Geology Reference
In-Depth Information
Box 8.2 Clay minerals and ion exchange
Clay minerals are sheet silicate minerals with characteris-
tics that make them of environmental and industrial
sandwich - enclosing an octahedral layer as a 'sandwich
filling' in between. In this configuration, tetrahedral and
octahedral layers are combined in the proportion 2:1.
pairs may be stacked directly against each other as in the
mineral pyrophyllite (repeat distance 0.9 nm), or succes-
sive 'sandwiches' may be separated by a plane of larger
interlayer cations such as K + , as in the clay mineral illite ,
which has a repeat distance of 1.0 nm (Figure 8.2.1b).
all clay minerals can be considered as hydrous owing to
the involvement of Oh - anions in the octahedral layer.
however, clay minerals of the smectite group, such as
montmorillonite (Figure  8.2.1c), additionally incorporate
discrete sheets of h 2 O molecules between adjacent 'sand-
wiches', closely associated with the large interlayer cati-
ons. the presence of h 2 O increases the interlayer repeat
distance to about 1.4 nm. Water content - and therefore
repeat distance - varies with changing temperature and
humidity, giving smectites their characteristic capacity to
swell and shrink according to conditions. the swelling
behaviour poses significant geotechnical challenges when
building on smectite-rich ground.
existing as fine-grained aggregates, they play an impor-
tant role in determining soil texture, fertility, permeabil-
ity and moisture retention.
Some clay minerals (the smectite group) can absorb
variable amounts of water, swelling as they do so.
Conversely they lose water and contract in dry cond-
itions, leading to the polygonal cracks that characterize
muddy soils during drought.
their surface properties give clay minerals a high
cation-exchange capacity.
Clay minerals have numerous technological and environ-
mental applications, from the obvious example of pottery
and ceramics (for which natural clay deposits - kaolinite -
are the raw material) to uses such as paper fillers and
coatings, cat litter, drilling muds, and barrier materials for
landfill sites and mine tailings.
Structure and types
Clay mineral crystals are assemblages of three types of
layer (Figure 8.2.1):
Cations such as K + , Mg 2+ , Ca 2+ and Nh 4 + (ammonium) - all
important plant nutrients - are attracted to the negatively
charged surfaces 1 of clay mineral particles and attach to
them electrostatically, a geochemically important type of
surface reaction called adsorption. Soils act as store-
houses of plant-available nutrients, and the large aggregate
surface area provided by fine clay particles regulates the
availability of such elements and constitutes - as a result
of adsorption - a key part of the soil nutrient reservoir.
adsorption on mineral surfaces is not restricted to cat-
ions. as we saw at the end of Chapter 4, the behaviour of
arsenic in groundwater is dictated by the formation of poly-
anions like arsenate and arsenite, and their capacity to
adsorb on to the surfaces of oxide mineral grains. anion
adsorption requires mineral surfaces with positive surface
charge, which is more likely to occur in acidic (low-ph)
layers of SiO 4 tetrahedra (with al substituting for Si in
some varieties) - 't' in Figure 8.2.1;
layers of al 3+ , Mg 2+ or Fe 2+ ions, octahedrally co- ordinated
by Oh - or O 2- anions - 'O' in Figure 8.2.1;
sheets of large 'interlayer' cation sites (analogous to
'a'-sites in amphiboles - Box  8.5) that accommodate
ions like K + , Na + and Ca 2+ - 'L' in Figure 8.2.1.
Like varieties of sandwich on a buffet tray, the above layers
can be combined in various ways (Figure 8.2.1). the sim-
plest is a stack of tetrahedral sheets, all facing the same
way, interspersed on a 1:1 basis with octahedral al(Oh) 3
layers (Figure 8.2.1a). this is the structure of the kaolinite
group of clay minerals, characterized by an interlayer
repeat distance (measured by X-ray diffraction - see
Box 5.3) of 0.7 nm.
More commonly, the tetrahedral layers combine in
inward-facing pairs - like buttered bread slices in a real
the causes of electrostatic charge on mineral surfaces are
complex and beyond the scope of this topic. Summaries are
given by Krauskopf and Bird (1995) and White (2013).
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