The stability of Fe-Mg chlorites in hydrothermal solutions - I. Results of experimental investigations

Three natural chlorites have been equilibrated with kaolinite (+/- quartz +/- gibbsite +/- hematite) in aqueous MgCl2 or NaCl solutions at saturated vapor conditions (P-V=P-H2O). The compositions of the chlorites, (Fe0.603+Fe5.432+ 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), (Al(2.33)Fe(1.00)(2+)Fe(1.00)(2+)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 (Si5.26Al2.74)(Al(2.74)Ti(0.28)Fe(6.16)(2+)Fe(0.56)(3+)Mn(0.05)Mg(2.05)Ca(0.31)P(0.19)V(0.01)square(0.46))O-20(OH)(16) were determined from X-ray fluorescence (XRF) and Mossbauer spectroscopy. The experiments were conducted in sealed bottles (LDPE or PTFE) or Teflon-lined (PTFE) reaction vessels, and the starting solution compositions were constructed such that equilibrium boundaries were approached from high and low values of log, a(Mg24)(0.5)/a(H)(+), log a(Na)(+)/a(H)(+) and log a(SiO(aq)). The attainment of stable equilibrium is indicated by the reversibility of fluid-mineral equilibria, agreement of results obtained in the different aqueous media, and the variance of the 6-component system, MgO-Al2O3-SiO2Fe2O3-FeO-H2O. To fully reflect the complexity of the equilibria being investigated, a number of reactions (with corresponding equilibrium constants) have been used to model the behavior of each chlorite under isothermal, isobaric conditions. These solution equilibration data also validate the applicability of solubility product conventions to chlorite solubility data and thus contradict the general presumption that the behaviors of complex layer silicates in aqueous solutions are not amenable to the law of mass action. (C) 2002 Elsevier Science Ltd. All rights reserved.