Thermochemistry of Jarosite-Alunite and Natrojarosite-Natroalunite Solid Solutions

Home , Alunite, Jarosite

Geochimica et Cosmochimica Acta, Vol. 68, No. 10, pp. 2197Ð2205, 2004 Copyright © 2004 Elsevier Ltd Pergamon Printed in the USA. All rights reserved 0016-7037/04 $30.00 ϩ .00 doi:10.1016/j.gca.2003.12.001 Thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions

1 1 2 2 1, CHRISTOPHE DROUET, KATRINA L. PASS, DIRK BARON, SARA DRAUCKER, and ALEXANDRA NAVROTSKY * 1Thermochemistry Facility and NEAT ORU, University of California at Davis, Davis, California 95616, USA 2Department of Physics and Geology, California State University at Bakersﬁeld, Bakersﬁeld, California 93311, USA

(Received July 10, 2003; accepted in revised form December 2, 2003)

Abstract—The thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions was investigated. Members of these series were either coprecipitated or synthesized hydrothermally and were characterized by XRD, FTIR, electron microprobe analysis, ICP-MS, and thermal analysis. Partial alkali substitution and vacancies on the Fe/Al sites were observed in all cases, and the solids studied can be described by the

general formula K1-x-yNay(H3O)xFezAlw(SO4)2(OH)6Ð3(3-z-w)(H2O)3(3-z-w). A strong preferential incorpora- ⌬ tion of Fe over Al in the jarosite/alunite structure was observed. Heats of formation from the elements, H¡f, were determined by high-temperature oxide melt solution calorimetry. The solid solutions deviate slightly from thermodynamic ideality by exhibiting positive enthalpies of mixing in the range 0 to ϩ11 kJ/mol. The ⌬ ϭϪ Ϯ heats of formation of the end members of both solid solutions were derived. The values H¡f 3773.6 ⌬ ϭϪ Ϯ ⌬ ϭϪ Ϯ ⌬ ϭϪ Ϯ 9.4 kJ/mol, H¡f 4912.2 24.2 kJ/mol, H¡f 3734.6 9.7 kJ/mol and H¡f 4979.7 7.5 kJ/mol were found for K0.85(H3O)0.15Fe2.5(SO4)2(OH)4.5(H2O)1.5,K0.85(H3O)0.15Al2.5(SO4)2(OH)4.5(H2O)1.5, Na0.7(H3O)0.3Fe2.7(SO4)2(OH)5.1(H2O)0.9, and Na0.7(H3O)0.3Al2.7(SO4)2(OH)5.1(H2O)0.9 respectively. To our ⌬ knowledge, this is the ﬁrst experimentally-based report of H¡f for such nonstoichiometric alunite and natroalunite samples. These thermodynamic data should prove helpful to study, under given conditions, the partitioning of Fe and Al between the solids and aqueous solution. Copyright © 2004 Elsevier Ltd

1. INTRODUCTION have been described (Brophy et al., 1962; Ha¬rtig et al., 1984; Alpers et al., 1992; Long et al., 1992; Stoffregen et al., 2000). Jarosites and alunites are members of the alunite supergroup Brophy et al. (1962) and Ha¬rtig et al. (1984) studied mixing in of minerals and can be described by the ideal formula synthetic solids. 3ϩ AB3(SO4)2(OH)6, with B-sites being occupied by Fe and Jarosites and alunites crystallize in the R3øm space group that ϩ Al3 . Although numerous cations can occupy the A-site, po- can be described by a hexagonal unit cell (Hendricks, 1937; tassium and sodium are incontestably the most commonly Menchetti and Sabelli, 1976). Deficiencies in iron and alumi- encountered in natural solids of this group as well as in syn- num have been reported (Hendricks, 1937; Parker, 1954; Rip- thetic phases (Dutrizac and Jambor, 2000). meester et al., 1986; Drouet and Navrotsky, 2003). Charge Natural specimens have been found in many places on Earth, neutrality for such nonstoichiometric samples is conserved by some dating from several million years (Hofstra et al., 1999; protonation of hydroxyl groups in the structure, leading to Stoffregen et al., 2000). These recent to ancient occurrences structural water molecules (Ripmeester et al., 1986). Recently, raise interest in the archeology and paleoclimatology fields. As Drouet and Navrotsky (2003) have reported, in the case of stated by Stoffregen et al. (2000), “the multiple crystallo- K-Na-H3O jarosites, that although such “excess water” was graphic sites available for measurement of stable isotopes in part of the structure, it was only weakly bound. Partial substi- alunite and jarosite make weathering-related samples desirable tution of hydronium for potassium or sodium in A-sites is often candidates for paleoenvironmental studies.” These materials observed for both synthetic and natural samples (Kubisz, 1970; can indeed lead to important information about the geochro- Dutrizac and Kaiman, 1976; Ripmeester et al., 1986). nology and climate of the area in which they are found. A limited set of data is available on the thermochemistry of Jarosite is common in acid rock drainage environments and stoichiometric jarosites and alunites (see Stoffregen et al. results from the weathering of sulfide ore deposits. The forma- (2000) for review), and very few on nonstoichiometric samples tion of hydrothermal jarosite phases (Dutrizac and Jambor, or their solid solutions (Bohmhammel et al., 1987). Yet, ther- 2000; Stoffregen et al., 2000), including the so-called “sour gas modynamic data for these phases are required to draw accurate jarosites” (Lueth et al., 2000; Lueth and Rye, 2003; Lueth et al., phase diagrams in the Fe-Al-S-O-H system. Such data are also 2003), is also documented in the literature. Alunite occurs in needed to constrain the conditions under which natural samples weathering, lacustrine, and hydrothermal environments as an formed. Other applications in the geochemistry field consist of alteration product of Al-containing minerals such as feldspars estimating aqueous concentrations from the chemical compo- (Stoffregen and Alpers, 1992; Stoffregen et al., 2000). Natural solids with intermediate compositions between jarosite and sitions of the solids present and vice versa. alunite, as well as occurrences of these two minerals together, In this work, we focused on the determination of the enthalpies of formation of jarosite-alunite and natrojarosite-natroalunite solid solutions. This study was performed by solution * Author to whom correspondence should be addressed (anavrotsky@ calorimetric experiments carried out in a molten oxide solvent ucdavis.edu). at 700¡C, on highly characterized synthetic samples. 2197 2198 C. Drouet et al.

Table 1. Composition of starting synthesis solutions.

Synthesis solution composition Sample

ID Vol. (mL) K2SO4 (g) Na2SO4 (g) Fe2(SO4)3 (g) Al2(SO4)3 18H2O (g) Synthesis method

K series: jarosite-alunite solid solution II-2* 100 4.4 0 10.5 40.5 Brophy et al. (1962) II-10 100 4.4 0 6.2 45.7 Brophy et al. (1962) II-7 100 4.4 0 3.6 49.0 Brophy et al. (1962) II-4 100 4.4 0 3.1 49.5 Brophy et al. (1962) II-5 100 4.4 0 1.3 51.5 Brophy et al. (1962) Na series: natrojarosite-natroalunite solid solution C-1 100 0 3.1 3.6 2.0 Coprecipitation C-2 100 0 5.7 8.4 3.1 Coprecipitation C-3 100 0 5.0 4.8 6.2 Coprecipitation H-1 2 0 0.41 0.15 0.31 Hydrothermal H-2 2 0 0.44 0.07 0.58 Hydrothermal H-3 2 0 0.60 0 0.60 Hydrothermal Alunite-natroalunite solid solution T-1 90 0.84 0.60 0 4.8 Stoffregen and Cygan (1990) T-2 90 0.24 1.20 0 4.8 Stoffregen and Cygan (1990) T-3 90 0.14 1.35 0 5.4 Stoffregen and Cygan (1990)

* Same sample names as those given by Brophy et al. (1962).

