Silicon mantle rocks

The crust, hydrosphere and atmosphere formed mainly by release of materials from within the upper mantle of the early Earth. Today, ocean crust forms at midocean ridges, accompanied by the release of gases and small amounts of water. Similar processes probably accounted for crustal production on the early Earth, forming a shell of rock less than 0.0001% of the volume of the whole planet (Fig. 1.2). The composition of this shell, which makes up the continents and ocean crust, has evolved over time, essentially distilling elements from the mantle by partial melting at about 100 km depth. The average chemical composition of the present crust (Fig. 1.3) shows that oxygen is the most abundant element, combined in various ways with silicon, aluminium (Al) and other elements to form silicate minerals. 

Geochemistry treats the earth as a chemical system. One of the main tasks for this rather young science is to determine the chemical composition of the earth. Another is to formulate the rules that control (and have controlled) the movements of the elements and their distribution between different minerals and rocks in the crust The result is that geochemistry is to a high degree the chemistry of sihcates. (Sihcate chemistry is treated in Chapter 40 Silicon.) For the earth as a whole, size and composition are estimated and the same is true for its different parts the core, the mantle, the crust the hydrosphere and the atmosphere. 

Silicates and aluminosilicates are the most abundant minerals, which are main constituents of various rocks in the crust and mantle. Si" " combines with 0 to form silicate anion, SiOa" " in silicate crystal. The SiOa" has tetrahedral form (Fig. 1.11). The SiOa tetrahedron is the basic building block of silicates in which silicon is situated at the center of a tetrahedron of four oxygen atoms. SiOq" tetrahedra exist as discrete units or is joined via the O atoms, sharing their particles with other tetrahedra (Fig. 1.12). Silicate minerals mainly consisting of oxygen and... 

Complex mixtures of mafic rock melt so that the lighter elements aluminum, silicon, sodium, and potassium are liberated first. These are lighter on average and more diffuse in density, so they tend to rise through the molten mantle. When they collect at the surface, light gray granite forms. 

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