Previous attempts to use illite as a geothermometer have failed: no general relation between the mineral’s chemical composition and temperature of crystallization has been found. Here, chemical compositions of 27 illite samples from five different geothermal fields (the data on four of which were drawn from the literature) were compared with their crystallization temperatures. As previously reported by Cathelineau (1988), the K content was found to be the only variable yielding a suitable correlation, but only when applied to one geothermal field; when various geothermal systems were considered, the correlation weakened considerably. Introduction of a correction algorithm to the K content of the illite has made it possible to draw a single line to fit the data from all the studied samples, yielding a good correlation coefficient (r = 0.84).

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