Mixed oxidation states in CalcZAF

[jeffchen]

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?
john

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.

[Probeman]

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.
john

[John Donovan]

It has taken some time but with the help of a number of friends (Andrew Locock, Anette von der Handt, John Fournelle, and Emma Bullock), we have finally realized Brian Joy's most excellent suggestion to implement a ferrous/ferric calculation into the matrix correction physics.

The implementation of this feature in Probe for EPMA will be described in this post and others soon to follow:

https://probesoftware.com/smf/index.php?topic=92.msg8591#msg8591

But I thought I'd start here with the implementation in CalcZAF. Though both PFE and CalcZAF are using exactly the same code for the ferrous/ferric calculation and subsequent treatment in the ZAFSmp routine as shown here (and hosted on Github):

Code: [Select]

' Calculate element relative to stoichiometric oxygen based on previous iteration calculation of oxygen
If zaf.il%(zaf.in0%) = 0 Then    ' if calculating oxygen by stoichiometry
For i% = 1 To zaf.in1%
If zaf.il%(i%) = 15 Then
r1!(i%) = (r1!(zaf.in0%) / zaf.atwts!(zaf.in0%)) * sample(1).StoichiometryRatio! * zaf.atwts!(i%)
zaf.ksum! = zaf.ksum! + r1!(i%)
zaf.krat!(i%) = r1!(i%)
End If
Next i%

' Calculate amount of stoichiometric oxygen and add to total
r1!(zaf.in0%) = 0#
For i% = 1 To zaf.in1%
r1!(zaf.in0%) = r1!(zaf.in0%) + r1!(i%) * zaf.p1!(i%)
Next i%

' Calculate equivalent oxygen from halogens and subtract from calculated oxygen if flagged
If UseOxygenFromHalogensCorrectionFlag Then
r1!(zaf.in0%) = r1!(zaf.in0%) - ConvertHalogensToOxygen(zaf.in1%, sample(1).Elsyms$(), sample(1).DisableQuantFlag%(), r1!())
If ierror Then Exit Sub
End If

' Calculate excess oxygen from ferric Fe
If sample(1).FerrousFerricCalculationFlag Then
Call ConvertFerrousFerricRatioFromComposition(zaf.in0%, zaf.Z%(), zaf.atwts!(), r1!(), sample(1).numcat%(), sample(1).numoxd%(), zaf.p1!(), sample(1).DisableQuantFlag%(), sample(1).FerrousFerricTotalCations!, sample(1).FerrousFerricTotalOxygens!, tFerricToTotalIronRatio!, tFerricOxygen!, tFe_as_FeO!, tFe_as_Fe2O3!)
If ierror Then Exit Sub
r1(zaf.in0%) = r1(zaf.in0%) + tFerricOxygen! / 100#
End If

' Add to sum
zaf.ksum! = zaf.ksum! + r1!(zaf.in0%)
End If

So here's an example in CalcZAF of a magnetite unknown using magnetite as a standard:



And calculating Fe as FeO we see the expected low totals due to some of the iron being Fe2O3:

Magnetite

STANDARD PARAMETERS (TOA= 40):

 ELEMENT  STDNUM STDCONC STDKFAC   Z-BAR  ABSCOR  FLUCOR  ZEDCOR  ZAFCOR                 
   Fe Ka      39  72.359   .6810 21.0246   .9969  1.0000  1.0660  1.0626

 ELEMENT STP-POW BKS-COR   F(x)e   F(x)s      Eo      Ec   Eo/Ec
   Fe Ka  1.0935   .9748   .9846   .9877   15.00  7.1120  2.1091


SAMPLE: 2, TOA: 40, ITERATIONS: 1, Z-BAR: 21.99148

 ELEMENT  ABSCOR  FLUCOR  ZEDCOR  ZAFCOR STP-POW BKS-COR   F(x)u      Ec   Eo/Ec    MACs uZAF/sZAF
   Fe ka   .9974  1.0000  1.0530  1.0503  1.0753   .9793   .9871  7.1120  2.1091 53.4587  .9883946

 ELEMENT   K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL                                       
   Fe ka 1.00023  .68111  71.536  92.031  50.000   1.000   15.00                                       
   O                        .000    .000    .000    .000
   O                      20.495   -----  50.000   1.000
   TOTAL:                 92.031  92.031 100.000   2.000

So by calculating Fe as FeO we get 71.536 wt% Fe, but are missing about 7 wt% excess oxygen from ferric iron. Now clicking on the Calculation Options button in CalcZAF we see a new checkbox and two text fields for the mineral stoichiometry.