2. EXPERIMENTAL 2.2. Characterization

2.1. Synthesis Powder X-ray diffraction patterns were recorded on a Rigaku Bi- Plane diffractometer for the jarosite-alunite and the alunite-natroalunite series and on a Scintag PAD-V diffractometer for the natrojarosite- 2.1.1. Jarosite-alunite solid solution natroalunite series. In all cases, Cu K␣ radiation was used (␭ ϭ 1.54056 Aû ). Before each experiment, the diffractometer was calibrated Five compositions in the solid solution between jarosite, with quartz as a standard. KFe3(SO4)2(OH)6, and alunite, KAl3(SO4)2(OH)6, were synthesized The chemical composition of the jarosite-alunite and alunite-natroal- following a procedure described by Brophy et al. (1962). Varying ⅐ unite samples was determined by ICP-MS. Small amounts of the precip- amounts of K2SO4,Fe2(SO4)3 and Al2(SO4)3 18H2O were placed in itates were digested in 20% trace-metal grade HNO3 and analyzed for K, 100 mL of 0.2 N H2SO4 and kept at 100¡C and 1 atm for three days Na, Al, Fe, and S using a Perkin Elmer Elan 6100. The instrument was using reﬂuxing condensers. The compositions of the starting solutions calibrated using NIST-traceable standards. The uncertainty associated with are summarized in Table 1. After three days, the resulting products such measurements is 5%. The composition of the natrojarosite-natroal- were separated by ﬁltering, washed thoroughly with ultrapure water (18 . unite samples was drawn from electron microprobe analysis. The mea- Mohm cm), and dried at 110¡C for 24 h. surements were carried out with a Cameca SX-50 apparatus. Each sample was pressed intoa5mgpellet and immobilized with epoxy resin. The microprobe results were used to determine the potassium, sodium, iron, 2.1.2. Natrojarosite-natroalunite solid solution aluminum, and sulfur contents, assuming that each unit formula contains two sulfate groups. The standard deviation on each determination (not Six members of the solid solution between natrojarosite, reported here for the sake of clarity) was close to 5%. For all experiments,

NaFe3(SO4)2(OH)6, and natroalunite, NaAl3(SO4)2(OH)6, were synthe- total weight percents of the analyzed elements are in good agreement with sized from varying weights of reagent grade Na2SO4,Fe2(SO4)3 and expected totals. ⅐ Al2(SO4)3 18H2O using two synthesis methods (Table 1). The first Thermal analyses (TG/DSC) were performed on a Netzsch STA 449 synthesis route consisted of a coprecipitation of the above reactants at C, with argon as the carrier gas. The heating rate was 10¡C/min, and the 90¡C for 4 h, with sulfuric acid 0.1 N. The precipitate was then filtered, gas phase was constantly monitored by Fourier transform infrared washed with deionized water, and dried at 130¡C for 4 h. In the second spectroscopy using a Bruker EQUINOX 55 spectrometer (MCT detec- method, the samples were obtained by hydrothermal synthesis at tor). Transmission infrared spectra were recorded on the same spec- 200¡C, in a sealed silica glass vial. After 2 d, the furnace was allowed trometer, but using a DTGS detector. Pellets of 13 mm diameter (150 to cool down, and the precipitate was then filtered, washed and dried as mg) were pressed under 300 bar for 1 min. The spectrometer was above. Note that sodium sulfate was added in excess due to difficulty flushed with nitrogen to avoid contamination. Spectra were collected in encountered in obtaining high-sodium contents. the 400Ð4000 cmϪ1 range with a resolution of 4 cmϪ1. A baseline correction was made before interpretation.

2.1.3. Alunite-natroalunite solid solution 2.3. High-temperature Calorimetry

To cross-check the results obtained for the jarosite-alunite and natro- High temperature drop solution calorimetry was carried out at 700¡C jarosite-natroalunite solid solutions, we prepared three solids from the in a custom-built Tian-Calvet twin calorimeter. The technique was alunite-natroalunite solid solution, with varying Na/K ratios. These described in detail elsewhere (Navrotsky, 1977 and 1997). In this work, ⅐ compounds were synthesized according to the procedure used by sodium molybdate, 3Na2O 4MoO3, was used as solvent. Stoffregen and Cygan (1990) and the Na/K ratios that we selected were The pelletized samples (5Ð15 mg) were dropped into a platinum similar to those used by Parker (1962). The starting solutions (see Table crucible containing the solvent, located in the hot zone of the calorim- 1) were placed in a sealed vessel and heated to 150¡C for 48 h. The eter (700¡C). The end of the reaction was judged by the return of the precipitates were ﬁltered, washed with ultrapure water, and dried for baseline to its initial value. The ﬁnal state in these calorimetric exper- ⅐ 24hat110¡C. The resulting solids were then heated to 300¡Cfor1h. iments is a dilute solution in 3Na2O 4MoO3 of the sulfates of Na, K, Jarosite-alunite thermochemistry 2199

diffraction file) standard files 22Ð0827 for jarosite, 71Ð1776 for alunite, 36Ð0425 for natrojarosite, and 41Ð1467 for natroalunite. Figure 1 is related to samples C-2 and H-3, which are close in composition to natrojarosite and natroalunite respectively. In all cases, the Fe-rich samples could easily be identi- fied as jarosite group minerals, and Al-rich samples as alunite group minerals. Evolution of the XRD patterns from jarosite- like to alunite-like was observed as the aluminum content increased, as expected for solid solutions. The unit cell parameters were determined for all samples (Table 2). For both the jarosite-alunite and natrojarosite- natroalunite solid solutions, the unit cell parameter “a” was found to increase monotonically with the iron content. In contrast, the parameter “c” remained almost constant for these series, close to 17.1 Aû for the potassium-containing samples Fig. 1. XRD patterns for samples C-3 and H-3, PDF standards for and 16.6 Aû for the Na samples, regardless of the Fe/Al ratio. natrojarosite and natroalunite. These results are consistent with those reported by Stoffregen et al. (2000) for Fe-Al substitution in jarosite-like species.

