4.5 billion years ago

Danilo Antón

It all started about 4.5 billion years ago in the solar system in formation. A large number of solid bodies and gaseous masses were derived in the gravitational fields of the sun and subordinate matter. With the passing of millions of years the aggregation and compaction of these gaseous bodies and masses (asteroids, comets and meteorites, commonly denominated "planetesimals") took place, giving rise to several protoplanets, one of which is the one we call today "Earth".

In that period, the various impacting and impacted bodies underwent mineralogical changes of their components due to their previous history and the physical conditions in which the aggregation and impacts occurred. These changes tended to give rise to the formation of compact crystalline structures forming dense or "impacted" minerals. This influenced the initial composition, predominantly silicate, and the impact strength. In the first millions of years these crystalline structures remained relatively stable due to the increasing pressure of the planetary mass in formation.

The minerals that originally formed (dense silicates)
The main minerals that integrated this proto-planetary cortege were dense silicate. Several crystalline species are known that probably were part of the original composition of the planet. From the above it can be deduced that silicate minerals predominate throughout the mantle and in the crust.
Among the high-density silicate minerals identified are the following:
Coesite: SiO2, mineral with monoclinic symmetry, density: 2.92 (the density of the quartz of the same composition is 2.65).
Stishovite: SiO2, mineral with tetragonal symmetry, density: 4.35 (much higher than the quartz density of the same composition: 2.65).
K-Na Holandite: (K, Na) AlSi3O8, minerals with monoclinic symmetry, prismatic, density about 4.5 (much higher than the density of feldspar Na-albite (2.61) and feldspar K-Ortosa (2.56).
Perovskite silicate: is a stable phase of perovskite. It is a high density mineral composed of silicates of magnesium and iron (Mg, Fe) SiO3 and calcium silicates (CaSiO3). Approximate Density: 4.0.
Post-perovskite (it is a high pressure phase of the magnesium silicate: MgSiO3; pPv). Approximate Density: 4.0.
Another source of information for the knowledge of the probable mineralogical composition of the planet in its origins is provided by meteorites, in particular the most common, called L-Condritas which include various silicate minerals among which the principal, in order of abundance, Are as follows:
Olivine: (Mg, Fe) 2SiO4 Density: 3.27-3.37
Hypersten (Mg, Fe) SiO3 Density: 3.3-3.9
Troilite FeS Density 4.67-4.79
Chromite FeCr2O4 Density: 4.5-4.8
Sodium feldspar: NaSi3AlO8 Density; 2.61
Calcium phosphate: Ca (H2PO4) 2 Density: 2.2 approx.
This chondritic composition gives an approximate idea of ​​the original terrestrial mantle reaffirming the notion of the predominance of silicate minerals with abundant iron and magnesium contents. To a lesser extent are sulphides and oxides, sodium aluminosilicates and calcium phosphates.
Expansion and formation of the crust
The initial mineralogical composition constituted mainly by dense minerals was relatively unstable. This instability was expressed more intensely in the outer layers of the planet subjected to lower pressures and temperatures than deep levels.
It was there, in these layers close to the surface that a variation in the crystalline structures of the silicate minerals from the dense phases (coesite, stishovite, K-holandite, Na-holandite, perovstarkite, psot- Perovskite, etc.) to less dense phases (quartz, potassium feldspar, sodium, calcium, olivine, pyroxenes). These changes implied an increase of volume due to the decrease of the density, causing an expansion in the rocky masses superficial1.
Due to the expansion of the upper mantle, an expanded surface area of ​​lower density was generated. It corresponds to the structural element that we call "crust". In this sense, it is possible to define the earth's crust as the portion of the mantle where the density decreased due to the phase changes of the minerals.
These processes of mineral transformation with decrease of density increased the volume of the planet giving rise to the swelling of certain zones. These areas were probably the regions where ancient geological shields were established