Since this is a magnetite, that would be 3 total cations and 4 totals oxygens. Other common minerals as follows (I'm no geologist so please let me know if I got these wrong):

Mineral   Cations  Oxygens
hematite     2        3
olivine      3        4
ilmenite     2        3
garnet       8       12
feldspar     5        8
pyroxene     4        6

With this flag checked and the 3 cations and 4 oxygens specified we now get this result:

Magnetite

STANDARD PARAMETERS (TOA= 40):

 ELEMENT  STDNUM STDCONC STDKFAC   Z-BAR  ABSCOR  FLUCOR  ZEDCOR  ZAFCOR                 
   Fe Ka      39  72.359   .6810 21.0246   .9969  1.0000  1.0660  1.0626

 ELEMENT STP-POW BKS-COR   F(x)e   F(x)s      Eo      Ec   Eo/Ec
   Fe Ka  1.0935   .9748   .9846   .9877   15.00  7.1120  2.1091

SAMPLE: 2, TOA: 40, ITERATIONS: 2, Z-BAR: 21.02464

 ELEMENT  ABSCOR  FLUCOR  ZEDCOR  ZAFCOR STP-POW BKS-COR   F(x)u      Ec   Eo/Ec    MACs uZAF/sZAF
   Fe ka   .9969  1.0000  1.0660  1.0626  1.0935   .9748   .9877  7.1120  2.1091 55.6448  1.000000

 ELEMENT   K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL                                       
   Fe ka 1.00023  .68111  72.376  93.111  42.857   1.000   15.00                                       
   O                        .000    .000    .000    .000
   O                      27.647   -----  57.143   1.333
   TOTAL:                100.023 100.023 100.000   2.333

Note that not only did our total get much better, but our Fe wt% went from 71.536 to 72.376, just from adding the ferric oxygen into the matrix correction!  This is simply because Fe Ka is absorbed more by oxygen, than by iron and therefore needs to be included in the matrix correction phsyics as suggested originally by Brian Joy. 

But of course the cool part is the new output as seen here:

FerricIronToTotalIronRatio:    .6667
Ferrous Oxide (FeO):         30.6769
Ferric Oxide (Fe2O3):        68.1855
Excess Oxygen from Fe2O3:     6.8316

Right now it outputs this twice (once for each iteration loop), so I'll need to fix that, but the new code calculates the ferric to total iron ratio, the ferrous and ferric iron, and the excess oxygen from the ferric iron.

Please download and try it out and let me know what you think.  I have no idea how much of an effect this will have on ones calculated temperatures and pressures, so let us know if you find anything interesting.

[Ben Buse]

Jarosewich et al 1980 report this analysis with a comment (#3: Total Fe reported as FeO). However, I agree with others that it is common for spinel-group mineral to have substitution of Al2O3 or Cr2O3 by Fe2O3. You can have a glimpse (estimate) of the FeO vs. Fe2O3 by using an oxygen AND cation normalization (i.e., you fix as a constraint both the total number of anion, 4 for chromite, and the total number of cation, 3). With this, you can perform a charge balance (how many positive charges, assuming only FeO, do you have to compensate for the 8 negative charges? If not enough positive charges, then you "convert" some FeO to Fe2O3 to balance the charge).

I've played with the chromite analysis given in Jarosewich. Here are the results (1st line = cation AND oxygen normalization, 2nd line = oxygen normalization only assuming all Fe as Fe2+):

         Al2O3   Cr2O3   Fe2O3   FeO   MgO   MnO   Total   
Cation   9.92   60.50   3.45   9.93   15.20   0.11   99.12   
Oxygen   9.92   60.50   0.00   13.04   15.20   0.11   98.77   
                        
         Al      Cr      Fe3+   Fe2+      Mg      Mn      Cation      O
Cation   0.376   1.540   0.084   0.267   0.730   0.003   3.000   4.000
Oxygen   0.380   1.556   0.000   0.355   0.737   0.003   3.032   4.000


Interestingly enough, with the estimate of Fe2+ / Fe3+, the total number of trivalent cation is now EXACTLY 2.000 (versus 1.937 if you assume all Fe2+ as Fe3+). This is EXACTLY the mineral formula of chromite:

(Fe2+ 0.267, Mg 0.730, Mn 0.003) (Cr 1.540, Al 0.376, Fe3+ 0.084) O4

Sum trivalent = 2.000
Sum divalent = 1.000

Now of course this is the CALCULATED (estimate!!!) of Fe2O3 - not measured. This addition of Fe3+ represent an excess of 0.346 wt-% oxygen, close to the 0.299 you report. Maybe the 0.299 actually reflect a non-published MEASUREMENT of Fe2O3 and FeO by Mössbauer or so...

