The origin of the silica of flints
The silicon so contained in the sea water results originally from the change of minerals silicatés establishing the continental crust and is brought by streams or by winds (dusts). This silicon can also result from oceanic volcanoes (silica lithogénique) or of phytolithes (continental biogenic silica).
Because of the climate in the Cretaceous, on continents develop red grounds latéritiques which hold the Aluminum (Gibbsite, Boehmite) and the Iron (Goethite) and free the silicon (If (OH) 4), contrary to what takes place today when prevail the grey grounds which hold the Aluminum and the Silicon and free the Iron. Thus the concentration of the sea water in Silicon be to be clearly superior to that of today.
In normal conditions of pH, the silicon is dissolved in the sea water in the form of acid orthosilicique So (OH)4 (Still written H4SiO4) Or train monomer. This molecule works as an acid tétraprotoné, 4 protons breaking up successively and producing a series of ions monosilicates: SiO (OH)3 1- SiO2(OH)2 2-SiO3(OH)3-SiO4 4-.
- If (OH)4 ? SiO (OH)3 1- + Hour+
- SiO (OH)3 1-? SiO2(OH)2 2- + Hour+
- SiO2(OH)2 2-? SiO3(OH)3- +H+
- SiO3(OH)3-? SiO4 4- + Hour+
In a pH of 8,1, a neutral shape So (OH)4 Is dominant (96.2 % of total Si).
In a pH included between 9,75 and 12,2, it is SiO (OH)3 1- Which prevails.
In concentrations in silica more raised than in the sea water, it is the molecule H7Yes2O7 Or shape dimère which prevails.
The primary silicon, under its various declensions, is consumed by certain animal or plant bodies for the manufacturing of a test which supports them or which protects them (oceanic biogenic silica). Most of them live in the plankton (diatomées with a frustule, radiolaires, ébriédiens, silicoflagellés with an endosquelette), but there are metazoans benthiques, spongiaires hexactinellidés and lithistidés, which have a skeleton trained of a felting of spicules siliceous. These animals or these vegetables (diatomées, silicoflagellés) very former knew various periods of outbreak during the geological times. The upper Cretaceous is one of these privileged periods, so the gaize albienne is largely established of spicules either still the chalk cénomanienne contains a big quantity of toweling protected in flint. Diatomées, which diversify in the Tertiary sector, establishes at present a pump with important silica. Occurring less in the Cretaceous, the concentration in acid silicique some sea water be to be ten times superior at least.
The biogenic haste of the silica is dominant. This silica, said has opal, is a weakly crystallized or amorphous silica containing until 10 % of water. It is the enzymatic activity of the body that allows the production and the stabilization of this opal. Because of the sub-saturation in silica of the sea water, when the individual dies, his test is put back in solution. The biohasty silica, although benefiting from a rather strong production, thus is only a temporary phase, because in the great majority of the cases, it restarts in solution, ready to be reused in another cycle. This dissolution of the tests takes place in the superficial centimeters of the sediment. The interstitial fluids grow rich in dissolved silica in the form of monomers So (OH)4 Which get back generally to the sea water. However, under certain conditions, this silica can re-precipitate and begin an evolution diagénétique which will protect her to the solid state.