Solubility of the magnetite + hematite buffer assemblage and iron speciation in sodium chloride solutions at 300°C and 500 bars

Details Solubility of the magnetite + hematite buffer assemblage and iron speciation in sodium chloride solutions at 300°C and 500 bars Solubility of the magnetite + hematite buffer assemblage and iron speciation in sodium chloride solutions at 300°C and 500 bars

Aug 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989gecoa..53.1875z&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 53, Issue 8, pp.1875-1882

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The solubility of the buffer assemblage magnetite + hematite in 0.10, 1.00 and 3.00 mol/kg NaCl solutions with various pH values was determined by use of a Dickson gold cell hydrothermal apparatus at 300°C and 500 bars. Activities of H + and Cl - in the NaCl hydrothermal solution for each determination were approximated. The experimental results show that the solubility is strongly dependent on the pH of the solutions and appears to have no explicit relation to NaCl molality in acidic solutions. The correlation of the measured solubility with pH at 300°C suggests a change in Fe speciation in solution at pH 5.3-5.4. Treatment of the experimental data by a ridge regression program and an iterative procedure indicates that the dominant species in solution under the run condition are FeOH + , Fe(OH) 3 Cl - , Fe(OH) 3 0 and Fe(OH) 4 - . Gibbs free energy of formation for these species at 300°C and 500 bars were derived: G f , Fe ( OH ) + 0 = -250.4 kJ/mol; G f , Fe ( OH ) 3 Cl - 0 = -644.7 kJ/mol; G f , Fe ( OH ) 3 0 0 = -552.6 kJ/mol; and G f , Fe ( OH ) 4 - 0 = -652.7 kJ/mol. The present study demonstrates that the change in acidity of hydrothermal ore-forming solutions exerts an important influence on the transport and deposition of Fe in nature.

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