Sericite, either as illite or illite/ smectite, is ubiquitous in geothermal systems. Theoretical Ca- and Na-smectite contents of non-expanding geothermal sericites have been calculated from published electron microprobe analyses. Geothermal sericites can be modeled as solid solutions of muscovite and smectite. For those sericites that fit the model, the amount of smectite in solid solution is related to temperature by the expression: ToC = 1000/(0.45Lo$Xamectitc t 2.38) - 273 The temperature dependence of illite interlayer chemistry suggests a related temperature dependence of the K, Na and Ca content of geothermal fluids. The original data used by Fournier and Truesdell (1973) to derive the empirical Na-K-Ca geothemometer for geothermal fluids can be modeled equally well by an equation incorporating the equilibrium constant for the reaction of smectite to illite: T OC = 1.145*10~/([0.35Lo~a+0.175~o&a-0.75Log~] where the concentration units are molalities. This supports the hypothesis that illite and illite/smectite are important controls on the concentrations of Na, K and Ca in geothermal fluids.

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