Solid-solution aqueous-solution reactions between jarosite (KFe 3 (SO 4 ) 2 (OH) 6 ) and its chromate analog

Details Solid-solution aqueous-solution reactions between jarosite (KFe 3 (SO 4 ) 2 (OH) 6 ) and its chromate analog Solid-solution aqueous-solution reactions between jarosite (KFe 3 (SO 4 ) 2 (OH) 6 ) and its chromate analog

Sep 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002gecoa..66.2841b&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 66, Issue 16, pp.2841-2853

Computer Science

2

Scientific paper

The sulfate mineral jarosite (KFe 3 (SO 4 ) 2 (OH) 6 ), its chromate analog (KFe 3 (CrO 4 ) 2 (OH) 6 ), and seven precipitates with intermediate compositions (KFe 3 (Cr X S (1-X) O 4 ) 2 (OH) 6 ) were synthesized. The unit cell volume of the precipitates is closely represented by a linear function of composition, suggesting a continuous solid solution. This solid solution dissolves stoichiometrically according to KFe 3 (Cr X S (1-X) O 4 ) 2 (OH) 6 + 6H + K + +3Fe 3+ + 2X CrO 4 2- + (2 - 2X) SO 4 2- + 6H 2 O and reaches stoichiometric saturation after approximately 40 d. Log K SS values calculated from samples taken after 1090 d are consistently lower than what would be expected for an ideal solid solution, indicating that the excess free energy of mixing, G E , is negative. G E calculated from the log K SS values can be closely modeled by the one-parameter Guggenheim expansion G E = X CrJar X Jar RT a 0 where a 0 is -4.9 ± 0.8, X CrJar and X Jar are the mole fractions of KFe 3 (CrO 4 ) 2 (OH) 6 and KFe 3 (SO 4 ) 2 (OH) 6 in the solids, R is the gas constant, and T the absolute temperature. Based on the calculated excess free energy, a Lippmann diagram with a modified abscissa was constructed.

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