How to calculate FeO from Fe2O3 (Whole rock analysis?). Whether, this FeO can be used in a classification diagram (Jensen Plot) for an altered metavolcanics?
The Fe2O3/FeO ratio is a challenging variable of chemical analysis where all Fe is measured as Fe2O3. Depending on the initial nature of your rocks of interest it will vary. A common ratio used for basaltic rocks is Fe2O3/FeO=0.15 after Brooks (1976), 'The Fe203/FeO ratio of basalt analyses:an appeal for a standardized procedure'. However in Jensen plot is FeOt so just convert as above and normalize to 100%. For a meta-volcanic rock, many major elements may have been mobile so be a bit cautious of any classification scheme that uses certain major elements for these. Alternatively look for a trace element scheme that utilizes more immobile elements such as Ti/Zr/Y/Nb, perhaps check Morrison (1978) for a discussion on the mobility of trace elements in metabasalts http://www.sciencedirect.com/science/article/pii/0012821X78900298
The calculation is simple, multiply the Fe2O3 value by by 0.9 (i.e. 71.85/79.85). Evaluating the actual ratio is a very different matter. For one thing, it depends on whether you want the iron valence for the altered rock or the protolith. Many geologists just use a standard ratio as described in the answer above, but this is simply guesswork.
Hi Kamal
Your rock samples contain both Fe2O3 and FeO. The ration between the Fe2O3 and FeO is depended on how much O2 in your system.
If you have XRF analysis, the iron total value is the total iron that you have in your rock but as Fe2O3. To convert the Fe2O3 to FeO, you can use the following equation:
FeO = Fe2O3 * 0.8998
This equation will convert all the iron content in your analysis to FeO
The following table is used when weight based analytical results need to be converted. For example 56.0% CaO might be obtained during a microprobe calcite analysis. To convert this oxide to carbonate look at the table below. Find the conversion for CaCO3 to CaO and use the multiplying factor beside CaCO3. The result is 56 x 1.7848 = 99.94% CaCO3.
A factor is the sum of the atomic weights of the elements in the required formula divided by the sum of the atomic weights in the original formula. For example the factor to convert Fe2O3to 2FeO is 2 x (55.85 + 16)/(55.85 x 2 + 16 x 3) = 0.8998 while the factor to convert 2FeO to Fe2O3 is 1/0.8998 = 1.1113.
http://www.jepspectro.com/electron_microprobe/oxide_conversion.htm
You may refer the paper of Kress and Carmichael (1991) regarding the ferric/ferrous ratio calculation - that is a function of T, X, and fO2 based on experimental work and empirical estimate. It's a tuff task though.
If possible, use wet chemical analysis - titration - to analyze bath FeO and Fe2O3. Unfortunately, modern analytical methods, e.g. XRF, ICP-OES, only provide total iron oxide, either as FeOT or Fe2O3T.
Middlemost (1989) paper recommends a set of standard ratios for basalt to rhyolite with different alkalinity. That paper may give you a general guide how to adapt a proper ferric/ferrous ratios for volcanic rocks of different compositions.
lf wet chem analysis is not possible, consult the paper by Middlemost (1989) as suggested by Dr. Xue-Ming Yang. But that is also an approximation only. You can contact me for details.
Its just a ratio of Fe in Fe2 O3 by atomic weights. So calculate the molecular/ atomic weights depending on whether to convert to FeO or Fe. Take the ideal ratio of Fe/ Fe2O3 . Multiply the ratio with the result obtained as Fe2O3. You would get result as Fe.
There is no general way to recalculate Fe2+ and Fe3+. It is too difficult in many silicates. There are undetermined other elements, vacancies, uncertain water/OH content, more elements with different oxidation state etc. Sometimes you can use "brute force" - like calculating garnet on 12 anions and 8 cations with valence calculation - but of course results of this may be quite far from reality.
The wet analysis is impossible for mixed, altered or too small minerals. You may avoid undetermined elements by using LA-ICP-MS, sometimes just this helps to get the formula. Sometimes the structure can help, eg. if you can say there is no Fe in position typical for Fe3+ - Raman and IR spectroscopy or xray diffraction help with this. The most efficient method is to use Mossbauer spectroscopy which can directly analyse and distinguish Fe2+ and Fe3+.
To get FeO to Fe2O3 just multiply FeO by 1.11111 Fe2O3 to FeO is just reverse, divide Fe2O3 by 1.11111 (or multiply by 0.89998).
My recent paper 'Ferric-ferrous iron oxide ratios: Effect on crystallization pressure of granites estimated by Qtz-geobarometry' published in Lithos, 2021, v. 380-381, 105920 (DOI: 10.1016/j.lithos.2020.105920) deals with this problem of calculation Fe3+/Fe2+ ratios from whole rock data that only provide total FeO or Fe2O3 wt%.
The related Excel spreadsheet is attached for referring.
You can use these equations for whole rock analysis.
Fe = FeOx0.7773
Fe = Fe2O3x0.6994
FeO = Fe2O3x0.8998
Fe2O3 = FeOx1.111
Fe2O3 = Fex1.43
Best Regards,
It is unlikely that the Fe2 / Fe3 ratio of a prepared powder will be representative of the Fe2/Fe3 ratio of the rock or the original magma. Heating during powder preparation is likely to lead to some oxidation, as is heating during determination of loss on ignition and if you are determining your composition by XRF on fused discs, there will be even more oxidation - likely complete. If there was alteration, this is also likely to lead to a change in the redox state of the iron. So this is not a problem that can be approached by calculation - if you really want to know the Fe2/Fe3 ratio of the rock you will need to look at some specialised analytical techniques.
However, if you simply want to convert Fe2O3 to FeO in order to plot a particular diagram, you can multiply Fe2O3 by 0.89981.
Middlemost (1989) paper recommends a set of standard ratios for basalt to rhyolite with different alkalinity. That paper may give you a general guide how to adapt a proper ferric/ferrous ratios for volcanic rocks of different compositions.
FeO=Fe2O3 *0.8998
Whereas, FeO total can calculate from this equation
FeO+0.8998*Fe2O3