Electron microscopy study of volcanic tuff alteration to illite-smectite under hydrothermal conditions

Experimental alteration of volcanic tuff from Almeria, southeastern Spain, was performed in solutions with different Na/K ratios (0.01, 1, 10, and 100 ), different total salt concentrations (0.01, 0.1, 0.2, 0.33, and 1 M), and in deionized water, at 60, 80, 120, and 160 degrees C, for periods of 60, 90, 180, and 360 d. Two particle size fractions of volcanic tuff were used: 10-200 and 20-60 mu m. Alteration products were examined by X-ray diffract ion (XRD), Fourier-transform infrared spectroscopy (FTIR), laser-particle s ize analysis, scanning electron microscopy equipped with an energy dispersi ve X-ray spectrometer (SEM-EDS), image computer analysis, and transmission electron microscopy with microanalysis (TEM-AEM). XRD detected neoformed ph ases only in the products from experiments of 180-360 d at high temperature s (120-160 degrees C), and with Na/K ratios above unity and in deionized wa ter. The synthesized phase is a random mixed-layer illite-smectite (I-S) wi th 75% smectite. The quantity of newly formed I-S, determined by FTIR, rang ed between 3-30%. There was no apparent change in grain size and shape of t he grains after the experiments as compared to before. SEM-EDS and TEM-AEM revealed the following alteration sequence: 1) intense etching on glass-grain surfaces; 2) formation of hemispherical morphologies on grain surfaces; 3) precipitation of very thin, individual flakes of ill ite-smectite on glass-grain surfaces; 4) development of I-S at the edges of glass grains; and 5) development of I-S honeycomb structures either coveri ng large areas of the glass grains or resulting from the complete alteratio n of glass grains. A direct transformation of glass to I-S seems to be the major reaction mechanism, although there also is evidence of glass dissolut ion and subsequent I-S precipitation.