3ϩ The unit cell parameters obtained for the samples from the Al and Fe (as ions in the melt) and gaseous H2O, all at 700¡C. Indeed, preliminary studies on simple sulfates and jarosite phases alunite-natroalunite solid solution (Table 2) show, on the con- (Majzlan et al., 2002; Drouet and Navrotsky, 2003) have shown that all trary, constant “a” values, close to 6.98 Aû , but increasing “c” the sulfur was retained in the melt rather than emitted in the gas phase. values in the range 16.7Ð17.2 Aû as the sodium content in- During the experiments, oxygen is ﬂushed through the gas space creases. This is in accord with reported data on alunites and above the melt (ϳ35 mL/min) and bubbled through the solvent (ϳ5 mL/min) to maintain oxidizing conditions, stir the melt, and remove the natroalunites (Stoffregen et al., 2000), and the “a” and “c” evolved water. variations observed here also agree with earlier results on ⌬ Throughout the text, the notation H¡f will represent the enthalpy of A-site potassium-sodium substitution in jarosites (Drouet and formation from the elements of a given species. For heats of formation determined from the oxides/sulfates, the corresponding notation will be Navrotsky, 2003). ⌬ The chemical compositions of the solids synthesized were H¡f,ox. determined from ICP-MS and/or electron microprobe analyses 3. RESULTS AND DISCUSSION (Table 2). The K ϩ Na content of all solids was lower than 1 3.1. Characterization mol/2 mol of sulfate, implying some substitution of hydronium ions for potassium and sodium. For the sodium-containing The X-ray diffraction patterns obtained with the samples samples, the hydronium content was not a simple function of precipitated in this work were compared to the PDF (powder the amount of sodium in the reactant mixture. It was affected by

Table 2. Chemical composition and unit cell parameters of the samples prepared in this work.

Chemical compositiona Unit cell parameters Sample ϩ ϩ ϩb 3ϩ 3ϩ 2Ϫ Ϫc c ID K Na H3O Fe Al SO4 OH H2O a(Å)c(Å)

K series: jarosite-alunite solid solution II-2 0.79 0 0.21 1.99 0.44 2 4.29 1.71 7.248 Ϯ 0.003 17.087 Ϯ 0.006 II-10 0.79 0 0.21 1.61 0.79 2 4.20 1.80 7.209 Ϯ 0.003 17.090 Ϯ 0.007 II-7 0.83 0 0.17 1.32 1.12 2 4.32 1.68 7.175 Ϯ 0.004 17.108 Ϯ 0.011 II-4 0.89 0 0.11 1.19 1.41 2 4.80 1.20 7.146 Ϯ 0.003 17.090 Ϯ 0.008 II-5 0.99 0 0.01 0.63 2.19 2 5.46 0.54 7.064 Ϯ 0.004 17.049 Ϯ 0.009 Na series: natrojarosite-natroalunite solid solution C-1 0 0.68 0.32 2.66 0.07 2 5.17 0.83 7.327 Ϯ 0.001 16.654 Ϯ 0.002 C-2 0 0.69 0.31 2.60 0.07 2 5.00 1.00 7.328 Ϯ 0.001 16.654 Ϯ 0.002 C-3 0 0.70 0.30 2.44 0.20 2 4.91 1.09 7.316 Ϯ 0.001 16.653 Ϯ 0.002 H-1 0 0.66 0.34 1.24 1.46 2 5.09 0.91 7.137 Ϯ 0.003 16.631 Ϯ 0.007 H-2 0 0.68 0.32 0.74 1.88 2 4.86 1.14 7.048 Ϯ 0.002 16.623 Ϯ 0.005 H-3 0 0.66 0.34 0 2.65 2 4.97 1.03 6.987 Ϯ 0.001 16.635 Ϯ 0.002 Alunite-natroalunite solid solution T-1 0.70 0.08 0.22 0 2.69 2 5.08 0.92 6.988 Ϯ 0.001 17.123 Ϯ 0.002 T-2 0.48 0.29 0.23 0 2.63 2 4.89 1.11 6.985 Ϯ 0.036 16.905 Ϯ 0.088 T-3 0.24 0.59 0.17 0 2.74 2 5.23 0.77 6.981 Ϯ 0.003 16.776 Ϯ 0.008

a 2 Normalized composition, containing two SO4 ions per unit formula. Standard deviation on each value is close to 5%. b ϩ ϩ ϭ Derived from K Na H3O 1 in A-site. c ϭ ϭ ϭ Derived from Fe 3-z and OH 6-3z and H2O 3z. 2200 C. Drouet et al.

natroalunite series by using a hydrothermal synthesis route. As a general observation, Fe was incorporated preferentially to Al in B-sites, as for the potassium series. Hydrothermal synthesis enabled us to synthesize Al-rich samples with Al/Fe ratios close to the target compositions. These ﬁndings are in agreement with the observations made by Brophy et al. (1962) showing that increasing the temperature and the pressure and lowering the acidity of the starting solutions may lead to higher Al/Fe ratios. The solids were analyzed by transmission FTIR spectroscopy. Infrared spectra for jarosite species have been reported and discussed elsewhere (Powers et al., 1975; Baron and Palmer, 1996; Drouet and Navrotsky, 2003). In particular, the FTIR spectra obtained here for Fe-rich samples were similar to

those obtained with K-Na-H3O jarosites in the literature. In the case of Al-rich samples, however, slight differences were observed, especially below 600 cmϪ1. Figure 3 gives the FTIR Fig. 2. Ternary diagram Fe/Al/B-site vacancies, for samples from the spectrum obtained with our sample H-3, that is close to natroal- K and Na series and derived end members. Dashed line corresponds to unite, as compared to a typical spectrum obtained for the natro- FeϩAl ϭ 2.5 and dotted line to FeϩAl ϭ 2.7. jarosite sample Na0.71(H3O)0.29Fe2.63(SO4)2(OH)4.89(H2O)1.11 during a previous study (Drouet and Navrotsky, 2003). The number of absorption bands is the same for both spectra: ϩ the Fe/Al ratio: the amount of H3O decreasing for higher iron only the position and, to some extent, the intensity of some of contents. In a similar way, the sum Fe ϩ Al was generally these bands differ. This is indeed often the case for solid smaller than 3 mol/2 mol of sulfate, indicating B-site vacancies solutions, where the progression from one structure to the next and the associated substitution of water molecules for part of is related to ion substitutions in equivalent crystallographic the OH- groups (protonation). The amount of such vacancies sites. It was, therefore, possible to assign the absorption bands decreased with increasing iron content, especially for samples observed here by comparison with the natrojarosite sample synthesized hydrothermally. (Fig. 3). The differences observed between the two spectra in We focused our syntheses on the preparation of two series of the region 400Ð600 cmϪ1 are most likely due to changes in

