The stability of Fe-Mg chlorites in hydrothermal solutions: II. Thermodynamic properties

dAbstract-The hydrothermal stabilities of a low-Fe clinochlore and a high-Mg chamosite, in the presence of kaolinite, were investigated recently at T less than or equal to 200degreesC and P-v = PH2O (Aja and Small, 1999; Aja and Dyar, obtained for the 2002). Standard state thermodynamic properties (S-298(0), Delta(f)H(1.298)(0)) and DeltafG(1.298)(0)) have been 0 + two chlorites whose structural formulae are (Al(2.33)Fe(1.00)(2+)Fe(0.14)(3+)Ca(0.02)Mn(0.01)Ni(0.02)Cr(0.01)Mg(8.40)square(0.07)) (Si5.66Al2.34)O-20(OH)(16) and Fe(0.60)(3+)Fe(5.43)(2+)Mg(2.30)Al(2.98)Mn(0.05)Ca(0.03)Zn(0.01)square(0.60))(Si5.63Al2.37)O-20(OH)(16). For the low-Fe clinochlore, the respective thermochernical properties are 430 J mol(-1) K-1, -8770.64+/-35.24 kJ mol(-1), and -8120.54+/-32.63 kJ mol(-1). Delta(f) H-1.298(0), Delta(f)G(1,298)(0) and S-298(0), 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(-1)K(-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 <175degreesC). The magnitudes of the excess thermodynamic properties calculated for these chlorites, within the ternary clinochlore-daphnite-sudoite system, suggest significant deviations from ideality.