High subcritical water-rock interaction for the formation of ferric minerals, in the absence of oxygen, UV light and microorganisms

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.