Environmental Engineering Reference
In-Depth Information
Cl
-
Cl
-
Cl
-
Na
+
Cl
-
Na
+
outer
Helmholtz
plane
Na
+
Na
+
Na
+
bound
layer
CLAY MINERAL
Figure 3.10
Schematic representation of the formation of the Helmholtz Double Layer
adjacent to a clay mineral surface (after Hill et al, 1997)
This means that the negative portions of the (neutral) crystals are on their
external surfaces.
Many rock-forming minerals do not have fixed chemical formulas, but
allow substitutions of cations, within their crystal lattices. In many silicate
minerals, this cation substitution involves the substitution of divalent (e.g.,
Fe
+2
) for trivalent (e.g., Al
+3
) cations, or tri-valent (e.g., Al
+3
) for quadra-
valent (e.g., Si
+4
) cations, in the crystal lattice, resulting in a net negative
surface charge on the crystal surface. This net negative charge density on
the mineral crystal surfaces attracts the positive poles of water molecules
and dissociated cations, while repelling the negative poles of water mol-
ecules and dissociated anions in solution (as shown in figure 3.10).
3.8.3
Phillosilicates and Clay Minerals
Reservoir rocks often contain significant amounts of clays (clay-sized par-
ticles, and/or clay minerals) within their matrixes. Clay particle size grains
are usually phillosilicates or sheet-silicates, such as micas and clay miner-
als, which have net negative surface charges.
Phillosilicate and clay mineral crystals are composed of composite sheets
of AlO
6
octahedra and SiO
4
tetrahedra layers. The substitution of divalent
for trivalent cations, in the octahedra layers or trivalent for quadra-valent
cations, in the tetrahedra layers leaves strong net negative charge densities
on the flat mineral phillosilicate crystal surfaces. Different clay minerals
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