Abstract
The formation of ferric minerals on the anoxic early Earth is usually explained with the action of microorganisms or UV light in acidic conditions. Results show that amorphous and crystalline ferric oxides and silicates can form in the absence of oxygen, microorganisms and UV light, when rocks, located in the upper crust of the Earth until ca 1 km depth, contain ferrous silicates which interact with water called high subcritical, at 300-350 °C and 10-25 MPa. This conclusion is established following the analyses of Eh-pH diagrams for the Fe-H2O system and solubility diagrams for quartz and amorphous silica. It is shown that water below the critical point and not above can lead to the formation of ferric iron in geological terrains on Earth and extraterrestrial objects where anoxic alkaline high subcritical water reacts with rocks containing ferrous silicates.
Highlights
Formation of the ferric iron which is observed inside the minerals hosted by rocks located in anoxic environments such as the Precambrian rocks up to the early Paleoproterozoic period is presently explained either with UV solar irradiation on the surface of acidic ocean water or with Fe(II)-oxidizing or oxygen-producing microorganisms
A new hypothesis is presented to form ferric iron in alkaline conditions, in the upper crust of the Earth or extraterrestrial objects, where water can be near its critical point and not above
The method for the analysis of minerals is a logical association and composition of my theoretical analyses of diagrams and experimental results published by other scientists, of my thermodynamic calculations on hydrolyses and carbonations of the olivine and pyroxene endmembers [3-6] and of my Raman analysis of minerals observed in a core drilled in the Barberton greenstone belt [7]
Summary
Formation of the ferric iron which is observed inside the minerals hosted by rocks located in anoxic environments such as the Precambrian rocks up to the early Paleoproterozoic period is presently explained either with UV solar irradiation on the surface of acidic ocean water or with Fe(II)-oxidizing or oxygen-producing microorganisms. The observation of the products led to the conclusion that magnetite was “surprisingly present in only trace amounts”, and Mössbauer spectrometry analysis showed the presence of both Fe2+ and Fe3+ in the product serpentine with 58% of the total Fe as ferrous iron and 42% as ferric iron. The following analyses and calculations led to the conclusion that ferric iron can be produced in anoxic geological environment where the conditions of temperature and pressure of water are those called high subcritical, hsubc
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