DESCRIPCIÓN GENERAL DE LA PLANTA

The first thing to me noted in the composition of table 3.5 is that the total mass of the minerals contained in the upper crust is greater than 100%. As mentioned above, the oxygen and hydrogen quantities in the crust have been left free. In the chemical compositions of the crust given by Rudnick et al. [292], Wedepohl [404]

or McLennan [215], no H or O values are provided. However the value for O can

be determined from the first two authors, as some of the elements are given as the corresponding oxides. That is the case for SiO₂, T iO₂, Al₂O3, FeO, M nO, M gO,

C aO, N a2O, K2Oand P2O5. The oxygen concentration resulting from those oxides

gives in Rudnick’s and Wedepohl’s models of the crust 2, 95_{× 10}−2 mole/g. In the model developed in this PhD, the concentration of oxygen is 4,8% greater: 3, 10_× 10−2 mole/g, while in Grigor’ev’s 0,7% greater: 2, 97 × 10−2 mole/g. Hence, if the chemical composition of the crust is right, then there is an excess of oxygen in the minerals of our model and that of Grigor’ev. This oxygen could be in the form of molecules O₂ or H₂O, which would be in the concentration of 1, 5_{× 10}−2 and 1, 1_{× 10}−3mole/g, respectively. An excess of oxygen could be attributed to the fact that the minerals considered may not be electronegatively neutral. Nevertheless, we have assured the neutrality of the charges of every mineral considered. If the oxygen quantity is fixed in our model, then the Si content is significantly smaller than the one given by Rudnick: 1, 01_×10−2instead of 1, 10_×10−2g/mole. With the chemical composition of the minerals given in table3.5, there is no possible solution to Eq.3.2

if the concentration of oxygen is also fixed. Hence, it seems that the problem comes from the chemical formulae used. It must be pointed out, that many of the minerals given by Grigor’ev do not have a fixed chemical composition. They represent a variety of minerals with changing concentrations of certain elements. That is the case for biotite, apatite, phosphate rock, etc. We have tried to take into account an average chemical formula, given by the empirical formula recorded under [172],

mineral. Nevertheless, many different formulas are possible. Therefore, this aspect should be checked in further developments of the model.

Another aspect that should be taken into account, is that the chemical composition of the earth in terms of elements has been assumed to be correct and not the one generated by Grigor’ev. The decision to do so was because the first one has been subject of many research studies throughout history, while the last one has just begun to be analyzed. Nevertheless, in some cases the procedure developed in this PhD could serve as a tool for assuring the chemical composition of the crust. The low concentration of calcite obtained in our model for example, has set the alarms for the concentration value of C in the crust, which might have been underestimated by Wedepohl.

3.7

Summary of the chapter

In this chapter, a revision of the studies concerning the mineralogical composition of the earth’s crust has been carried out. It has been verified, that the literature about this topic is very limited and inaccurate, due to the heterogeneity and complexity of the crust. Nevertheless, one single author, the Russian geochemist Grigor’ev has been very recently the first one in giving a comprehensive mineralogical composition of the upper crust.

With the help of Eq. 3.1, we were able to check the satisfaction of the mass balance between the minerals proposed by Grigor’ev and the better known chemical compo- sition in terms of elements of the crust. The no satisfaction of the mass balance, lead us to propose a new composition, based on the Grigor’ev’s semi-empirical analysis. The methodology used minimizes the difference between Grigor’ev’s and our pro- posed compositions under the constraint of assuring chemical coherence with the average chemical composition of the earth’s crust in terms of elements. We have made assumptions based on the literature for those important minerals not taken into account in Grigor’ev’s analysis, and included them in our model. As a result, we have obtained a mean mineralogical composition of the upper crust, consisting of the 292 most abundant minerals.

This composition does not have to be taken as final and closed, since many assump- tions had to be made. Nevertheless, it is the first step for obtaining a coherent mineralogical composition of the crust.

In chapters 2 and 3, we have tried to describe the composition of the earth as a whole. From the global components of the earth, only a few are used by man. The next chapter is focused on describing that part of the earth useful to man: the natural resources.

The resources of the earth

4.1

Introduction

In this chapter, a deeper look at the earth’s components useful to man is undertaken. For that purpose, a revision of energy and non-energy resources is carried out. The energy resources have been divided into energy coming from the solid earth, i.e. nuclear and geothermal energy; tidal energy; and energy coming from the sun, in- cluding solar, water, wind, ocean power and hydrocarbons.

In addition to the energy resources, mineral resources are also studied, stressing out their abundance and average crustal concentration.