Stability relations of jarosite and natrojarosite at 150-250°C

Details Stability relations of jarosite and natrojarosite at 150-250°C Stability relations of jarosite and natrojarosite at 150-250°C

Jun 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.2417s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 57, Issue 11, pp.2417-2429

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7

Scientific paper

The jarosite decomposition reaction KFe 3 ( SO 4 ) 2 ( OH ) 6 = 1.5 Fe 2 O 3 + K + + 2 SO 4 2- + 3 H + + 1.5 H 2 O and the analogous natrojarosite reaction have been studied in hydrothermal experiments of 1 to 1 1 weeks duration at 150-250°C. For a log mK 2 SO 4 in the coexisting aqueous phase of between -0.4 and -1.2, the log mH 2 SO 4 required to stabilize jarosite decreases from -0.35 ± 0.05 at 250° to -0.58 ± 0.12 at 200°C. At higher K 2 SO 4 concentrations the H 2 SO 4 concentration required to stabilize jarosite increases at both temperatures, consistent with the position of the jarosite-hematite boundary predicted with PHRQPITZ. Natrojarosite could not be produced from hematite at 250°C. At 200°C, the natrojarositehematite boundary occurs at log mH 2 SO 4 of -0.17 ± 0.08 at log mNa 2 SO 4 of -0.2 to -1.0. At log mNa 2 SO 4 below about -1.0, natrojarosite is unstable with respect to the assemblage hematite +Fe(SO 4 )(OH). Apparent standard molal Gibbs free energies computed from the experimental data are G 0 jarosite ,200°,100 bars = -3416.3 ± 1.7 kJ/mol and G 0 natrojarosite , 200°, 100 bars = -3371.9 ± 2.0 kJ/mol. The estimated errors reflect a lack of complete reversibility in the experiments, but do not consider other, potentially greater, sources of uncertainty such as those associated with calculation of aqueous activity coefficients. Additional experiments were used to study the exchange reaction KFe 3 ( SO 4 ) 2 ( OH ) 6 + Na + = NaFe 3 ( SO 4 ) 2 ( OH ) 6 + K + . Ln K D for this reaction is -3.1 ± 0.5 at 250°C, -3.7 ± 0.4 at 200°C, and -4.9 ± 0.2 at 150°C. Ln K D does not show a systematic variation with X Na at either 250 or 200°C, which suggests that jarosite and natrojarosite may be modeled as an ideal solid solution at these temperatures. At 150°C In K D does not vary from X Na = 0.1 to 0.6, but good constraints on its value could not be obtained outside this compositional range. The experimental data therefore do not rule out a significant departure from ideality at higher X Na at 150°C.

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