Okay, I am swallowing my pride to ask for help. Help!
I have reached the point in my career as an earth scientist where I must contend with the biotite problems, and I am confused on the exact procedure that is best.
Based off of reading previous biotite/amphibole forum posts, I understand that conceptually what we need to do is first calculate the amount of unanalyzed OH because biotite stochiometry dictates that the number of atoms of F+Cl+OH = 2. But I get a little lost in the sauce when it comes to then calculating the amount of ferric iron - we know the relevant substitution mechanisms (e.g. Fe+3 in the octahedral site charge balanced by tetrahedral Al+3 replacing a divalent octahedral cation and a tetrahedral Si+4 ; and Ti-related substitution mechanisms operating - which I acknowledge can get messy quickly)
Am I correct in my understanding that the biotite subroutine doesn't calculate the ferric iron based off of other substitution mechanisms? Is there a simple way to get around this?
I guess my question is: what is the most rigorous way to treat H and Fe+3 in biotite?
Quote from: macosta on April 07, 2025, 03:21:32 PMAm I correct in my understanding that the biotite subroutine doesn't calculate the ferric iron based off of other substitution mechanisms? Is there a simple way to get around this?
I guess my question is: what is the most rigorous way to treat H and Fe+3 in biotite?
By "the biotite subroutine" are you asking about the Brimhall and Ague biotite output option from the Calculation Options dialog? The code described here:
https://smf.probesoftware.com/index.php?topic=1143.msg7789#msg7789
Besides this output option, we currently only handle excess oxygen from ferric iron for minerals and amphiboles (Locock, et al.):
https://smf.probesoftware.com/index.php?topic=40.msg10853#msg10853
One can also export a tab delimited file using the right click menu in the Analyze! window, for import into Andrew Locock's spreadsheet. But I don't know if he has one for biotites...
But you can specify hydrogen by stoichiometry to another element in the same dialog if you know the formula. You can also specify an Fe to O ratio using the cation/oxygen parameters in the Elements/Cations dialog if calculating as an oxide. But let's let an actual mineralogist chime in...
Hi John,
Yes, that's what I meant by "biotite subroutine" - I don't see that it's differentiating between Fe+3 and Fe+2 in the output though, so I'm guessing it doesn't?
I think first we need to constrain the amount of OH given the F and Cl concentrations (which is basically the same as specifying H by stoichiometry), and then the amount of Fe+3/Fe+2 using what we know about plausible substitution mechanisms and site assignments. It wouldn't be a constant Fe:O ratio, nor would it be specifying a formula (because the formula changes depending on how much Fe+3 you have b/c of charge balance constraints). I can imagine doing this by hand or in excel but what hoping someone had already done it. I also know that there are cases where this logic would break down (e.g. biotite deprotonation)...
The amphibole spreadsheet might be more what I'm looking for. Will investigate further.
Quote from: macosta on April 07, 2025, 08:09:20 PMThe amphibole spreadsheet might be more what I'm looking for. Will investigate further.
Andrew Locock is out of town for a couple of weeks, but wrote this to me on why calculating excess oxygen from ferric Fe (using charge balance) is difficult with biotites:
QuoteThe amphibole spreadsheet is not appropriate for mica.
In general, it is not possible to calculate exact charge balance for Mica because of
1 - possible dehydogenation
2- possible vacancies on the octahedral sites
3 - possible vacancies on the K site
As we don't have a fixed number of cations, and we don't have a fixed anionic charge, it is not possible to have a fixed charge balance ratio.
Yavuz has some publications about micas.
Hi John,
Yeah, I thought that the deprotonation would be a problem and was not too worried about K-site vacancies as I was thinking I could focus only on the other cation site.
I'm going to have an undergrad make a bunch of bivariate element plots from the data to identify which exchange mechanisms are operating, then I might try to do the ferric:ferrous iron site assignments by hand.
Does the "calculate biotite" routine add the water in before the matrix corrections, since I've learned from you that water makes a big difference in matrix corrections?
Quote from: macosta on April 08, 2025, 12:14:47 PMDoes the "calculate biotite" routine add the water in before the matrix corrections, since I've learned from you that water makes a big difference in matrix corrections?
You can look at the biotite code in the link above, but no, it's just an off-line code (translated from FORTRAN) from Brimhall and Ague to calculate some geology parameters, all outside of the matrix correction.
So, yes, you might want to find a way to include water in the analysis, either by stoichiometry, difference or specified as a fixed concentration. See the options in the Calculation Options dialog.