The relationships between the composition and the crystallization temperature of chlorites and illites have been investigated in different geothermal fields and in particular the Los Azufres system in Mexico, considered to be a natural analogue to experimental laboratories, as the main changes in physical and chemical conditions and mineralogy are related to progressively increasing temperature with depth. Temperature was estimated from combined geothermometric approaches, and especially from fluid inclusion studies on quartz coexisting with clays. The Al_(IV) content in the tetrahedral site of chlorites, and the K content and total interlayer occupancy of illites increase with temperature. These chemical changes are mainly related to the marked decrease in the molar fraction of the Si_(IV)-rich end-members (kaolinite for chlorites, and pyrophyllite for illites) which become negligible at ∼300°C. Other chemical changes, such as the variation in Fe and Mg contents, are partly influenced by temperature, but are strongly dependent on the geological environment, and consequently on the solution composition. The empirical relationships between chemical variables and temperature were calibrated from 150–300°C, but extrapolations at lower and higher temperatures seem possible for chlorites. Such geothermometers provide tools for estimating the crystallization temperature of the clays, and are important for the study of diagenetic, hydrothermal and low-T metamorphic processes.

The hydrothermal leaching of quartz from granite is frequently associated with sodium and/or potassium metasomatism, which promotes important alterations in the primary mineralogy, together with concomitant changes in the whole-rock composition. The differential stability of granite minerals other than quartz during alteration, and the variety of vug infillings, explains the range of the observed chemical changes. Theoretical chemical compositions and parameters may be calculated from the mineral parageneses. Thus, by comparing theoretical and natural trends in chemical mineralogical diagrams it is possible to define and to classify major alteration processes. The P-T conditions during metasomatism range from 0·3 to 1·5 kb, and from 250 to 450°C. The metasomatic fluids are aqueous and of low salinity. Alteration results from a series of disequilibrium reactions between fluids and host rocks. Undersaturation of quartz in the fluids results from drastic temperature changes in the range 450-350°C, and in most cases the K or Na metasomatism is controlled by the subsequent disequilibrium between fluids and host rocks.