Geoscience Reference
In-Depth Information
Fig. 1.7 Representative EDS spectra of Na-montmorillonite. Cu peaks arise from the TEM grid.
Reprinted with permission from Yaron-Marcovich et al. (
2005
)
silicate particles (about 10-50 nm wide and 50-400 nm long) with no obvious
orientation are visible. Closer inspection of the microstructure clearly discloses a
layered structure with layers occurring as well-defined stacks separated by a
regular van der Waals gap along the c-axis. The interlayer spacing measured from
HRTEM images was 0.138 nm; this result (compared to 0.149 nm obtained by
XRD) may be explained by the influence of interlayer hydration on the d-spacing
of montmorillonite. The interlayer compositional variation of the same clay
sample determined by X-ray energy-dispersive spectroscopy (EDS) is shown in
Fig.
1.7
. It may be observed that the nanoparticles are of similar composition,
containing Na, K, Al, Mg, and O. The presence of copper, iron, and carbon signals
can be ignored, as they arise from the analytical technique.
Infrared analysis of smectites provides considerable information on their struc-
tural properties. Silicate minerals have strong Si-O bands near 600 and 1,000 cm
-1
,
which can be affected by substitution of silicon atoms by Al atoms. The OH bending
vibrations produce the absorption characteristics of the octahedral sheet. When Al is
present, the absorption is near 920 cm
-1
, and when only Fe is present, absorption is
near 820 cm
-1
. A mixture of Al, Fe, and Mg leads to intermediate values. Hydroxyl
stretching vibrations absorb in the 3,000-3,800 cm
-1
region. Farmer and Russell
(
1967
) show that this band appears to broaden as a result of Al substitution for Si in
the tetrahedral sheet and, at the same time, reflect the type of saturating cation and
the hydration status in the interlayer space. The surface oxygen atoms are weak
electron donors and form weak hydrogen bonds. As a consequence and as a result of
their association with exchangeable cations, water molecules on the smectite
