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Clays and Clay Minerals; October 2002; v. 50; no. 5; p. 591-600
© 2002 Clay Minerals Society
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THE STABILITY OF Fe-Mg CHLORITES IN HYDROTHERMAL SOLUTIONS: II. THERMODYNAMIC PROPERTIES

Stephen U. Aja

Department of Geology, Brooklyn College of the City University of New York, Brooklyn, NY 11210-2889, USA

* E-mail address of corresponding author: emenike_nduzo{at}msn.com

The hydrothermal stabilities of a low-Fe clinochlore and a high-Mg chamosite, in the presence of kaolinite, were investigated recently at T ≤ 200°C and Pv = P H2O(Aja and Small, 1999; Aja and Dyar, 2002). Standard state thermodynamic properties (S2980 {Delta}fH1,2980 and {Delta}fG1,2980) have been obtained for the two chlorites whose structural formulae are (Al2.33Fe1.002+Fe0.143+Ca0.02Mn0.01Ni0.02Cr0.01Mg8.40{square}0.07) (Si5.66Al2.34)O20(OH)16 and (Fe0.603+Fe5.432+Mg2.30Al2.98Mn0.05Ca0.03Zn0.01{square}0.60) (Si5.63Al2.37)O20(OH)16. For the low-Fe clinochlore, the respective thermochemical properties are 430 J mol–1 K–1,–8770.64±35.24 kJ mol–1, and –8120.54±32.63 kJ mol–1. {Delta}fH1,2980 {Delta}fG1,2980, and S2980, similarly obtained for the Windsor chamosite are –7851.29±23.14 kJ mol–1, –7271.01±21.43 kJ mol–1 and 668±5 J mol–1K–1, respectively. Ideal site-mixing models of chlorite composition, along the chamosite-clinochlore binary, fail to model satisfactorily these chlorite-fluid equilibria only at lower temperatures (T <175°C). The magnitudes of the excess thermodynamic properties calculated for these chlorites, within the ternary clinochlore-daphnite-sudoite system, suggest significant deviations from ideality.

Key Words: Chamosite • Clinochlore • Solid-solution • Sudoite • Thermodynamic Properties




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