Geology Reference
In-Depth Information
Example 3.4: Investigation of the stability relationship between gibbsite
and kaolinite in natural waters at pH o 7.
Consider the acid hydrolysis weathering reaction undergone by Na-
feldspars (pH o 7):
NaAlSi 3 O 8(s) þ H (aq) þ 7H 2 O
(aq) þ 3OH (aq)
þ 3Si(OH) 4(aq)
This equilibrium expression represents congruent dissolution of Na-
feldspar but the solution quickly becomes saturated with respect to
the solid phase Al(OH) 3 (gibbsite K SP ¼ 10 33.9 ) and so the secondary
mineral, gibbsite, is formed.
NaAlSi 3 O 8(s) þ H (aq) þ 7H 2 O
Na (aq) þ Al 3 þ
"
Al(OH) 3(s) þ Na (aq) þ 3Si(OH) 4(aq)
The concentration of silicic acid in solution is a key parameter
influencing the stability of gibbsite. In the presence of even low
concentrations of silicic acid, e.g.10 4 mol L 1 , kaolinite would be
formed instead.
2NaAlSi 3 O 8(s) þ 2H (aq) þ 9H 2 O
"
Al 2 Si 2 O 5 (OH) 4(s) þ 2Na (aq)
þ 4Si(OH) 4(aq)
In natural systems, the role of water as a transporting agent is
important. Geochemical conditions promoting removal of silicic acid
from the solid-water interface (e.g. water flow) favour the formation
of gibbsite over kaolinite.
(Adapted from Drever. 8 )
"
3.2.2.2 Solubility. Dissolution of minerals during chemical weath-
ering releases species into solution but aqueous phase concentrations
are limited by the solubility of solid phases, such as amorphous silica,
gibbsite, and metal salts. For example, the solubility of Al III and
Ca II may be limited by the formation of gibbsite and gypsum, respec-
tively.
Al 3 þ
(aq) þ 3OH (aq)
Al(OH) 3 gibbsite
"
(3.23)
Ca 2 þ
(aq) þ SO 2 (aq) þ 2H 2 O
CaSO 4 .2H 2 O gypsum
"
(3.24)
The equilibrium solubilities are defined by the respective solubility
products, K SP (values from ref. 30).
K SP ¼ {Al 31 }{OH } 3 ¼ 10 33.9
(3.25)
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