Geology Reference
In-Depth Information
11.2.3 The thermodynamic properties of the upper continental
crust
Table 11.7 shows the standard thermodynamic properties of the major minerals
that compose the upper continental crust, according to the model developed in
Chap. 10. Only the references of the experimental values of H
f i
and G
f i
are
provided in the aforementioned table. The remaining values were obtained via 11
different estimation methods described in Valero D. et al. (2012). The error ("
%) associated with the different methods varies from 0 to 10%. Note that it was
assumed that Thanatia's composition could be approximated to that of the average
mineralogical of the upper crust and hence the chemical exergy obtained for the
upper crust is equivalent to that of Thanatia.
The average standard enthalpy, Gibbs free energy and chemical exergy of the
upper continental crust, for an average molecular weight of the crust equal to
MW
cr
= 155:2 g/mol is (Valero D. et al., 2011):
(H
0
f
)
cr
= 2300:37 kJ/mol
(G
0
f
)
cr
= 2162:56 kJ/mol
(b
ch
)
cr
= = 3:63 kJ/mol
It should be stated, that the above values are subject to different sources of
error. Firstly, the composition of the crust used and the relative proportion of each
of its minerals
i
is based on the crust of the Crepuscular Earth Model developed
in Chap. 10. It was built ensuring consistency between the chemical composition
of the crust in terms of elements (which is reasonably well known thanks to the
contribution of a good number of geochemical studies in the last century), and
the composition in terms of minerals (for which with the exception of the work
of Grigor'ev (2007), no significant research had been carried out due to the com-
plexity and the heterogeneity of the crust). The model was complemented with
different hypotheses based on geological and geochemical observations. Hence, it is
expected that
i
will be updated in the future as better geological and geochemical
information becomes available.
The second source of error stems from assuming only one chemical composition
per mineral. In fact, any one mineral can have slight variations in composition.
For instance, the general chemical formula of oligoclase is (Na;Ca)(Si;Al)
4
O
8
and
under that formula, many different chemical compositions of oligoclase may appear.
In this topic it has been assumed that oligoclase (and the remaining minerals) is
represented by its empirical formula, which is derived from the mineral's structural
(molecular) formula by using the published analysis of the mineral or deriving the
analysis using a basic set of rules (Barthelmy, 2007).
The third source of error is associated with the different estimation methods
explained in Valero D. et al. (2012) for obtaining the enthalpy and Gibbs free
energy of mineral formation.
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