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Chemical hydrogeology 115

+
Table 3.13 Estimated average mineralogical composition of In this weathering reaction, H ions dissociated
the upper continental crust (volume %). The composition of the from H CO hydrate the silicate surface and naturally
2
3
exposed crust differs from the upper continental crustal estimate
buffer the infiltrating soil water or groundwater. The
primarily in the content of volcanic glass. After Nesbitt and Young 2+
ionic bonds between Ca and the SiO tetrahedra are
(1984). 2+ 4
easily broken, releasing Ca into solution resulting in
Upper continental Exposed continental a Ca-HCO water type.
3
crust crust surface Secondly, for the Na-rich plagioclase feldspar
(albite) the incongruent reaction producing kaolinite
Plagioclase feldspar 39.9 34.9 + −
Potassium feldspar 12.9 11.3 and releasing Na and HCO ions is:
3
Quartz 23.2 20.3
Volcanic glass 0.0 12.5 2NaAlSi O + 9H O + 2H CO →
3 8(s) 2 2 3(aq)
Amphibole 2.1 1.8 Al Si O (OH) 4(s) + 2Na + (aq) + 2HCO − 3(aq) +
5
2 2
Biotite mica 8.7 7.6 4Si(OH) eq. 3.46
Muscovite mica 5.0 4.4 4(aq)
Chlorite 2.2 1.9
Pyroxene 1.4 1.2 Further examples of weathering reactions for some
Olivine 0.2 0.2 common primary minerals are listed in Table 3.14.
2+
Oxides 1.6 1.4 When ferrous iron (Fe ) is present in the lattice, as
Others 3.0 2.6
in the case of biotite mica, oxygen consumption
may become an important factor affecting the rate
of dissolution that results in Fe-oxide as an insoluble
weathering product. For example, in the following
results in very dilute solutions, the weathering of silic- equation, biotite weathers to gibbsite and goethite:
ate minerals is estimated to contribute about 45% to
1
the total dissolved load of the world’s rivers, under- KMgFe AlSi O (OH) + /2O + 3CO +
2 3 10 2(s) 2(aq) 2(aq)
lining the significant role of these processes in the 11H O → Al(OH) 3(s) + 2Fe(OH) 3(s) + K + (aq) +
2
2+ −
overall chemical denudation on the Earth’s surface Mg + 3HCO + 3Si(OH) eq. 3.47
(aq) 3(aq) 4(aq)
(Stumm & Wollast 1990).
Upper crustal rocks have an average composition Differences in solution rates between the silicate
similar to the rock granodiorite (Table 3.13) that is minerals lead to their successive disappearance as
composed of the framework silicates plagioclase weathering proceeds. This kinetic control on the
feldspar, potassium feldspar and quartz, with plagio- distribution of primary silicates is known as the
clase feldspar being the most abundant. Depending Goldich weathering sequence (Fig. 3.22). As shown
on the nature of the parent rocks, various secondary in Fig. 3.22, olivine and Ca-plagioclase are the most
minerals such as gibbsite, kaolinite, smectite and illite easily weathered minerals while quartz is the most
are formed as reaction products. In all cases, water resistant to weathering.
+
and carbonic acid (H CO ), which is the source of H , The leaching of different weathering products
2 3
are the main reactants. The net result of the reactions depends not just on the rate of mineral weathering,
2+ 2+ + +
is the release of cations (Ca , Mg , K , Na ) and the but also on the hydrological conditions. Mont-
−
production of alkalinity via HCO . morillonite (Na Al Mg Si O (OH) ) is formed
3 0.5 1.5 0.5 4 10 2
The two following reactions (eqs 3.45 and 3.46) preferentially in relatively dry climates, where the
provide examples of important silicate weathering flushing rate in the soil is low, and its formation
processes. Firstly, taking Ca-rich plagioclase feldspar is favoured when rapidly dissolving material such
(anorthite) the incongruent weathering reaction as volcanic rock is available. By contrast, gibbsite
resulting in the aluminosilicate residue kaolinite is (Al(OH) ⋅3H O) forms typically in tropical areas with
3 2
written (Andrews et al. 2004): intense rainfall and under well-drained conditions.
Here, gibbsite and other aluminium hydroxides may
CaAl Si O 8(s) + 2H CO 3(aq) + H O → form a thick weathering residue of bauxite. Because
2 2
2
2
2+
Ca + 2HCO + Al Si O (OH) eq. 3.45 of differences in residence times, water in areas where
(aq) 3(aq) 2 2 5 4(s)