Formation and growth of smectites in bentonites: A case study from KimolosIsland, Aegean, Greece

The low-temperature alteration of a rhyolitic rock from Kimolos Island. Aeg ean, Greece. yielded an alteration profile characterized by gradual transit ion from fresh glass to bentonite containing homogeneous Chambers-type mont morillonite and then to a mordenite-bearing bentonite. Both mordenite and s mectite were formed from poorly crystalline precursors, which probably had compositions comparable to that of the crystalline end-product. However, th eir composition may have been modified to some degree after reaction with t he fluid phase. Particle length and width measurements of smectite crystals yielded lognormal profiles, which suggest supply-controlled crystal growth in an open system or random ripening in a closed system. The former mechan ism is in accordance with the observed sustained supply of Mg and Fe by the fluid phase throughout the alteration profile and is believed to be the do minant formation mechanism of smectites in bentonites in general. In the mo rdenite-bearing zone, random ripening is expected in domains with low perme ability, in which the system was essentially closed, favoring the formation of mordenite. The level of supersaturation with respect to smectite was pr obably lower in the mordenite-bearing zone. Smectite probably affected pore -fluid chemistry either through ion exchange or via dissolution of initiall y formed K-bearing smectite. The latter process raised the K+/(Na+ + Ca2(+) ) activity ratio of the pore-fluid favoring K-bearing mordenite.