Fe-Al compounds. Samples from the first series were members octahedra vibrations between FeO6 and AlO6. Indeed, absorp- of the jarosite-alunite solid solution having similar potassium tion bands assigned to FeO6 vibrations were reported at 477, contents and amounts of B-site vacancies. To a first approxi- 513, and 575 cm-1 for jarosite (Baron and Palmer, 1996), and mation, the only compositional variable was the iron-to-alumi- similar bands have been observed for the jarosite-natrojarosite num ratio. This series will be referred to as the “potassium solid solution (Drouet and Navrotsky, 2003). In the present series.” The second family of compounds was the parallel to the case, three bands are seen below 600 cmϪ1 for Al-rich samples first one, but with sodium replacing potassium (i.e., natro- (Fig. 3), at ca. 490, 515, and 592 cmϪ1. Taking into account the jarosite-natroalunite solid solution), therefore leading to a “so- above, these bands are most likely assignable to AlO6 octahe- dium series.” The general chemical formulas of the potassium dra vibrations. Although the band at 592 cmϪ1 is surprisingly and sodium series can be approximated to K0.85(H3O)0.15 intense, the positions of these three bands (490, 515, and 592 Ϫ1 Ϫ1 Fe2.5-wAlw(SO4)2(OH)4.5(H2O)1.5 and Na0.7(H3O)0.3Fe2.7-w cm ) are close to those (486, 528, and 595 cm ) reported by Alw(SO4)2(OH)5.1(H2O)0.9, although some variations in the K Powers et al. (1975) for alunite. series are observed. Figure 2 shows the composition of the In the region above 3000 cmϪ1, the broad band correspond- samples synthesized in this work for both series, in the plane ing to OH stretching (3456 cmϪ1) appears at higher frequencies Fe/Al/B-sites vacancies. The composition of the intermediate solids from the potassium series (jarosite-alunite solid solution) shows that Fe is incorporated preferentially over Al into the crystal structure. This is apparent from comparison of the Al/Fe ratios in the starting solutions and the resulting solids. For example, the synthesis solution for sample II-4 has an Al/Fe mole ratio of ϳ10:1, but leads to a precipitate with an Al/Fe ratio close to 1.2:1 (Tables 1 and 2). Such preferential incorporation of Fe has been reported by Brophy et al. (1962) for similar compositions. The Fe-rich samples from the sodium series were synthesized by coprecipitation. We experienced some difficulty in the preparation of Al-rich samples with this technique, even when using elevated Al/Fe ratios in the starting solutions. We suc- Fig. 3. FTIR spectra of sample H-3 (this work) and a natrojarosite- ceeded in preparing Al-rich samples in the natrojarosite- like sample from Drouet and Navrotsky (2003). Jarosite-alunite thermochemistry 2201

deed been conﬁrmed by Menchetti and Sabelli (1976),inthe case of jarosite/alunite: these authors showed that Al-OH bond lengths in alunites were close to 1.879 Aû as compared to 1.975 for Fe-OH in jarosites. It is, moreover, possible to estimate, in a semiquantitative way, the enthalpy of decomposition of the (dehydrated) natrojarosite/natroalunite samples from integration of peak 1, after ⌬ calibration of the DSC. The corresponding values of Hdecomp ⌬ range between 193 and 235 kJ/mol (Table 3). Hdecomp becomes more endothermic as the Al content increases, conﬁrm- ing that the decomposition of the jarosite/alunite network requires more energy as the samples become richer in Al. Thus, the higher decomposition temperature for Al-rich samples has a thermodynamic basis.

3.2. Calorimetry ⌬ The enthalpies of formation from the elements ( H¡f)ofthe samples prepared in this work were obtained from high-temperature oxide melt calorimetric experiments as was done ear-

lier for K-Na-H3O jarosites (Drouet and Navrotsky, 2003). The ⌬ values of H¡f were determined from experimental drop solu- ⌬ tion enthalpies ( Hds) by application of the thermodynamic Fig. 4. TG/DSC curves for sample C-2 and evolution of DSC peak cycle described in Table 4. maxima with Al content. In this cycle, x, y, z, and w respectively stand for the hydronium, sodium, iron, and aluminum contents in the solids, than for K-Na jarosites (ca. 3360Ð3390 cmϪ1, Drouet and the potassium content being 1-x-y. As was mentioned above, Navrotsky, 2003). This is in accord with literature data for a the sum z ϩ w was generally lower than 3, leading to modi- K-Na alunite sample (Okada et al., 1982) as well as with ﬁcations in the OH sites as compared to the theoretical com- previous results comparing alunite and jarosite spectral features position AB3(SO4)2(OH)6 where B represents Fe or Al. In this (Powers et al., 1975). regard, the general chemical formula of these compounds is Thermal analyses of the samples showed the presence of K1-x-yNay(H3O)xFezAlw(SO4)2(OH)6-3(3-z-w)(H2O)3(3-z-w). “excess water,” indicated by a peak close to 250¡C(Fig. 4). In addition to the enthalpies of drop solution of these solids, Also, the two intense endothermic peaks that are typical during the thermodynamic cycle given in Table 3 involves the phases ␣ ␣ jarosite thermal decomposition were also observed here for K2SO4,Na2SO4, -Fe2O3 (hematite), -Al2O3 (corundum), both the K series and the Na series. Considering earlier works SO3(g), and H2O(l). Thermodynamic data for these compounds on the thermal decomposition of jarosites (Kubisz, 1971; were taken from the literature (Robie and Hemingway, 1995; Drouet and Navrotsky, 2003; and references therein) and al- Majzlan et al., 2002; Drouet and Navrotsky, 2003), and they are unites (Pysiak and Glinka, 1981; Bohmhammel et al., 1987; given in Table 5. Numerical values for the enthalpies of drop ⌬ Stoffregen and Alpers, 1992), one can assign the ﬁrst of these solution Hds measured in this work, as well as the derived ⌬ intense peaks (peak 1) in the range 400Ð600¡C to dehydroxy- enthalpies of formation H¡f, have also been included in this ⌬ lation, leading among other phases to yavapaiite-like com- table. Note that each of these H¡f values have been deter- pounds (AB(SO ) ,Aϭ K, Na, B ϭ Fe, Al). The second mined using the actual x, y, z, and w values corresponding to 4 2 ⌬ intense peak (peak 2), at 700Ð820¡C, can be attributed to the each sample. Therefore, the uncertainties in H¡f take into thermal decomposition of such yavapaiite-type structures. account the variations in chemical composition as well as the ⌬ The temperature dependence of these DSC peaks was fol- uncertainties on the experimental determination of Hds. lowed as a function of the Fe/Al ratio in the sodium series, which is less documented than the potassium series in the Table 3. DSC peak maxima for the natrojarosite-natroalunite solid literature. As shown in Table 3, the temperature of the maxima solution, and enthalpy of decomposition. of these peaks increases as the samples become richer in aluminum, and differences of up to 113¡C are seen between Tmax (¡C) Al-rich and Fe-rich samples. This is in agreement with earlier Sample Fe/Al Peak 1 Peak 2 ⌬H (kJ/mol)a results (Brophy et al., 1962), showing that the dehydroxylation decomp of alunites requires higher temperatures than the dehydroxyla- C-1 38.0 435.6 730.3 202 tion of jarosites. Note that the same general trend is observed C-2 37.1 438.1 729.2 193 for Fe and Al oxyhydroxides (Wolska et al., 1992; Kloprogge C-3 12.2 444.1 731.2 225 et al., 2002; Mitov et al., 2002): the higher the Al content, the H-1 0.9 494.4 750.8 224 H-2 0.4 507.7 769.2 235 higher the decomposition temperatures. According to Brophy et H-3 0 548.3 812.7 218 al. (1962), these observations may be indicative of stronger bonds formed between Al and hydroxyl groups. This has in- a Estimated from area of DSC peak 1. 2202 C. Drouet et al.

Table 4. Thermodynamic cycle used for the determination of the enthalpy of formation of K-Na-H3O alunite-jarosites.

