, an experimental blank was collected before adding Na-jarosite. After adding the Najarosite, 10 mL samples were collected at regular intervals and filtered through a 0.2 lm syringe filter. The timing interval for sample collection was based on the predicted reaction progress from previous results (Elwood Madden et al., 2012). Experiments were replicated 2–4 times to ensure reproducibility of the results. Samples were stored in a refrigerator until the concentration of Na+ was analyzed using flame Atomic Absorption Spectrophotometer (AAS). Settings on a Perkin Elmer AAnalyst 800 were determined according to the manufacturer’s instructions and standards were matrixmatched, including potassium chloride to suppress Na ionization. Samples were analyzed three times and average values used for rate calculations. Rate of dissolution was determined by plotting the reported Na+ concentration versus reaction time of each experiment and then fitting the curve with a second order polynomial (chosen based on the goodness of the fit). The second term of the fit of this curve gives the instantaneous slope at t = 0, providing an initial rate of dissolution (Rimstidt and Newcomb, 1993). Na+ release was used as the metric for jarosite dissolution for three reasons: (1) Fe3+ has a low solubility in aqueous solutions at pH > 2, leading to precipitation of abundant nanophase iron oxides including hematite, maghemite, and goethite in previous K-jarosite experiments (Elwood Madden et al., 2012). Therefore, most of the aqueous Fe released during jarosite dissolution is immediately removed from solution. (2) SO2 4 readily sorbs to these positively charged iron oxide surfaces (Fukushi and Sverjensky, 2007; Hug, 1997; Watanabe et al., 1994), again leading to removal of SO2 4 from solution following dissolution. (3) Since most previous studies have used the A-site cation as the metric for jarosite dissolution (Smith et al., 2006; Elwood Madden et al., 2009, 2012), Na+ concentration versus time data are required to directly compare K- and Na-jarosite dissolution rates. The rate equation was derived by multiple linear regression of log r versus pH and 1/T using JMP statistical software from SAS

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Institute Inc., Cary, NC. t-tests were used to test the significance of the fitted parameters and residual plots were carefully scrutinized to ensure that there were no outliers in the data set. R2 for the fit is 0.949. The uncertainty in each of the fitted parameters is reported as 1 standard error. Details about fitting strategies for rate equations are given in Rimstidt et al. (2012). 3. Results The dissolution rates of Na-jarosite over a pH range from 1 to 10 and temperature from 277 to 323 are listed in Table 1, as well as Fig. 2. Na-jarosite rates increase with increasing pH at pH > 3.8, but also increase at low pH, forming a ‘‘V’’ shape (Fig. 2). Previous K-jarosite experiments yielded similar results; however, the Na-jarosite dissolution rates are slightly faster than comparable K-jarosite rates at similar temperature and pH conditions. The derived rate equation for Na-jarosite dissolution at pH > 3.8 based on multilinear regression of log rate as a function of both 1/T and pH is

log r ¼ 0:639ð2:744Þ þ 0:317ð0:027ÞpH  2730ð762Þ=T

ð1Þ

where r is the rate of Na+ release (mol m2 s1), T is temperature in Kelvin and 1 standard error for the each coefficient is shown in parentheses. Na-jarosite dissolution at pH P 3.8 yields an activation energy (Ea) = 52.3 (±14.6) kJ/mol, slightly less than the activation energy for K-jarosite (73.9 ± 4.7 kJ/mol) measured under similar conditions. Multilinear regression of Na-dissolution at pH 6 3.8 produces the following rate equation:

log r ¼ 1:024ð1:958Þ  0:830ð0:118ÞpH  1406ð593Þ=T yielding an activation energy of 26.9 (±11.4) kJ/mol. The combined rate equation for Na-jarosite dissolution from pH 1–10 and 280– 323 K in dilute solutions is: 26;921 RT

r ¼ 0:0946e

52;272 RT

ðaHþ Þ0:830 þ 4:36e

ðaOH Þ0:317

ð2Þ

 where R is the ideal gas constant and aþ H and aOH represent the activity of hydronium and hydroxyl ions in solution (Fig. 3a).

Table 1 Na-jarosite and K-jarosite dissolution rates. Initial solution pH

T (K)

Solution

Initial slurry pH

Log r

Source

Na-jarosite 5.7 5.7 5.7 8.1 8.1 1.0 1.0 2.0 2.0 8.0 8.0 8.0 10.0 10.0 10.0 5.7 5.7 5.7 5.7

277 277 277 295 295 295 295 295 295 295 295 295 295 295 295 323 323 323 323

Water Water Water Water Water H2SO4 H2SO4 H2SO4 H2SO4 Tris + HCl Tris + HCl Tris + HCl Tris Tris Tris Water Water Water Water

3.8 3.8 3.8 3.5 3.5 1.0 1.0 2.0 2.0 8.0 8.0 8.0 10.0 10.0 10.0 3.0 3.0 3.0 3.0

9.50 9.27 9.1 8.89 8.80 7.02 6.94 7.1 6.6 7.35 7.27 7.05 6.97 6.89 6.58 8.22 8.10 7.78 7.45

