Early stages of volcanic tuff alteration in hydrothermal experiments: Formation of mixed-layer illite-smectite

Volcanic tuff from the Cabo de Gata region in Almeria, southeastern Spain, was altered under hydrothermal conditions at different temperatures (60 to 180degreesC), reaction times (60 to 360 days), and reacting solutions (deionized water and NaCl and KCl solutions with Na/K ratios from 0.01 to 100, and a total salt concentration of 0.1 to 1 M). X-ray diffraction (XRD) patterns of the reacted samples revealed a very weak, broad peak at similar to 13 Angstrom that migrated to 17 Angstrom upon glycolation. Comparison between simulated (NEWMOD) and experimental XRD patterns indicated that the neoformed phase is a random mixed-layer illite-smectite (I-S) with 75% expandable layers. Fourier transform infrared (FTIR) spectroscopy showed that I-S formation was most extensive for high pH (8-9) solutions, corresponding to dilute solutions and, especially, to deionized water. Analytical electron microscopy (AEM) analyses of isolated I-S particles showed that most of them are smectite-rich I-S regardless of the experimental conditions, in agreement with XRD results. The I-S particles had a wide range of octahedral Mg contents. The pH and Na, K. Ca and Mg concentrations in the final solutions suggested cation (including H+) exchange as a major process in the alteration experiments. Analysis of aqueous activity diagrams (109 a(K)/a(H) vs. log a(SiO2)) showed that some solution compositions are consistent and some are inconsistent with I-S formation. These results, combined with complementary electron microscopy analyses (de la Fuente et al., 2000a), are interpreted to be due to direct transformation of the glass into I-S in a process controlled by glass composition.