Reactiona ⌬H (kJ/mol)b

3 Ϫ Ϫ ϩ ⌬ (K1؊x؊yNay(H3O)xFezAlw(SO4)2(OH)6؊3(3؊z؊w)(H2O)3(3؊z؊w) (s, 298) (1 x y)/2 K2O (soln, 973) y/2 Na2O Hds (sample (1) ϩ ϩ ϩ ϩ ϩ Ϫ Ϫ (soln, 973) z/2 Fe2O3 (soln, 973) w/2 Al2O3 (soln, 973) 2SO3 (soln, 973) (15 3x 3z 3w)/2 H2O (g, 973) 3 ϩ ⌬ (2) K2SO4 (s, 298) K2O (soln, 973) SO3 (soln, 973) Hds (K2SO4) 3 ϩ ⌬ (3) Na2SO4 (s, 298) Na2O (soln, 973) SO3 (soln, 973) Hds (Na2SO4) ϩ ϩ 3 ⌬ (4) 2 K (s, 298) S (s, 298) 2O2 (g, 298) K2SO4 (s, 298) H¡f (K2SO4) ϩ ϩ 3 ⌬ (5) 2 Na (s, 298) S (s, 298) 2O2 (g, 298) Na2SO4 (s, 298) H¡f (Na2SO4) ␣ 3 ⌬ ␣ (6) -Fe2O3 (s, 298) Fe2O3 (soln, 973) Hds ( -Fe2O3) ϩ 3 ␣ ⌬ ␣ (7) 2 Fe (s, 298) 3/2 O2 (g, 298) -Fe2O3 (s, 298) H¡f ( -Fe2O3) ␣ 3 ⌬ ␣ (8) -Al2O3 (s, 298) Al2O3 (soln, 973) Hds ( -Al2O3) ϩ 3 ␣ ⌬ ␣ (9) 2 Al (s, 298) 3/2 O2 (g, 298) -Al2O3 (s, 298) H¡f ( -Al2O3) 3 ⌬ c (10) SO3 (g, 298) SO3 (soln, 973) Hds (SO3(g)) ϩ 3 ⌬ (11) S (s, 298) 3/2 O2 (g, 298) SO3 (g, 298) H¡f (SO3(g)) 3 ⌬ (12) H2O (1, 298) H2O (g, 973) Hds (H2O(1)) ϩ 3 ⌬ (13) H2 (g, 298) 1/2 O2 (g, 298) H2O (1, 298) H¡f (H2O(g)) Formation of a (K, Na)-jarosite-alunite: Ϫ Ϫ ϩ ϩ ϩ ϩ ϩ ϩ ϩ ⌬ (12) (1 x y) K (s, 298) y Na (s, 298) z Fe (s, 298) w Al (s, 298) 2 S (s, 298) (14 x)/2 O2 (g, 298) H¡f (sample) ϩ Ϫ Ϫ 3 (3x 3z 3w)/2 H2 (g, 298) K1؊x؊y Nay (H3O)x FezAlw (SO4)2 (OH)6؊3(3؊z؊w) (H2O)3(3؊z؊w) (s, 298 15) ⌬ o ϭϪ⌬ ϩ Ϫ Ϫ ⌬ ϩ ⌬ ϩ Ϫ Ϫ ⌬ ϩ ⌬ ϩ ⌬ ϩ ⌬ ϩ ⌬ Hf (sample) H1 (1 x y)/2 H2 y/2 H3 (1 x y)/2 H4 y/2 H5 z/2 H6 z/2 H7 w/2 H8 ϩ ⌬ ϩ ϩ ⌬ ϩ ϩ ⌬ ϩ ϩ Ϫ Ϫ ⌬ ϩ ϩ Ϫ Ϫ ⌬ w/2 H9 (3 x)/2 H10 (3 x)/2 H11 (15 3x 3z 3w)/2 H12 (15 3x 3z 3w)/2 H13

a s ϭ solid; g ϭ gas; soln ϭ in solution (in sodium molybdate). b ⌬ ⌬ ⌬ Hds, H¡f and Hhc are respectively the drop solution enthalpy, standard enthalpy of formation, and heat content. c ⌬ Hds (SO3(g)) as determined by Majzlan et al. (2002).

⌬ Ϫ As the ratio Fe/Al decreases, H¡f varies from 3963 to heats of formation (from the elements) of corundum and he- Ϫ4778 kJ/mol for the potassium series, and from Ϫ3760 to matite themselves differ by a factor two (Table 5), it is inter- Ϫ4980 kJ/mol for the sodium series. In other words, the en- esting to calculate the enthalpies of formation from the oxides/ ⌬ ⌬ thalpies of formation from the elements become less exother- sulfates, H¡f,ox. For the K series, H¡f,ox is deﬁned here as the mic as samples become richer in iron. However, because the enthalpy accompanying the reaction (at 298 K):

Table 5. Thermodynamic data for the solid phases used in this work (uncertainties calculated from thermodynamic cycle using two standard deviations of the mean).

⌬ ⌬ d ⌬ e ⌬ Compound Hds (kJ/mol) H¡f (kJ/mol) H¡f,ox (kJ/mol) Hmix (kJ/mol)

Ϯ a Ϫ Ϯ b Na2SO4 155.7 2.3 (6) 1387.8 0.4 Ϯ a Ϫ Ϯ b K2SO4 153.4 1.8 (8) 1437.7 0.5 ␣ Ϯ a Ϫ Ϯ b -Fe2O3 95.0 1.8 (8) 826.2 1.3 ␣ Ϯ f Ϫ Ϯ b -Al2O3 105.0 1.2 (8) 1675.7 1.3 Ϫ Ϯ c Ϫ Ϯ b SO3(g) 205.8 3.7 395.7 0.7 Ϫ Ϯ b H2O(1) 285.8 0.0 Jarosite-alunite solid solution: K0.85 (H3O)0.15 Fe2.5-w Alw (SO4)2 (OH)4.5 (H2O)1.5 II-2 513.0 Ϯ 8.0 (6) Ϫ3963.4 Ϯ 10.4 Ϫ377.2 Ϯ 10.1 6.8 Ϯ 8.6 II-10 521.9 Ϯ 5.3 (6) Ϫ4118.0 Ϯ 8.4 Ϫ384.4 Ϯ 8.1 10.4 Ϯ 5.3 II-7 533.2 Ϯ 6.0 (6) Ϫ4270.1 Ϯ 8.7 Ϫ394.0 Ϯ 8.5 10.9 Ϯ 6.0 II-4 544.3 Ϯ 6.8 (6) Ϫ4410.3 Ϯ 9.2 Ϫ403.7 Ϯ 9.1 10.1 Ϯ 6.8 II-5 574.5 Ϯ 4.5 (6) Ϫ4778.4 Ϯ 7.6 Ϫ430.0 Ϯ 7.5 7.7 Ϯ 4.5

