Solidsolution aqueoussolution reactions between jarosite (KFe _{3}(SO _{4}) _{2}(OH) _{6}) and its chromate analog
Abstract
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 _{(1X)}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 _{(1X)}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 oneparameter 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.
 Publication:

Geochimica et Cosmochimica Acta
 Pub Date:
 September 2002
 DOI:
 10.1016/S00167037(02)008803
 Bibcode:
 2002GeCoA..66.2841B