QED.

Hi,

Anyone know why the total is so low for this chromite.

The interesting thing about chromites (particularly when considering oxidation state) is that the Mg and Al concentrations are very sensitivity to matrix correction and MACs used, and can change values by 1-2wt.% absolute.

Is there any chromite standard that has been independently measured?

Thanks

Ben

[Probeman]

Jarosewich et al 1980 report this analysis with a comment (#3: Total Fe reported as FeO). However, I agree with others that it is common for spinel-group mineral to have substitution of Al2O3 or Cr2O3 by Fe2O3. You can have a glimpse (estimate) of the FeO vs. Fe2O3 by using an oxygen AND cation normalization (i.e., you fix as a constraint both the total number of anion, 4 for chromite, and the total number of cation, 3). With this, you can perform a charge balance (how many positive charges, assuming only FeO, do you have to compensate for the 8 negative charges? If not enough positive charges, then you "convert" some FeO to Fe2O3 to balance the charge).

I've played with the chromite analysis given in Jarosewich. Here are the results (1st line = cation AND oxygen normalization, 2nd line = oxygen normalization only assuming all Fe as Fe2+):

         Al2O3   Cr2O3   Fe2O3   FeO   MgO   MnO   Total   
Cation   9.92   60.50   3.45   9.93   15.20   0.11   99.12   
Oxygen   9.92   60.50   0.00   13.04   15.20   0.11   98.77   
                        
         Al      Cr      Fe3+   Fe2+      Mg      Mn      Cation      O
Cation   0.376   1.540   0.084   0.267   0.730   0.003   3.000   4.000
Oxygen   0.380   1.556   0.000   0.355   0.737   0.003   3.032   4.000


Interestingly enough, with the estimate of Fe2+ / Fe3+, the total number of trivalent cation is now EXACTLY 2.000 (versus 1.937 if you assume all Fe2+ as Fe3+). This is EXACTLY the mineral formula of chromite:

(Fe2+ 0.267, Mg 0.730, Mn 0.003) (Cr 1.540, Al 0.376, Fe3+ 0.084) O4

Sum trivalent = 2.000
Sum divalent = 1.000

Now of course this is the CALCULATED (estimate!!!) of Fe2O3 - not measured. This addition of Fe3+ represent an excess of 0.346 wt-% oxygen, close to the 0.299 you report. Maybe the 0.299 actually reflect a non-published MEASUREMENT of Fe2O3 and FeO by Mössbauer or so...

QED.

Hi,

Anyone know why the total is so low for this chromite.

The interesting thing about chromites (particularly when considering oxidation state) is that the Mg and Al concentrations are very sensitivity to matrix correction and MACs used, and can change values by 1-2wt.% absolute.

Is there any chromite standard that has been independently measured?

Thanks

Ben

Hi Ben,
I'm not sure where some of these numbers came from, but my USNM chromite shows this composition along with a 99.64% total, including 0.299 wt% excess oxygen.

St  455 Chromite USNM 117075
TakeOff = 40.0  KiloVolt = 15.0  Density =  4.750  Type = oxide

Analysis (wet chemistry) by Gene Jarosewich
Oxide and Elemental Composition

Average Total Oxygen:       33.208     Average Total Weight%:   99.640
Average Calculated Oxygen:  32.909     Average Atomic Number:   17.187
Average Excess Oxygen:        .299     Average Atomic Weight:   27.414

ELEM:    Cr2O3     FeO   Al2O3     MgO     CaO     MnO    TiO2     NiO    SiO2       O
XRAY:      ka      ka      ka      ka      ka      ka      ka      ka      ka      ka
OXWT:   60.502  13.040   9.920  15.200    .120    .230    .120    .160    .049    .299
ELWT:   41.395  10.136   5.250   9.166    .086    .178    .072    .126    .023  33.208
KFAC:    .3859   .0890   .0317   .0493   .0009   .0016   .0008   .0011   .0002   .2384
ZCOR:   1.0727  1.1388  1.6541  1.8607   .9619  1.1152   .9396  1.1224  1.3950  1.3931
AT% :   21.903   4.993   5.353  10.376    .059    .089    .041    .059    .023  57.103
24 O:    9.206   2.099   2.250   4.361    .025    .037    .017    .025    .009  24.000

Of course everyone knows my feelings about natural standard materials- they just aren't pure enough, nor homogeneous enough, nor inclusion free enough for my "tastes".   

https://probesoftware.com/smf/index.php?topic=301.msg2405#msg2405

I wonder if there's someone out there that could synthesize a pure end member chromite (FeCr2O4) for us?  Wouldn't that be wonderful?

