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given substances do in fact fall within the threshold, the most stable will be taken as
the R.S. as long as the “Earth similarity criterion” is not contradicted. The stability
threshold does not have a fix value and depends on each element considered, since
it is subject to geological uncertainties 16 . Consequently, Szargut's R.E. is similar
to the real physical environment and should provide a good representation of those
products formed in the interaction between the components of the natural envi-
ronment and the waste products of processes. In such a case, the most probable
products of this interaction should be chosen as reference species.
Drawing inspiration from Szargut's model, Ranz (1999) developed a R.E. based
on complete stability which contains the most stable minerals appearing in the
Earth's upper continental crust. According to this author, minerals found in the
upper continental crust, are not necessarily stable and do not represent, as proposed
by Szargut (1989), the products of an interaction between the components of the
natural environment and the waste products of industrial processes. Both Szargut's
and especially Ranz's criteria for establishing their R.E. are more in accordance with
the idea of the commercial end of the planet (Thanatia). Yet they are not and can
never substitute the Crepuscular Earth Model due to their impractical assumption
of only one substance per chemical element 17 .
Finally, another very important difference between a R.E. and Thanatia is that
the former only provides the environment's chemical composition. The concentra-
tion factor is a very important issue for assessing the loss of the mineral endowment
on Earth as will be seen later in the chapter. This is because the exergy of a mine-
ral deposit increases exponentially with its ore grade i.e. the greater the difference
between the concentration of the mineral in the mine and in the dispersed crust,
the greater the exergy of the deposit. Hence, not only is the composition of the
substances in the “dead environment” required but also their concentration.
In short, the reference environment and Thanatia are separate entities, albeit
that the authors recognise that they are closely related concepts. It should be
noted at this stage, that the final state of a R.E. is not particularly relevant nor
a prerequisite of the calculations presented here, as one of the purposes of this
topic is to ascertain the difference between the exergy of the mineral deposits at
their current state and that of Thanatia, the world's commercial end, as a means
to evaluate the loss of the global mineral endowment. Nevertheless, conventional
reference environments are still needed (in fact, Thanatia has a chemical exergy
with respect to a defined R.E.) and constitute a tool for the calculation of chemical
exergies.
16 Thus for example in the case of Sb, the substance Sb 2 S 3 is more abundant than Sb 2 O 5 , never-
theless, as Sb 2 O 5 , is much more stable, the latter was chosen as reference. Likewise the nitrates
such as Ca(NO 3 ) 2 , NaNO 3 , KNO 3 were discarded because although stable they are not abundant
in the natural environment and would thus break the similarity criterion if taken as the R.S.
17 For instance, the multiple natural substances in which silicon is present will hardly turn, within
the time span of human civilisation, into its reference substance SiO 2 because even if possible from
a thermodynamic point of view, the kinetics of the reactions literally block the transformation.
 
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