Geology Reference
In-Depth Information
Box 8.2 (Continued)
equilibria like reaction 8.2.1 and contribute to a clay min-
eral's overall capacity for ion exchange.
Geological materials (clay minerals, sediments, soils)
vary widely in their propensity to store and exchange cat-
ions, which depends upon composition and mineral struc-
ture as well as environmental conditions such as ph. the
natural variability among environmental materials is illus-
trated by typical values of
cation exchange capacity
(CeC)
given in table 8.2.1:
CeC is usually defined and measured in terms of how
much Nh
4
+
a sample can take up from a 1 M ammonium
acetate solution at ph = 7.0. Being dependent upon a
number of factors, it is not a precise quantity but it
nonetheless provides a useful measure for comparing
the ion exchange properties of minerals, soils and
sediments.
Table 8.2.1
Cation exchange capacities of
geological materials
Soil constituent
Cation exchange
capacity/CEC units*
hydrous oxide minerals
~4
Kaolinite
2-10
Illite
10-40
Montmorillonite
80-150
Vermiculite
120-200
Soil organic matter
150-300
*CeC is usually expressed in arcane units of
meq
per
100 g of mineral or soil.
Note that even the highest CeC values of clays are often
exceeded by those of organic colloids in soil.
large cavities and channels through which cations and
even molecules can diffuse quite readily.
As there are no non-bridging oxygens in the frame-
work silicate structure (
p
= 0), the Z:O ratio is 1:2. The
simplest composition is SiO
2
, whose stable form at
room temperature is quartz (specific gravity, SG 2.65);
at elevated temperatures the more open structures of
the polymorphs tridymite (SG 2.26) and cristobalite
(SG 2.33) crystallize in its place. The three-dimensional
network structure is reflected in the poor or non-
existent cleavage of the silica minerals; quartz, for
example, has a conchoidal fracture.
Substitution of Al into some of the tetrahedral sites
in place of Si makes possible a huge variety of
alum-
inosilicate
minerals
2
including the feldspars (the most
abundant mineral group in the crust), and their silica-
deficient cousins the feldspathoids. Replacing Si
4+
with
trivalent Al
3+
without changing the oxygen content, of
course, requires other cations to be introduced to main-
tain the charge balance. Owing to the openness of the
framework structure, these compensating cations can
be quite large (Na
+
, K
+
, Ba
2+
).
Among the most open of the aluminosilicates are the
zeolites. Unlike the feldspars they are hydrous, the
water being held loosely in large intercommunicating
cavities which can be up to 1 nm across. This framework
is so rigid that the zeolites possess the remarkable abil-
ity to expel this water continuously and reversibly when
heated, without their structure breaking down. Zeolites
(natural and artificial) have many uses in chemical eng-
ineering, as ion exchangers and as 'molecular sieves'
that can separate small molecules according to their
size. They perform a vital function as catalysts in the
petroleum industry.
The same range of silicate polymers can be found in
silicate melts (Box 8.3) too, but with one key differ-
ence: whereas each crystalline silicate mineral incor-
porates
one
(or in rare cases two) types of silicate
polymer,
several
polymeric types may coexist within
the more disordered structure of a chemically homo-
geneous silicate melt.
Cation sites in silicates
Not to be confused with the
aluminium silicate
(Al
2
SiO
5
) min-
erals kyanite, andalusite and sillimanite introduced in Chapter 2,
in which at least some of the Al
3+
ions are in
octahedral
co-ordina-
tion. This important distinction is often misunderstood.
2
In most silicates (excluding the framework silicates)
the tetrahedra fit together compactly, producing an
orderly three-dimensional array of fairly closely-packed
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