[AndrewLocock]

Hi,

Anyone know why the total is so low for this chromite.

The interesting thing about chromites (particularly when considering oxidation state) is that the Mg and Al concentrations are very sensitivity to matrix correction and MACs used, and can change values by 1-2wt.% absolute.

Is there any chromite standard that has been independently measured?

Thanks

Ben

Hello,
Forsythe & Fisk (Proceedings of the Ocean Drilling Program, Scientific Results Vol 135, pp. 585-594, 1994) in their Table 2 reported the following additional oxides (n=62, with standard deviations in parentheses):
SiO2 0.07 (0.01), TiO2 0.12 (0.02), ZnO 0.06 (0.03), NiO 0.17 (0.02), V2O5 0.10 (0.02), Na2O 0.01 (0.01), whose sum is 0.53 wt%.

My own in-house analyses of Tiebaghi chromite USNM 117075 (n=75) give the following additional oxides:
- below the limit of detection are SiO2, CaO, Nb2O5, SO3, CoO, Na2O, and K2O;
- found were TiO2 0.11 (0.01), ZnO 0.03 (0.02), NiO 0.17 (0.01), V2O3 0.08 (0.01), sum 0.38 wt%.
These results are in generally good agreement with Forsythe & Fisk (1994).
[Note that 0.08 V2O3 is essentially equivalent to 0.10 V2O5.]

In particular, I find no CaO above detection, in contrast with the 1980 report of Jarosewich et al.
However, this is in agreement with the most recent on-line posting of the Smithsonian Microprobe Standards datasheets, where CaO is omitted from the chromite entry.

The implication is an additional ~0.4 wt% of elements in addition to those listed on the current SMS datasheet.
And the consequent atomic proportion of ferric iron would be around 24% (around 3.5 wt% Fe2O3).

It would be interesting to know if others have observed these minor elements.

Best regards,
Andrew

[Brian Joy]

My own in-house analyses of Tiebaghi chromite USNM 117075 (n=75) give the following additional oxides:
- below the limit of detection are SiO2, CaO, Nb2O5, SO3, CoO, Na2O, and K2O;
- found were TiO2 0.11 (0.01), ZnO 0.03 (0.02), NiO 0.17 (0.01), V2O3 0.08 (0.01), sum 0.38 wt%.
These results are in generally good agreement with Forsythe & Fisk (1994).
[Note that 0.08 V2O3 is essentially equivalent to 0.10 V2O5.]

In particular, I find no CaO above detection, in contrast with the 1980 report of Jarosewich et al.
However, this is in agreement with the most recent on-line posting of the Smithsonian Microprobe Standards datasheets, where CaO is omitted from the chromite entry.

The implication is an additional ~0.4 wt% of elements in addition to those listed on the current SMS datasheet.
And the consequent atomic proportion of ferric iron would be around 24% (around 3.5 wt% Fe2O3).

It would be interesting to know if others have observed these minor elements.

Best regards,
Andrew

I often use Tiebaghi chromite as a secondary standard.  I find that it is inhomogeneous in Fe and Mg.  Using synthetic eskolaite (Cr), synthetic spinel (Mg, Al), synthetic magnetite (Fe), synthetic hausmannite (Mn), natural rutile (Ti), natural gahnite (Zn), synthetic liebenbergite (Ni), and natural wollastonite (Si, Ca) as standards, I get the following averages (and standard deviations) from a subset of 44 analyses collected on nine occasions:

SiO₂:  bdl
TiO₂:  0.12 (0.02) wt%
Al₂O₃:  9.75 (0.05)
Cr₂O₃:  61.10 (0.42)
V₂O₃:  0.09 (0.02)
Fe₂O₃:  3.25 (0.44)
FeO:  10.14 (0.31)
MnO:  0.20 (0.01)
MgO:  15.11 (0.21)
ZnO:  0.05 (0.03)
NiO:  0.18 (0.01)
CaO:  bdl
Total:  99.98

I’ve used the PAP atomic number and absorption corrections with Heinrich’s (1987) mass absorption coefficients and Reed’s (1990) characteristic fluorescence correction.  I’ve recalculated for Fe₂O₃ by charge balance within the iterations used to calculate the matrix correction factors.