Natrojarosite-natroalunite solid solution: Na0.7 (H3O)0.3 Fe2.7-w Alw (SO4)2 (OH)5.1 (H2O)0.9 C-1 472.5 Ϯ 5.3 (6) Ϫ3760.2 Ϯ 8.7 Ϫ361.2 Ϯ 8.5 8.8 Ϯ 7.2 C-2 491.7 Ϯ 2.1 (7) Ϫ3776.8 Ϯ 7.2 Ϫ380.4 Ϯ 6.9 Ϫ10.4 Ϯ 2.1 C-3 488.4 Ϯ 2.9 (6) Ϫ3826.8 Ϯ 7.4 Ϫ376.4 Ϯ 7.2 Ϫ0.4 Ϯ 2.9 H-1 547.4 Ϯ 9.6 (3) Ϫ4417.5 Ϯ 11.7 Ϫ429.1 Ϯ 11.5 5.3 Ϯ 9.6 H-2 563.3 Ϯ 4.0 (6) Ϫ4606.3 Ϯ 7.7 Ϫ442.9 Ϯ 7.5 11.0 Ϯ 4.0 H-3 612.1 Ϯ 3.3 (9) Ϫ4979.7 Ϯ 7.5 Ϫ487.9 Ϯ 7.2 1.7 Ϯ 3.3

Alunite-natroalunite solid solution: K0.8-x Nax (H3O)0.2 Al2.7 (SO4)2 (OH)5.1 (H2O)0.9 T-1 593.4 Ϯ 4.4 (6) Ϫ4996.4 Ϯ 7.9 T-2 604.9 Ϯ 4.1 (10) Ϫ4974.7 Ϯ 7.7 T-3 609.5 Ϯ 4.0 (6) Ϫ5023.0 Ϯ 7.6

a Drouet and Navrotsky (2003). In parentheses are the numbers of experiments performed. b Robie and Hemingway (1995). c Majzlan et al. (2002). d Enthalpy of formation from the elements. Uncertainties are two standard deviations of the mean (experimental). e Enthalpy of formation from the oxides/sulfates. f This work. Jarosite-alunite thermochemistry 2203

By deﬁnition, the enthalpy of mixing for an intermediate com-

position KFe3-wAlw(SO4)2(OH)6 between the end-members jarosite, KFe3(SO4)2(OH)6, and alunite, KAl3(SO4)2(OH)6,is given by the equation: ⌬ ϭ ⌬ Hmix(Fe3-wAlw) H¡f(Fe3-wAlw) Ϫ ⌬ Ϫ ⌬ (3-w)/3 . H¡f (jarosite) w/3 . H¡f (alunite) (5) or, equivalently: ⌬ ϭ ⌬ Hmix(Fe3-wAlw) (3-w)/3 . Hds (jarosite) ϩ ⌬ Ϫ ⌬ w/3 . Hds (alunite) Hds(Fe3-wAlw) (6) The use of Eqn. 6 is preferred to that of Eqn. 5 because the magnitude of the heats of formation is often very large; in addition, the drop solution enthalpies are the experimental ⌬ ⌬ Fig. 5. Hds and H¡f for the potassium series (jarosite-alunite solid solution) and for end members (this work, Drouet and Navrotsky, values directly measured here, and thus calculation using Eqn. 2003). 6 gives smaller uncertainties. Application of Eqn. 6 to the potassium and sodium series studied in this work requires the knowledge of the enthalpies of ϩ 0.425 K2SO4(s) (2.5-w)/2 Fe2O3(s) drop solution of four end members: jarosite, natrojarosite, alunite, and natroalunite. However, because our samples show ϩ w/2 Al O ϩ 3.975 H O ϩ 1.575 SO 3 2 3(s) 2 (l) 3(g) deviations from the theoretical stoichiometric formulas, as

K0.85(H3O)0.15Fe2.5-wAlw(SO4)2(OH)4.5(H2O)1.5(s) (1) many natural samples do, we consider here end members with A- and B-site contents close to those of the samples studied. Likewise, for the Na series, the reaction is Values of the enthalpies of drop solution (in sodium molyb- 0.35 Na SO ϩ (2.7-w)/2 Fe O date) of the jarosite K0.72(H3O)0.28Fe2.60(SO4)2(OH)4.80(H2O)1.20 2 4(s) 2 3(s) ⌬ ϭ Ϯ ( Hds 504.1 3.9 kJ/mol) and the natrojarosite ϩ w/2 Al O ϩ 3.9 H O ϩ 1.65 SO 3 ⌬ ϭ 2 3(s) 2 (l) 3(g) Na0.68(H3O)0.32Fe2.83(SO4)2(OH)5.49(H2O)0.51 ( Hds 477.7 Ϯ 5.0 kJ/mol) have been reported in a previous study (Drouet Na0.7(H3O)0.3Fe2.7-wAlw(SO4)2(OH)5.1(H2O)0.9(s) (2) and Navrotsky, 2003). Taking into account their chemical These enthalpies can be derived from the enthalpy of drop compositions, these compounds can be chosen as the nonstoi- solution of the samples and the thermodynamic cycle in Table chiometric jarosite and natrojarosite end members needed here. 4 by the equations: The remaining two end members, namely nonstoichiometric alunite K (H O) Al (SO ) (OH) (H O) and natro- ⌬ ϭ Ϫ⌬ ϩ ⌬ 0.8 3 0.2 2.7 4 2 5.1 2 0.9 H¡f,ox (K series) Hds(sample) 0.425 Hds(K2SO4) alunite Na0.8(H3O)0.2Al2.7(SO4)2(OH)5.1(H2O)0.9, were ϩ ⌬ ϩ ⌬ ⌬ (2.5-w)/2 Hds(Fe2O3) w/2 Hds(Al2O3) extrapolated from the linear variation of Hds with the sodium content for the alunite-natroalunite solid solution ϩ ⌬ ϩ ⌬ 3.975 Hds(H2O(l)) 1.575 Hds(SO3(g)) (3) K0.8-xNax(H3O)0.2Al2.7(SO4)2(OH)5.1(H2O)0.9 (samples T-1, ⌬ ϭ Ϯ and T-2, T-3, Fig. 7). The numerical values are Hds 593.3 2.9 ⌬ ϭ Ϯ kJ/mol for the K-Al end member and Hds 616.4 5.9 ⌬ ϭ Ϫ⌬ ϩ ⌬ H¡f,ox (Na series) Hds(sample) 0.35 Hds(Na2SO4) kJ/mol for the Na-Al end member. ϩ ⌬ ϩ ⌬ (2.7-w)/2 Hds(Fe2O3) w/2 Hds(Al2O3) ϩ ⌬ ϩ ⌬ 3.9 Hds(H2O(l)) 1.65 Hds(SO3(g)) (4) ⌬ Numerical values of H¡f,ox, obtained by application of Eqn. 3 and Eqn. 4, are listed in Table 5. For both series, these values ⌬ show that, like H¡f from the elements, the heats of formation from the oxides/sulfates are more endothermic for Fe-rich samples. These results indicate that the effect of Al/Fe ratio on ⌬ H¡f is not solely due to the difference in heats of formation of hematite and corundum. Figures 5 and 6 depict the variation of the enthalpies of drop solution and of formation (from the elements) for the potassium and sodium series respectively, as a function of the Al content in B-sites. In both cases, a deviation from linearity is observed and the curves can best be ﬁtted to second-degree polynomials. This trend points out that these solid solutions depart from ⌬ ⌬ Fig. 6. Hds and H¡f for the sodium series (natrojarosite-natroal- thermodynamic ideality, and it is indicative of non-zero enthal- unite solid solution) and for end members (this work, Drouet and ⌬ pies of mixing, Hmix. Navrotsky, 2003). 2204 C. Drouet et al.

