Publication: Electron microscopic study of the illite–
smectite transformation in the bentonites
from Cerro del Aguila (Toledo, Spain)
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Publication Date
1998
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Mineralogical Society / Blackwell Scientific publications
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Abstract
A mineralogical and microstructural study of bentonites from ‘Cerro del Aguila’
located in the Tagus Basin (Toledo, Spain) was carried out using XRD, SEM and TEM observations
and chemical data obtained by TEM-EDX. The bentonites are mainly composed of trioctahedral
smectite with a unit-cell formula (Si3.76Al0.24)(Al0.44Fe3+
0.26Mg1.81)Ca0.05K0.19O10(OH)2 and small
amounts of illite. The relationships between illite and smectite particles observed by SEM and TEM
allowed the study of the weathering process of illite to form smectite through possible intermediate
stages. The transformation begins as an exfoliation normal to the stacking direction and develops by
opening of the interlayer spacing, the replacement of K+ by hydrated interlayer cations and slight
reorganization of the 2:1 layer structure. The so-called solid-state transformation process then
proceeds by further dissolution and the formation of a colloidal phase acting as raw material in the
growth of new phyllosilicates such as smectite.