Characterization of the Subsurface Environment - Contaminant Geochemistry - page 12

Fig. 1.8 High-resolution
electron micrographs of thin
sections showing electron
optical fringes a indicative of
inclusion of mica in kaolinite,
and b interstratification of
kaolinite and other layer
silicates. Fringes shown
indicate the spacings of basal
planes viewed from the edge
(Dixon 1989 )
surface are more acidic than the interlayer water (Borchardt 1989 ). Hydrogen
migration from exchangeable positions into the tetrahedral sheet may occur.
Heating of Al- and H-saturated montmorillonite results in fading out of the
1,700 cm -1 band as a result of proton migration (Yariv and Heller-Kallai 1973 ).
Of particular interest for chemical transport into a predominantly smectite
medium is the shrink-swell property of the clay material. The swelling properties
of smectites are explained by two concepts. The first one, developed by Sposito
( 1973 ), shows that smectite swelling is caused by the hydration and mobility of the
cations, which in turn balance the negative charge of the layer silicates. The
second concept, presented by Low ( 1981 ), emphasizes the direct interaction of
water molecules with the silicate surface. Both viewpoints fit the common
observation that smectite swells in a high-hydration environment and at low
electrolyte concentrations and shrinks when water is lost and salt is added to the
bulk solution.
A mixture of intercalating clays is generally found in the subsurface. Inter-
stratification of kaolinite and smectite has been reported in some cases (e.g.,
Schultz et al. 1971 ; Lee et al. 1975a , b ; Yerima et al. 1985 ). This fact is reflected in
an XRD spacing of about 0.8 nm (Dixon 1989 ) or in HRTEM micrographs
(Fig. 1.8 ), which show the inclusion of mica (1:2 layer) and other layer silicates in
kaolinite (Lee et al. 1975a , b ).
Environmentally induced processes occurring in the subsurface (e.g., leaching,
acidification) may induce weathering processes and structural changes in natural
clays. For example, in soils with low organic matter, moderate leaching and a pH
of about 5 causes smectites to be transformed into pedogenetic chlorites (Barnhisel
and Bertsch 1989 ). Laboratory studies proved a direct transformation of smectites
into kaolinite during intense weathering. In natural environmental conditions, the