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American Mineralogist; November-December; v. 88; no. 11-12; p. 1949-1954
© 2003 Mineralogical Society of America
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On the thermochemistry of the solid solution between jarosite and its chromate analog

Christophe Drouet1, Dirk Baron2 and Alexandra Navrotsky1,*

1 Thermochemistry Facility, University of California at Davis, One Shields Avenue, Davis, California 95616, U.S.A.
2 Department of Physics and Geology, California State University, Bakersfield, California 93311, U.S.A.

Correspondence: * E-mail: anavrotsky{at}ucdavis.edu

Minerals of the jarosite group can be a significant environmental sink for hexavalent Cr by substitution of chromate for sulfate. The thermochemistry of the synthetic solid solution between jarosite KFe3(SO4)2(OH)6 and its chromate analog KFe3(CrO4)2(OH)6 was investigated by high-temperature oxide-melt solution calorimetry. The enthalpies of formation ({Delta}H0f) of the latter, as well as of five intermediate compositions in the series KFe3(S1–zCrzO4)2(OH)6, were determined, where z corresponds to the Cr content. The variation of {Delta}H0f with Cr content deviates from ideality, and negative enthalpies of mixing between jarosite and its chromate analog are observed, suggesting some ordering of the sulfate/chromate groups in the solid solution. The measured enthalpy of formation from the elements of the end-member KFe3(CrO4)2(OH)6 is {Delta}H0f = –3762.5 ± 8.0 kJ/mol. In view of this work, and considering literature data, {Delta}G0f = –3305.5 ± 3.4 kJ/mol, {Delta}S0f = –1533.6 ± 29.2 J/(mol·K), and S0 = 487.7 ± 29.2 J/(mol·K) are recommended for KFe3(CrO4)2(OH)6.




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C. H. Yoder and J. P. Rowand
Application of the simple salt lattice energy approximation to the solubility of minerals
American Mineralogist, May 1, 2006; 91(5-6): 747 - 752.
[Abstract] [Full Text] [PDF]


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