1298 WATER: PROPERTIES, STRUCTURE, AND OCCURRENCE IN NATURE
(partial) dissolution of aluminium silicates which may be
represented schematically as follows:Me Al silicates(s) H CO H O
HCO H SiO Me Al silica23*
2
344 n n tte(s), (a)where Me is a cation such as Na^ ^ or Ca^2 ^. Table 5 shows
examples of this type as well as weathering reactions of lime-
stone (calcite).
In essence, primary minerals are converted into sec-
ondary minerals. The secondary minerals are frequently
structurally ill-defined or X-ray amorphous. The structural
breakdown of aluminium silicates is accompanied by a
release of cations and usually of silicic acid. As a result of
such reactions solution increases during the weathering pro-
cess, the solid residue has a higher acidity than the original
aluminium silicate.
Because H 2 CO 3 * is usually the acid in the attack of water on
the primary silicates, HCO 3 is the predominant anion in most
fresh waters. Because of respiration by organisms, the CO 2
composition in soils can be a few hundred times larger than
that in the atmosphere. Correspondingly ground waters tend
to contain higher concentrations of HCO 3 and other solutes.
Minerals of the kaolinite group are the main alteration products
of the weathering of feldspar. In addition to kaolinites, mon-
morillonites and micas are also found as intermediates or end
products. Mica (illite) has been identified as an intermediate in
the decomposition of potassium feldsparorthoclase.
The dissolution kinetics of feldspars may cause them to
reach their final products through temporally stable interme-
diates. Virtually all bonds must be broken in the tetrahedric
aluminium framework before the structure can be rearranged
to a lattice having six coordination bonds around the alu-
minium as in kaolinite. Understandably, the rate of reactionis very slow. Physical weathering processes, that is, pro-
cesses which by physical or mechanical means alter the size
and hence the specific surface area of the minerals, enhance
reaction rates since the weathering processes take place only
at the rock-water interface.
The composition of water is obviously influenced by these
weathering reactions. While Ca^2 ^ , HCO 3 , H^ ^ , and perhaps
Mg^2 ^ may be controlled by dissolution carbonate rocks, the
source of Na^ ^ , K^ ^ , and H 4 SiO 4 , and possibly also of Ca^2 ^ and
Mg^2 ^ , is the silicate minerals that make up 70% or more of the
rocks in contact with underground waters and streams.
Figure 10 gives in an abbreviated periodic table the
important elements typically found in fresh and sea water,
their dominant forms of occurrence and their representative
concentrations. A recent survey of the frequency distribution
of various constituents of terrestrial waters by Davies and
DeWiest (Figure 11) shows that many of the aquatic constit-
uents exhibit little natural variation in their concentrations.Sedimentary
Rocks Sediments SoilsIgneous and
metamorphic
rocksVolcanic
emanation
H 2 O, CO 2 , HCl,
H 2 S, S, SO 2 , SAtmo-
sphereWeathering
+
Transport
+
DepositionSolutes in waterFIGURE 9 Interaction between the cycles of water and
rocks.TABLE 5
Examples of typical weathering reactions aI. Congruent dissolution reactions
SiO (s) 2H O 2244 H SiOquartz
CaCO (s) H CO 323 *2Ca2HCO 3calcite
Al O 3H O(s) 2H O 23 ⋅ 2 2 2Al(OH) 4 2Hgibbsite(^) Ca (PO ) (OH)(s) 4H CO 5Ca 3HO 4HCO H O
543 23
*2
4
2
32
apatite
II. Incongruent dissolution reactions
NaAlSi O (s) H CO^3823 9 2H O
- 2
albite (Na-feldspar) =
Na HCO^344 2H SiO 1 2Al Si O (OH) (s)^2254
kaolinite
7NaAlSi O (s) 6H CO^3823 20H O
2
albite (Na-feldspar) =
6Na 6HCO^3 10H SiO^44 3Na0.33Al2.33Si3.67O (OH) (s)^102
Na-montmorillonite
Al Si O (OH) (s) 5H O^2254 ^2 2H SiO^44 Al O 3H O^23 ⋅^2
kaolinite gibbsite
CaMg(CO ) (s) Ca^32 Mg 2CaCO (s)
22
3
dolmite calcite
a After Stumm and Morgan (1981).
C023_004_r03.indd 1298C023_004_r03.indd 1298 11/18/2005 11:12:33 AM11/18/2005 11:12:33 AM