⌬ ⌬ ϭ This linearity of Hds versus Na content for the alunite- Table 6. Equation of curves Hds f (Al mole fraction) for the K natroalunite solid solution is not surprising as it parallels what and Na series. was observed in previous work (Drouet and Navrotsky, 2003) ϭ ϩ ϩ 2 ϭ⌬ y A Bx Cx ,y Hds and on the jarosite-natrojarosite solid solution. This suggests that x ϭ Al mole fraction the enthalpies of mixing for substitutions in A-sites are negli- gible regardless of the nature of the cation located in B-sites. In K series: A ϭ 504.4, B ϭ 44.8, C ϭ 78.2 ϭ ϭ ϭ contrast, a recent study (Drouet et al., in press) on the solid Na series: A 479.9, B 113.9, C 39.3 solution between jarosite and its chromate analog,

KFe3(CrO4)2(OH)6, showed non-zero (negative) enthalpies of mixing for the substitution of chromium (VI) for sulfur. the heats of formation of the corresponding end members. The ⌬ The above values of Hds for the four end members were ⌬ parabolic curves Hds vs. Al mole fraction are described by then used to calculate the enthalpies of mixing for all interme- second-order polynomials y ϭ A ϩ Bx ϩ Cx2 (Table 6). diate compositions in the potassium and the sodium series Variation in x (chemical composition), noted ⌬x, will therefore ⌬ using Eqn. 6. Numerical values of Hmix are given in Table 5. lead to a variation in y, ⌬y. The samples studied differ from the Despite fairly large uncertainties, mostly originating from ex- ideal series formulas by their FeϩAl content as well as their ⌬ perimental uncertainties on the Hds values, these enthalpies of KϩNa content. One can include these two sources of variation mixing are unambiguously different from zero and mostly ⌬ ⌬ ϭ ͑⌬ 2 in chemical composition by calculating xas x A range from 0 to ϩ11 kJ/mol. These positive enthalpies of ϩ ⌬ 2͒1/ 2 ⌬ ⌬ B where A and B are the differences between the mixing suggest a tendency toward clustering and exsolution. actual A-site and B-site contents, respectively, and their values It is possible to extrapolate the heats of formation, from the in the series formulas. ⌬y can then be derived from ⌬xby elements, of the end members of the K and Na series from the differentiation of the equation y ϭ A ϩ Bx ϩ Cx2 , giving: ⌬ curves Hds vs. Al content (Table 6) and application of the cy- ⌬y ϭ (B ϩ 2Cx) ⌬x . This leads, for the end members, to cle in Table 4. For the potassium series, the extrapolation gives additional uncertainty on ⌬H of Ϯ2.1 kJ/mol for the K-Fe end ⌬ ϭϪ Ϯ ds H¡f 3773.6 7.7 kJ/mol for K0.85(H3O)0.15Fe2.5(SO4)2 member, Ϯ18.5 kJ/mol for K-Al, and Ϯ1.4 kJ/mol for Na-Fe. ⌬ ϭϪ Ϯ (OH)4.5(H2O)1.5 and H¡f 4912.2 7.1 kJ/mol for The corresponding additional uncertainties on the heats of K0.85(H3O)0.15Al2.5(SO4)2(OH)4.5(H2O)1.5 (Fig. 5). For the so- formation ⌬H¡ are Ϯ1.7 kJ/mol for K-Fe, Ϯ17.1 kJ/mol for ⌬ ϭϪ Ϯ f dium series, a similar extrapolation gives H¡f 3734.6 K-Al, and Ϯ1.1 kJ/mol for Na-Fe, as determined from the 8.6 kJ/mol for Na0.7(H3O)0.3Fe2.7(SO4)2(OH)5.1(H2O)0.9 (Fig. uncertainties applying in the the thermodynamic cycle of Table ⌬ ϭϪ Ϯ 6). The value H¡f 4979.7 7.5 kJ/mol obtained for the 4. Therefore, the ﬁnal values for the extrapolated heats of sample H-3 can be taken as the heat of formation of the ⌬ ϭϪ Ϯ formation of these end members are H¡f 3773.6 9.4 end-member Na0.7(H3O)0.3Al2.7(SO4)2(OH)5.1(H2O)0.9 due to ⌬ kJ/mol for K0.85(H3O)0.15Fe2.5(SO4)2(OH)4.5(H2O)1.5, H¡f its chemical composition. The values obtained here for the ϭϪ Ϯ 4912.2 24.2 kJ/mol for K0.85(H3O)0.15Al2.5 K-Fe and Na-Fe end members compare well to earlier results ⌬ ϭϪ Ϯ (SO4)2(OH)4.5(H2O)1.5, and H¡f 3734.6 9.7 kJ/mol for on K-H3O and Na-H3O jarosites (Drouet and Navrotsky, 2003). Na0.7(H3O)0.3Fe2.7(SO4)2(OH)5.1(H2O)0.9. Note that the higher However, these extrapolations are based, for the K-Fe, K-Al uncertainty obtained for the nonstoichiometric alunite end and Na-Fe end members, on the assumption that the potassium member (K-Al) is due to the higher scatter observed for Al-rich and sodium series can be described by the formulas samples, especially sample II-5, in Figure 2. To our knowledge, K0.85(H3O)0.15Fe2.5-wAlw(SO4)2(OH)4.5(H2O)1.5 and Na0.7(H3O)0.3 this is the ﬁrst report of thermochemical data on nonstoichio- Fe2.7-wAlw(SO4)2(OH)5.1(H2O)0.9 respectively. As mentioned above, metric (K, Na)-alunite-jarosite phases. some variations to these formulas are observed in the samples, which leads to additional uncertainty for the extrapolation of 4. CONCLUSIONS

The thermochemistry of the jarosite-alunite and natrojarosite-natroalunite solid solutions was investigated by oxide melt solution calorimetry. The enthalpies of formation from the elements of 11 samples from these series and 3 samples from the alunite-natroalunite solid solution series were determined. ⌬ H¡f becomes more endothermic with the iron content. Slightly positive enthalpies of mixing were found, indicating that these solid solutions involving B-site Fe/Al substitutions are not thermodynamically ideal. The heats of formation of the end members of the series were extrapolated from the experimental data. These thermodynamic data should enable the estimation/prediction of the heats of