This This This This This This This This This This This This This This This This This This This

K-jarosite 6.4 6.8 5.9 6.4 0.9 2.0 3.0 4.0 5.9 6.4 7.1 9.6 10.9 6.3 7.9 4.3 4.9 2 8 3.76 4 3 4 3 5.5 2

280 296 313 323 296 296 296 296 296 296 296 296 296 296 296 296 296 298 298 298 298 298 298 298 298 298

Water Water Water Water H2SO4 H2SO4 H2SO4 H2SO4 NaOH NaOH NaOH NaOH NaOH MES MES Water Water HCl CaOH Water HCl HCl H2SO4 H2SO4 Water HCl

5.0 4.4 4.3 4.05 0.9 2.0 3.0 4.0 4.5 4.7 5.0 6.7 10.9 6.2 7.5

9.71 (0.16) 9.2 (0.05) 8.59 (0.03) 8.24 (0.10) 7.15 (0.08) 8.54 (0.06) 9.09 (0.05) 9.28 (0.02) 9.24 (0.23) 9.20 (0.10) 8.93 (0.05) 8.71 (0.09) 6.59 (0.07) 8.49 (0.07) 7.95 (0.13) 9.1 (0.4) 8.2 (0.5) 8.8 9.1 11.2 (0.1) 11.3 (0.9) 11.4 (0.4) 11.3 (0.5) 11.8 (0.3) 6.8 (0.4) 8.5 (0.03)

Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2012) Elwood Madden et al. (2009) Elwood Madden et al. (2009) Smith et al. (2006) (first 85 days) Smith et al. (2006) (first 85 days) Welch et al. (2008) Welch et al. (2008) Welch et al. (2008) Welch et al. (2008) Welch et al. (2008) Gasharova et al. (2005) Baron and Palmer (1996)

Units on r: (mol m2 s1) (± 1 std. dev.).

study study study study study study study study study study study study study study study study study study study

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-5

-6.0

A

-6

-7.0

log r (mol m-2 s-1)

log r (mol m-2 s-1)

-6.5

-7.5 -8.0 -8.5 -9.0

-7 -8 -9 -10

-9.5

-11

-10.0 0

2

4

6

8

10

0

12

2

4

1400

Fig. 2. Effect of pH on Na- and K-jarosite dissolution rates at room temperature. Najarosite dissolution rates determined in this study (solid triangles) follow the same trends with pH observed in a previous K-jarosite study (open circles). Both Na- and K-jarosite dissolution rates increase at low and high pH with minimum rates observed from pH 3–4. This suggests that jarosite dissolution occurs via two different mechanisms. The low pH mechanism is mediated by H+, while the high pH mechanism involved OH. Similar trends in dissolution rate with pH have also been observed in silicate mineral dissolution studies. Error bars represent one standard deviation from the mean of 2–4 replicate experiments.

8

10

12

B

1200

lifetime (yr)

1000 800 600 400

4. Discussion Rates of Na-jarosite dissolution are very similar to those reported for K-jarosite dissolution (Elwood Madden et al., 2012). This suggests that cation substitution in the A site does not significantly affect dissolution rates. The greater absolute value of enthalpy of hydration (DHhydration) for Na+ compared with K+ in aqueous solution (Table 2) may lead to preferential dissolution of Na+ from the structure. However, K+ has a larger ionic radius and thus lower ionic potential, which suggests the K–O bonds are weaker than Na–O bonds (Fig. 4). These two competing properties appear to offset one another, resulting in nearly equal rates of dissolution under the conditions studied here. The ‘‘V’’ shaped trend in dissolution rates over pH 1–10 conditions indicates that dissolution rates increase with increasing H+ or OH in solution. Similar ‘‘V’’ shaped trends have also been observed in silicate mineral and glass dissolution kinetics (e.g. Chou and Wollast, 1985; Chen and Brantley, 1997; Gislason and Oelkers, 2003; Kohler et al., 2003; Lowson et al., 2005), as well as dissolution of corundum (Carroll-Webb and Walther, 1988). This suggests that different dissolution mechanisms dominate at low and high pH. Under low pH conditions, Na-jarosite dissolves via the reaction:

NaFe3 ðSO4 Þ2 ðOHÞ6 þ 6Hþ ¼ Naþ þ 3Fe3þ þ 2SO2 4 þ 6H2 O

6

pH

pH

ð3Þ

consuming H+ as the reaction proceeds. However at pH conditions >3–4, the dissolution reaction consumes OH as secondary iron oxide/hydroxide minerals (represented by Fe(OH)3 in Eq. (4), however, the specific phase is likely dependent on the pH and the rate of formation) are precipitated due to lower solubility of Fe3+ at moderate to high pH (Elwood Madden et al., 2012).

