I try to calculate by using the FeO= Fe2O3T*0.8990…whether this correct????
If this correct, why the calculated values are less in comparison to other’s data???
I want to convert FeO for calculating Mg# of peritotites….
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.
The factor to convert Fe2O3 to 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.
Dear Madam
I could not understand the Equation you have written on the first line...
FeO=FeO 0.8991*Fe2O3
I believe its should be FeO=Fe2O3T*0.8991.....
As e.g., 1gram of FeO shows more Fe than 1 gram of Fe2 O 3 and also because of the differences in RMM, 1.00 mass % of the element Fe transforms into (RAM Fe + RAM O ) ÷ RAM Fe = (55.85 + 16.00) ÷ 55.85 = 1.29 % FeO or into (2 × 55.85 + 3 × 16.00) ÷ (2 × 55.85) = 1.43% Fe 2 O 3. The dividing Fe2O3 ratio by FeO ratio (1.4297 ÷ 1.2865 = 1.11) provides the conversion factor that must be taken into account in any calculation involving the notional transformation of the mass of one iron oxide into the other, or any summation of the two
To convert FeO to Fe2O3 = FeO*1.1113
To conver Fe2O3 to FeO = Fe2O3*0.8998
- Doing this, you have an assumption that all iron oxide is FeO when using Fe2O3T x 0.899. Obviously, this assumption is not right because we do not know how much iron occurs as FeO and how much as Fe2O3. The problem is attributed to that the whole-rock analytical data commonly provide either Fe2O3T or FeOT. This problem may be solved by using titration method to determine both FeO and Fe2O3 contents, or alternatively, by using the method presented in the paper I published recently in Lithos 2021, v. 380-381, 105920. DOI: 10.1016/j.lithos.2020.105920
- An Excel spreadsheet program related to this paper would help to calculate FeO and Fe2O3 contents from the whole-rock data Fe2O3T. Thus, Mg# can be calculated in terms of the equation Mg# = molar (MgO/(MgO+FeO). Note that Mg# is a molar ratio, rather than wt% ratio.
Xue-Ming Yang,
What about mafic rocks ? because your work focuses on granitic rocks ?
Thank you
Hi Bendimerad Chaouki:
Yes, you are right that the Lithos paper focuses on granitoid rocks, although it can potentially be applicable to mafic rocks because the method was based on the experimental work covering a range of composition from mafic to felsic (granitoid) rocks.
Regards,
Xue-Ming
Hi Ranjit,
If you multiply Fe2O3T by 0.8998 you will get FeOT. But to find FeO from Fe2O3T you need to know the proportion in which FeO and Fe2O3 occur in your sample. For unaltered volcanic rocks, a reasonable ratio of FeO : Fe2O3 is 85:15 (it might vary with changing rock types and degree of alteration). You need to check the ratio that is suitable for your rock. If for eg. we assume the Fe2O3/Fe2O3T = 15/100 = 0.15 (from above ratio), then we can multiply the Fe2O3T with 0.15 to get wt% of Fe2O3 in your sample. Then we simply subtract the calculated Fe2O3 from the Fe2O3T to get FeO. This FeO can be used for Mg# calculation.
It is also possible to calculate the conversion factor with the use of Cat-ion proportions. I have seen the factor of 0.899811 multiple times but I wanted to make sure this is correct.
In the added Excel sheet there is an explanation by the calculation to obtain the 0.899811 conversion factor.
Hope this helps other people as well to confirm the factor is 0.899811.
Dear Bendimerad Chaouki you can work on two sort of ratios (I try to use this general statement): mafic - 15/85 ultramafic and or alkaline mafic rocks (nephelinites, basanites etc) - 10/90
Dr. Xue-Ming Yang your explanation is excellent based on that the simples converter factor do not state the right value for the FeO out of the FeOT.
Cheers,
Sir, Xue-Ming Yang the excel sheet shared by you is very informative and helpful. However you have mentioned it can be used only for mafic or felsic rocks. Can it be used for a metapelitic or a metabasic rock as well?
Dear Parita Gorania:
The spreadsheet is designed to estimate Fe2O3/FeO ratio of a granitoid sample using bulk-rock geochemical data, i.e. Fe2O3T or FeOT. You may try it for metabasic rocks, but cannot use it for metapelitic rock.
Regards,
Xue-Ming
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