Page 133 - Hydrogeology Principles and Practice

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116 Chapter Three

Table 3.14 Reactions for incongruent dissolution of some aluminosilicate minerals (solid phases are underlined). After Freeze and
Cherry (1979).
Aluminosilicate mineral Reaction
Gibbsite-kaolinite Al O ·3H O + 2Si(OH) = Al Si O (OH) + 5H O
2 2 5
4
4
2
2
2 3
O (OH) + /3H + /6H O = /6Al Si O (OH) + /3Na + /3Si(OH)
Na-montmorillonite-kaolinite Na 0.33 Al 2.33 3.67 10 2 1 + 23 2 7 2 2 5 4 1 + 4 4
Si
O (OH) + /3H + /2H O = /3Al SiO O (OH) + /3Ca + /3Si(OH)
Si
Ca-montmorillonite-kaolinite Ca 0.33 Al 4.67 7.33 20 4 2 + 23 2 7 2 2 5 4 1 2+ 8 4
Illite-kaolinite K Mg 0.25 Al 2.30 3.5 10 2 11 + 63 2 23 2 2 5 4 3 + 1 2+ 6 4
Si O (OH) + /10H + /60H O = /30Al Si O (OH) + /5K + /4Mg + /5Si(OH)
0.6
2+
+
+
1
1
Biotite-kaolinite KMg AlSi O (OH) + 7H + /2H O = /2Al Si O (OH) + K + 3Mg + 2Si(OH) 4
3 10
3
2 2 5
2
4
2
+
+
1
9
Albite-kaolinite NaAlSi O + H + /2H O = /2Al Si O (OH) + Na + 2Si(OH) 4
2
2 2 5
4
3 8
+
20
6
3
O (OH) + /7Na + /7Si(OH)
Albite-Na-montmorillonite NaAlSi O + /7H + /7H O = /7Na 0.33 Al 2.33 3.67 10 2 6 + 10 4
Si
3 8
2
+
+
1
9
Microcline-kaolinite KAlSi O + H + /2H O = /2Al Si O (OH) + K + 2Si(OH) 4
4
3 8
2
2 2 5
+
Anorthite-kaolinite CaAl Si O + 2H + H O = Al Si O (OH) + Ca 2+
2
2 2 5
2 2 8
4
+
+
2+
1
3
3
1
11
Andesine-kaolinite Na Ca Al Si O + /2H + /4H O = /4Al Si O (OH) + /2Na + /2Ca + Si(OH) 4
4
2 2 5
0.5
0.5
1.5 2.5 8
2
+ 2
[Na ][ (OH ) ]
Si
4
K = eq. 3.48
−
albite kaolinite +
[H ]
where K is the equilibrium constant. In this
albite-kaolonite
approach, the activities of the mineral phases and
water are taken as unity. Expressing equation 3.48 in
logarithmic form gives:
+
log K = log [Na ] + 2 log [Si(OH) ] − pH
10 albite–kaolinite 10 10 4
eq. 3.49
or
+ ]⎞
[ ⎛ Na
+
2
log K = log 10 ⎜ log [ ( Si OH ] ]
−
10 albite kaolinite + ⎟ 10 4
Fig. 3.22 The Goldich weathering sequence based on ⎝ [H ] ⎠

observations of the sequence of disappearance of primary silicate
eq. 3.50
minerals in soils. After Goldich (1938).
which indicates that the equilibrium condition for the
montmorillonite is forming is high in dissolved ions albite-kaolinite reaction can be expressed in terms
+
while in areas with high rainfall with the formation of of pH and activities of Na and Si(OH) . Equilibrium
4
gibbsite, the dissolved ion concentrations are low relations such as this are the basis for the construc-
(Appelo & Postma 1994). tion of stability diagrams, examples of which are
Now, by taking a thermodynamic equilibrium shown in Fig. 3.23. These types of diagrams represent
approach, it is possible to gain further insight into minerals with ideal chemical compositions, which
some of the more specific results of groundwater may not accurately represent real systems, but, never-
interactions with feldspars and clays. For example, theless, are useful in the interpretation of chemical
considering the albite (NaAlSi O ) dissolution reac- data from hydrogeological systems. In igneous ter-
3 8
tion given in Table 3.14, then from the law of mass rain, nearly all groundwaters within several hundred
action: metres of the ground surface plot in the kaolinite