Geology Reference
In-Depth Information
(a)
Kaolinite (T:O = 1:1)
O
T
O
T
'Gibbsite' Al(OH)
3
layer
O
0.7 nm
T
T= layer of SiO
4
tetrahedra
as shown in Fig. 8.2(c)
O= layer of Al
3+
, Mg
2+
or Fe
2+
ions surrounded by OH
-
and O
2-
anions in
octahedral
co-ordination
(b)
Illite (T:O = 2:1)
T
O
1.0 nm
T
L
T
O
T
L
L = sheet of larger 'interlayer' cations (
e.g.
K
+
in illite, Na
+
or Ca
2+
in montmorillonite)
(c)
Montmorillonite (T:O = 2:1)
T
O
1.4 nm
variable
T
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
L
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
T
O
T
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
L
Figure 8.2.1
Simplified representation (not to scale) of the structures of the clay minerals (a) kaolinite, a '1:1 clay
mineral', (b) illite, and (c) montmorillonite ('2:1 clay minerals'). Silicate sheets are shown edge-on using the symbolism
of Figure 8.3. all three minerals contain octahedral layers (light orange) approximating to the composition of the mineral
gibbsite
, al(Oh)
3
; other clay minerals may instead contain octahedral layers resembling the mineral
brucite
, Mg(Oh)
2
.
Ion exchange
the population of soluble cations adsorbed on the nega-
tively charged surface of a clay particle interacts chemically
with any aqueous fluid in contact with it, exchanging cati-
ons in order to arrive at an equilibrium distribution. For
example, the reaction
illustrates an important control that colloidal clay particles
exert on the Ca
2+
content of river water entering the
oceans. Such
ion exchange reactions
play a key part in
regulating the composition of natural waters more
generally, for example in buffering the major element
composition of the groundwater in an aquifer. Moreover
the interlayer cations within clay mineral particles (the
so-called 'exchangeable cations') also participate in
Ca +2K
2+
+
Ca+2K
2+
+
(8.2.1)
aq
clay
clay
aq
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