formation of natural (K-Na-H3O)-jarosite-alunite samples, encountered, for example, in acid rock drainage environments, Fig. 7. ⌬H and ⌬H¡ for the alunite-natroalunite solid solution which generally depart from the AB3(SO4)2(OH)6 stoichiom- ds f etry. Also, under given conditions, the partitioning of Fe and Al K0.8-xNax(H3O)0.2Al2.7(SO4)2(OH)5.1(H2O)0.9 (samples T-1, T-2, T-3 and H-3). between the solids and the aqueous solution may be estimated Jarosite-alunite thermochemistry 2205 using these thermodynamic data and aqueous speciation pro- iron oxides in a hypersaline system: Lake Tyrrell, Victoria, Austra- grams, which were beyond the scope of the present paper. lia. Chem. Geol. 96, 183Ð202. The strong affinity of the jarosite-alunite structure for Fe, Lueth V. W., Rye R. O., and Peters L. (2000) Hydrothermal “sour gas” jarosite: ancient to modern acid sulfate mineralization events in the observed in this work, is consistent with observed metal uptake Rio Grande rift. Geological Society of America, 2000 Annual Meet- in industrial applications using jarosite precipitation, e.g., in ing, Abstracts and Programs, pp A49. zinc hydrometallurgy, to eliminate iron impurities from raw Lueth V. W. and Rye R. O. (2003) Sulfuric and hydrofluoric acid minerals (Dutrizac and Jambor, 2000). speleogenesis associated with fluorite-jarosite mineralization along the Rio Grande rift, New Mexico. New Mexico Geological Society Acknowledgments—This work was supported by the U.S. Department 2003 Spring Meeting, Abstracts and programs and New Mexico of Energy (grant DE-FG0397SF 14749). The authors also wish to thank Geology 25, pp 42. greatly Dr. C. Alpers for a thorough review of this contribution. Lueth V. W., Rye R. O., and Peters L. (in press) “Sour gas” hydrothermal jarosite: ancient to modern acid sulfate mineralization in the southern Rio Grande rift: chemical geology. Associate editor: D. Wesolowski Majzlan J., Navrotsky A., and Neil J. M. (2002) Energetics of anhy- drite, barite, celestine, and anglesite: A high-temperature and differ- REFERENCES ential scanning calorimetry study. Geochim. Cosmochim. Acta 66, Alpers C. N., Rye R. O., Nordstrom D. K., White L. D., and Bi-Shia 1839Ð1850. King (1992) Chemical, crystallographic and stable isotopic proper- Menchetti S. and Sabelli C. (1976) Crystal chemistry of the alunite ties of alunite and jarosite from acid-hypersaline Australian lakes. series; crystal structure refinement of alunite and synthetic jarosite. Chem. Geol. 96, 203Ð226. Neues Jahrbuch fuer Mineralogie. Monatshefte 9, 406Ð417. Baron D. and Palmer C. D. (1996) Solubility of jarosite at 4Ð35 ¡C. Mitov I., Paneva D., and Kunev B. (2002) Comparative study of the Geochim. Cosmochim. Acta 60, 185Ð195. thermal decomposition of iron oxyhydroxides. Therm. Acta 386, Bohmhammel K., Naumann R., and Paulik F. (1987) Thermoanalytical 179Ð188. and calorimetric investigations on the formation and decomposition Navrotsky A. (1977) Progress and new directions in high temperature of some alunites. Thermochim. Acta 121, 109Ð119. calorimetry. Phys. Chem. Miner. 2, 89Ð104. Brophy G. P., Scott E. S., and Snellgrove R. A. (1962) Sulfate studies Navrotsky A. (1997) Progress and new directions in high temperature II. Solid solution between alunite and jarosite. Am. Miner. 47, calorimetry revisited. Phys. Chem. Miner. 24, 222Ð241. 112Ð126. Okada K., Hirabayashi J., and Ossaka J. (1982) Crystal structure of Drouet C. and Navrotsky A. (2003) Synthesis, characterization, and natroalunite and crystal chemistry of the alunite group. Neues Jahr- thermochemistry of K-Na-H3O jarosites. Geochim. Cosmochim. buch fuer Mineralogie. Monatshefte 12, 534Ð540. Acta 67, 2063Ð2076. Parker R. L. (1954) Alunitic alteration at Marysvale, Utah. Ph.D. Drouet C., Baron D., and Navrotsky A. (2003) On the thermochemistry Thesis, Columbia University. of the solid solution between jarosite and its chromate analog. Am. Parker R. L. (1962) Isomorphous substitution in natural and synthetic Miner., in press. alunite. Am. Miner. 47, 127Ð136. Dutrizac J. E. and Jambor J. L. (2000) Jarosites and their applications Powers D. A., Rossman G. R., Schugar H. J., and Gray H. B. (1975) in hydrometallurgy. Reviews in Mineralogy and Geochemistry 40, Magnetic behavior and infrared spectra of jarosite, basic iron sulfate 405Ð452. and their chromate analogs. J. Sol. Stat. Chem. 13, 1Ð13. Dutrizac J. E. and Kaiman S. (1976) Synthesis and properties of Pysiak J. and Glinka A. (1981) Thermal decomposition of basic alu- jarosite-type compounds. Can. Mineral. 14, 151Ð158. minium potassium sulphate. Part I: Stages of decomposition. Ther- Ha¬rtig C., Brand P., and Bohmhammel K. (1984) Fe-Al isomorphie und mochimica Acta 44, 21Ð28. strukturwasser in Kristallen von jarosit-alunit typ. Z. Anorg. Allg. Ripmeester J. A., Ratcliffe C. I., Dutrizac J. E., and Jambor J. L. (1986) Chem. 508, 159Ð164. Hydronium in the alunite-jarosite group. Can. Miner. 22, 773Ð784. Hendricks S. B. (1937) The crystal structure of alunite and the jarosites. Robie R. and A. Hemingway B. S. (1995) Thermodynamic properties Am. Miner. 22, 773Ð784. of minerals and related substances at 298.15K and 1bar (105 Hofstra A. H., Snee L. W., Rye R. O., Folger H. W., Phinisey J. D., Pascals) Loranger R. J., Dahl A. R., Naeser C. W., Stein H. J., and Lewchuk pressure and at higher temperatures. U.S. Geological Survey Bull. M. T. (1999) Age constraints on Jerritt Canyon and other Carlin-type 2131. gold deposits in the western United States: Relationship to mid- Stoffregen R. E. and Alpers C. N. (1992) Observations on the unit-cell ␦ tertiary extension and magnetism. Economic Geol. 94, 769Ð802. dimensions, H2O contents, and D values of natural and synthetic Kloprogge J. T., Ruan H. D., and Frost R. L. (2002) Thermal decom- alunite. Am. Miner. 77, 1092Ð1098. position of bauxite minerals: infrared emission spectroscopy of Stoffregen R. E. and Cygan G. L. (1990) An experimental study of gibbsite, boehmite and diaspore. J. Mat. Sci. 37, 1121Ð1129. Na-K exchange between alunite and aqueous sulfate solutions. Am. Kubisz J. (1970) Studies on synthetic alkali-hydronium jarosites. I. Miner. 75, 209Ð220. Synthesis of jarosite and natrojarosite. Mineralogia Polonica 1, Stoffregen R. E., Alpers C. N., and Jambor J. L. (2000) Alunite-jarosite 47Ð57. crystallography, thermodynamics and geochronology. Reviews in Kubisz J. (1971) Studies on synthetic alkali-hydronium jarosites. II: Mineralogy and Geochemistry 40, 453Ð479. Thermal investigations. Mineralogia Polonica 2, 51Ð59. Wolska E., Szajda W., and Piszora P. (1992) Determination of solid Long D. T., Fegan N. E., McKee J. D., Lyons W. B., Hines M. E., and solution limits based on the thermal behavior of aluminum substi- Macumber P. G. (1992) Formation of alunite, jarosite and hydrous tuted iron hydroxides and oxides. J. Therm. Anal. 38, 2115Ð2122.