Abstract
In the volcanic region of Cabo de Gata (Almería, Spain) bentonite deposits were formed from pyroclastic rocks. There is evidence that the smectite composition changed from Fe-rich to Al-rich members as the degree of hydrothermal alteration increased. Because true initial stages of alteration do not occur in this region, artificial hydrolysis of rocks is needed to obtain early-stage smectites. Three typical volcanic rocks were hydrolyzed in Soxhlet extractors at 82°C for 1457 hr. with 243–302 l of percolating water. In these conditions, 11–12% of smectite was formed. Mineralogical and chemical data of synthetic smectites reveal that they are Fe-bearing beidellites. In fact, the mean value of octahedral Fe of synthetic smectite is 0.85 whereas for the natural one is 0.21. The tetrahedral charge is close to 0.6 and 0.2, respectively. The differences between the synthetic and the natural processes of formation are strictly kinetic and do not seem to affect either the mechanism of the hydrolytic reaction or the composition of neoformed smectites.
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