200 0 0

2

4

6

8

10

pH Fig. 3. Model Na- and K-jarosite dissolution rates (A) and 1 mm particle lifetimes (B) based on comprehensive rate equations from this study (Eq. (3)) and Elwood Madden et al. (2012 – eqs. (2) and (3)). Short dashed gray lines are estimates for low temperature K-jarosite dissolution since the activation energy was not determined for the low pH dissolution mechanism. Maximum particle lifetimes (30– 1350 years) are predicted at pH 3–4 where dissolution rate is minimized.

(pH > 3–4). Therefore, in closed systems, aqueous fluids in contact with jarosite may be buffered to pH 3–4 due to jarosite dissolution, effectively slowing the dissolution process as jarosite reacts with the fluid. 4.1. Na-jarosite lifetimes Lifetimes of 1 mm Na-jarosite particles were estimated using a shrinking sphere equation:

Dt ¼ d=2V m r

ð5Þ

where d is the diameter (m), Vm is the molar volume (Na-jarosite = 146.79, K-jarosite = 160.26 cm3 mol1), and r is the rate of dissolution (mol m2 s1) (Lasaga, 1998). The resulting lifetimes – t (s) – for 1 mm Na- and K-jarosite grains, the largest particle diameter observed within the jarosite-bearing matrix at Meridiani Planum (McLennan et al., 2005), are shown in Fig. 3b. Na-jarosite

NaFe3 ðSO4 Þ2 ðOHÞ6 þ 3OH ¼ 3FeðOHÞ3 þ Naþ þ 2SO2 4 þ 3H2 O ð4Þ These two concurrent mechanisms lead to the ‘‘V’’ shaped trend in dissolution rates since the rate of mechanism (3) increases with concentration of H+ ions and mechanism (4) increases with OH ions. The transition between which of these two mechanisms dominates occurs at pH  3.8. Based on these two mechanisms, jarosite dissolution will either raise the pH of acidic solutions (initial pH < 3–4) or lower the pH of sub-neutral to alkaline solutions

Table 2 Ionic properties of Na+ and K+.

a b

Ion

Ionic radius (Å)a

Ionic potential (z/r)

DHhydration (kJ/mol)b

Na+ K+

1.16 1.51

0.86 0.66

405 321

Shannon and Prewitt (1969). Burgess (1988).

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Fig. 4. K- and Na-jarosite crystal structures produced with CrystalMaker based on the full Fe-occupancy structural model of Basciano and Peterson (2007). Both K+ and Na+ fill the A site (8-fold coordination) within the generalized jarosite structure AB3(TO4)2(OH)6, where the B site in standard endmember jarosite is filled by Fe in octahedral coordination and the T (tetrahedral) site is filled by S. K+ has a slightly larger ionic radius (Table 2) than Na+, leading to slightly longer K–O bond distances (2.778 minimum– 2.974 maximum), compared to Na–O bond distances in Na-jarosite (2.679 minimum–2.935 maximum).

lifetimes are predicted to be longer in lower temperature and subneutral pH aqueous systems and shorter in higher temperature and both extremely low or high pH aqueous systems. Since Na-jarosite dissolution rates are slightly faster than previously determined Kjarosite dissolution rates, Na-jarosite would be not be expected to survive as long as K-jarosite particles in aqueous solutions. The relatively short geologic lifetimes of both Na-jarosite and Kjarosite over all the conditions studied here indicate that in order to preserve jarosite within rocks on Mars, post-jarosite aqueous diagenesis and/or weathering on Mars was limited to geologically short intervals. On the other hand, as minerals dissolve in a closed system, the solution approaches equilibrium with the mineral phase and the dissolution rate generally slows as the chemical driving force decreases (e.g. Langmuir, 1996). Therefore, in closed systems, mineral lifetimes may be extended as the fluids reach equilibrium with the primary mineral assemblage. However, if those fluids are further concentrated via evaporation and/or freezing, dissolution rates may increase due to secondary mineral precipitation and subsequent changes in anion speciation within the fluid (Pritchett et al., 2012). 4.2. Aqueous environment at Meridiani Planum Low temperature conditions produced the slowest dissolution rates, and hence the longest jarosite lifetimes. This suggests that jarosite at Meridiani Planum could have survived longer periods of low temperature aqueous alteration within permafrost (as proposed by Niles and Michalski (2009)) than higher temperature hydrothermal (Golden et al., 2008) or volcanic (McCollom and Hynek, 2005) alteration events. Dissolution rates are also minimized around pH 3–4, yielding longer particle lifetimes. While initial jarosite formation requires lower pH conditions (pH 1–4), the presence of hematite coexisting with the jarosite in the outcrops suggest mildly acidic to neutral fluids were present after jarosite formed (Elwood Madden et al., 2012). 5. Conclusions This study and previous studies indicate that Na- and K-jarosite dissolution rates depend upon temperature and pH. Both K- and Na-jarosite dissolution rates decrease with decreasing temperature and are minimized at pH 3–4. However, Na-jarosite dissolution rates at neutral pH are slightly faster than K-jarosite rates and yield shorter particle lifetimes in dilute aqueous systems. The overall similarity between Na- and K-jarosite dissolution rates indicates that substitution of univalent cations with similar ionic radii into the A-site within the general jarosite structure has little effect on

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