Just to build up on my previous success with oxygen origin in the atmosphere, I want to expand the question as was advised in the answers. The official heresy, despite denied in the comments and answers here, says that the atmosphere oxygen was produced by plants. Meantime, the oxidation of the atmosphere was delayed by oxidation of the iron, which was free at that time. Yet, if we look at the geology, we find very few Fe oxides in the crust. There is much more aluminium oxide Al2O3 (aka clay) and quartz (SiO2 aka sand). I wonder, when did the clay with sand were formed so that no phytoplankton was necessary for their formation and why was iron delayed its oxidation?

PS I have later realized that Ca oxidation is also an issue of this kind.


Because Al and Si were already oxidised to begin with.

When the Earth formed, it had some amount of metals (Fe, Si, Mg, Al, Ca, etc) and a fixed amount of oxygen to bond with those metals. Certain metals with bond with oxygen preferentially. Mg and Ca are usually the first to take up oxygen, followed by Al, Si, and then Fe comes last (out of the short list I just provided). Turns out that there is not enough oxygen left for all of the Fe!

As you probably know, Fe oxides come in two states, ferrous iron (Fe2+) and ferric iron (Fe3+). So to put it simply, the amount of oxygen left in the Earth after being used up in the other more-easily-oxidised metals was enough to oxidise just a small part of the Fe0 to Fe2+. That's why Earth has a metal core - that's where all of the Fe0 is. The crust, however, had only Fe2+ in it. Once you start pumping out oxygen to the atmosphere, this Fe2+ starts oxidising to Fe3+, which is the stable form of iron in the Earth's surface in the presence of oxygen (as you can tell by iron rusting everywhere quite rapidly). This buffers the amount of oxygen you had in the atmosphere until you run out of Fe2+ and then oxygen can really increase.

Some corrections: Al2O3 is not clay. It is an important component of clay minerals, but it is definitely not clay. Clay minerals have both Al2O3 and SiO2 in it.

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    $\begingroup$ Thanks, this dispels my hypothesis guessing that answer is related to the fact that the surface Fe that we mine and deplete has the meteorite origin. $\endgroup$ – Little Alien Jul 27 '16 at 17:06

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