ILLITE EQUILIBRIA IN SOLUTIONS .1. PHASE-RELATIONSHIPS IN THE SYSTEM K2O-AL2O3-SIO2-H2O BETWEEN 25 AND 250-DEGREES-C

Natural illite from Marblehead, Wisconsin (MH), USA, has been equilibrated with 0.2 and 2.0M KCl/KOH and KCl/HCl solutions in the presence of excess kaolinite or microcline and quartz or amorphous silica at temperatures between 25 and 250-degrees-C and P(v) = P(H20). Reversibility of univariant equilibria was demonstrated by approach from high and low a(K+)/a(H+) and from silica under- and super-saturation. Solutions were separated after experiments using immiscible displacement techniques. Isothermal, isobaric log a(K+)/a(H+) vs. log a(SiO2)2,aq diagrams have been constructed defining possible stability fields for kaolinite, microcline, gibbsite (or boehmite or diaspore), muscovite, and four illitic phases. Assuming an R+2-free stoichiometry, K-content per half cell, estimated from the slopes of univariant lines, are 0.29, 0.50, 0.69, and 0.85 K; these phases are compositional analogs of smectite (S), mixed-layer illite I/S (i.e., IS, ISII) and illite (I), respectively. Illitization reactions are strongly affected by temperature and porewater chemistry. At quartz saturation, direct conversion of smectite or kaolinite to endmember illite can occur at high pH; at low pH, these reactions are unlikely inasmuch as K+ requirements exceed concentrations observed in most natural pore waters. In silica-supersaturated solutions, illitization reactions proceed through crystallization of intermediate phases with compositions between smectite and endmember illite (I).