Author Topic: Mixed oxidation states in CalcZAF  (Read 5353 times)


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Re: Mixed oxidation states in CalcZAF
« Reply #30 on: February 11, 2018, 03:01:19 pm »
I understand the purpose of recalculating the oxidation state of multi-valent elements is to satisfy the "assumed chemical formula" of a crystalline phase. In the context of high temperature metallurgy, many phases do not really have fixed "formula", an example is magnetite. At high temperature(~1450oC) in air, magnetite can take significant more Fe3+ so that Fe3+/Fe2+ in it is greater than 2. 

I'd appreciate if someone could educate me to what extent mineral phases are close to their the theoretical chemical formulas in geological context.

Hi Jeff,
I am not a geologist so I probably can't help you, but I have a question.  Are you saying, for example, that in the case of magnetite Fe3O4, which is ideally composed of one FeO molecule and one Fe2O3 molecule, that at high temperatures, the ratio of FeO to Fe2O3 is no longer 1:1?  That there are more Fe2O3 molecules than FeO molecules in high temperature magnetite?

Hi John,

That's correct, I attached a FeO-Fe2O3 phase diagram at 1atm presssure. As you can see the magnetite can have more Fe2O3 than Fe3O4, it starts at around 1100oC and reaches maximum at around 1800oC.


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    • John Donovan
Re: Mixed oxidation states in CalcZAF
« Reply #31 on: February 11, 2018, 05:08:55 pm »
Hi Jeff,
I learn something new everyday!     :)

So in natural magnetites, what is the observed range of compositions?   I guess this is what you were asking about in your original post?   

Maybe I should ask, if stoichiometric Fe3O4 is 72.36 wt% iron and 27.64 wt.% oxygen, based on that phase diagram you posted, how much would we expect to see the Fe concentration change in natural materials (on Earth!)?

Again apologies for asking basic geology questions.
« Last Edit: February 11, 2018, 09:55:55 pm by Probeman »
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