ThestabilityofFe–Mgchloritesinhydrothermalsolutions—I. Results of experimental investigations Stephen U. Aja a, *,M.DarbyDyar b a Department of Geology, Brooklyn College of the City University of New York, Brooklyn, NY 11210-2889, USA b Department of Earth and Environment, Mount Holyoke College, South Hadley, MA 01075-6419, USA Received 13 November 2000; accepted 10 September 2001 Editorial handling by R.L. Bassett Abstract Three natural chlorites have been equilibrated with kaolinite (  quartz  gibbsite  hematite) in aqueous MgCl 2 or NaCl solutions at saturated vapor conditions (P V =P H 2 O ). The compositions of the chlorites, (Fe 0.60 3+ Fe 5.43 2+ Mg 2.30 Al 2.98 Mn 0.05 Ca 0.03 Zn 0.01 & 0.60 )(Si 5.63 Al 2.37 )O 20 (OH) 16 , (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 & 0.07 ) (Si 5.66 Al 2.34 )O 20 (OH) 16 and (Si 5.26 Al 2.74 )(Al 1.94 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 & 0.46 )O 20 (OH) 16 were determined from X-ray fluorescence (XRF) and Mo¨ssbauer spectroscopy. The experiments were conducted in sealed bottles (LDPE or PTFE) or Teflon-lined (PTFE) reaction vessels, and the starting solution compositions were con- structed such that equilibrium boundaries were approached from high and low values of log a 0:5 Mg 2þ a H þ , log a Na þ a H þ and log a SiO aq ð Þ . 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–Al 2 O 3 –SiO 2 - Fe 2 O 3 –FeO–H 2 O. To fully reflect the complexity of the equilibria being investigated, a number of reactions (with cor- responding 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. # 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction The need for a better understanding of chlorite equi- libria in aqueous low temperature hydrothermal solu- tionsstemspartlyfromtheeffectsofclayminerals(such as chlorites) on the petrophysical properties of siliclastic hydrocarbon reservoirs (Harper and Buller, 1986; Hurst and Archer, 1986; Kantorowicz et al., 1986). These clay minerals, for instance, exercise a tremendous influence on the porosity, permeability and wireline logs of hydrocarbon reservoirs and they commonly occur as pore-filling cements or grain coatings. Besides the per- vasive nature of pore-filling clay cements in sandstones, theproportions,typesandformsofclaymineralspresent in a siliclastic reservoir may be altered by enhanced hydrocarbon recovery techniques upon the injection of fluidsintothereservoirs(GunterandBird,1988).Ithas also been demonstrated that chlorite-forming diagenetic reactions may buffer CO 2 fugacities and thus the con- sequent generation of overpressures in such reservoirs; the typical reacting mineral compatibility include kaoli- nite-carbonate or illite-carbonate (McDowell and Paces, 1985) although non-clay carbonate compatibilities may occur in some deeply buried sandstones (Wilson, 1994). With respect to the former, the inferred reactions include (Smith and Ehrenberg, 1989), 3Al 2 Si 2 O 5 OH ð Þ 4 þ4:5Ca 2 FeMg CO 3 ð Þ 4 þ2H 2 O ¼ Fe 4:5 Mg 4:5 Al 6 Si 5 O 20 OH ð Þ 16 þSiO 2;cr þ 9CaCO 3 þ 9CO 2 ð1aÞ and 0883-2927/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(01)00131-7 Applied Geochemistry 17 (2002) 1219–1239 www.elsevier.com/locate/apgeochem * Corresponding author. Tel.: +1-718-951-4226. E-mail address: emenike_nduzo@msn.com (S.U. Aja).