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Chemistry of Geothermal Fluids 77

For the quartz in the pot, the situation is different. The value for K at 100°C is about 0.001
(Figure 5.2), which translates into a total SiO (aq) concentration of about 0.06 grams at equilibrium,
2
or approximately 60 parts per million. Clearly, in this case, we have added to the pot of water much
more quartz than would ever dissolve. In fact, so little would dissolve that it would not be noticeable
unless we carefully weighed it before and after our experiment.
The halite and quartz reactions are representative of dissolution and precipitation reactions that
will occur in geothermal systems. A fluid migrating through a porous or fractured medium will
react with the rock framework. If the fluid is undersaturated with respect to the minerals in the rock,
it will dissolve the minerals and thus increase the porosity and permeability along its flow path. If
the fluid happens to become supersaturated with respect to one or more minerals, it will precipi-
tate those minerals and decrease the porosity and permeability. Since most rocks are composed
of a suite of minerals, a wide range of reactions are possible thus leading to the possibility that
the solution could become undersaturated in some minerals and supersaturated in others, with the
consequence that precipitation and dissolution could occur simultaneously. Added to this is the pos-
sibility, which is often realized, that the solution can become supersaturated in a mineral phase not
present in the rock. In this case, growth of a new mineral phase along the flow path can occur. If
the solution composition evolves over time, a sequence of different minerals may grow on fracture
surfaces or in pore spaces, resulting in mineralogically zoned features. Given the millions of years
over which geothermal systems can persist, it is possible that the mineralogy seen today could be
completely different from that when the geothermal system first formed. Geothermal fluids thus can
be powerful agents of change.

ion exchanGe
Another important reaction process that has only minimal impact on porosity and permeability evo-
lution, but which has important implications for assessing geothermal systems, is ion exchange.
Although some minerals, such as quartz, have a fixed chemical composition (i.e., SiO ), other miner-
2
als can accommodate a variety of ions in their structure. One example of this is alkali feldspar, one of
the most common minerals in the Earth’s crust. There are two main end member chemical formulae
for alkali feldspar, NaAlSi O and KAlSi O . These two pure end member compositions are occa-
8
3
3
8
sionally found in nature, but more commonly the mineral is found to be a mixture of the two end
member molecules, with the chemical formula Na K AlSi O , where x is less than or equal to 1.0.
8
3
x
1-x
The relationship between the end members can be written as an exchange reaction:
+
+
NaAlSi O + K < = > KAlSi O + Na ,
3
8
8
3
+
where the Na and K are ions in a solution coexisting with the feldspar. In Figure 5.3 the log of the
+
equilibrium constant for this exchange reaction is plotted as a function of temperature.
The large variability of log K over a relatively small temperature interval raises an important
point. Namely, an aqueous solution in contact with and in equilibrium with alkali feldspar at a given
+
+
+
temperature should have a specific Na :K ratio. In other words, the ratio of Na to K in a solution
+
is temperature sensitive and has the potential to be a geothermometer (see, for example, Fouraier
1992). There are numerous caveats to applying this principle, but a variety of exchange reactions
involving numerous minerals have been evaluated for their potential as geothermometers. We will
consider these geothermometers in much more detail in Chapters 6 and 7.
Although these chemical arguments provide a means for evaluating what minerals may dissolve
or precipitate under a given set of conditions, and how reaction processes will influence fluid and
mineral compositions, they tell us nothing about the rate at which these processes will proceed. To
evaluate how quickly, or even whether equilibrium will be achieved, and to know how fast dissolu-
tion or precipitation will occur, we must consider reaction kinetics.