Hi Andrew & co,
I *might* agree that this can be confusing. Let's take back the example of good and bad analysis you give (I converted the Ca content in CaO):
Wt-% (O norm.), CaO, CO2, Total
Analysis 1, 56.0238, 43.9673, 99.9911
Analysis 2, 53.7431, 42.1774, 95.9205
Norm., Ca, C, Total cation, O
Analysis 1, 1.00000, 1.00000, 2.00000, 3.00000
Analysis 2, 1.00000, 1.00000, 2.00000, 3.00000
Effectively in both case you still have 1 C for 3 O. With this "simple" case of one cation, you will ALWAYS get 1 cation of Ca, 1 of C and one of O. This is because of the normalization process, which specify (in normal situation) a fixed amount of oxygen per cation to balance the charge. Now, if we consider carbonate, we can assume it must have one oxygen atom for each atom of calcium, and add to this one molecule of CO2, but the more logical way to see this problem is NOT to consider just C, or just CO2, but effectively
CO3. Of course, at the end, the recalculation effectively reports results as elemental C wt-% or CO2 wt-%. Key is that, through the mineral formula recalculation, the
“game” is always to balance the positive charges (= what is measured) with anions that are NOT measured. In most case, we simply compensate all positive charges with O2-, and the amount of O is defined by the fixed oxidation state for each cation. In the case of carbonate, the negative charge is effectively (CO3)2-, which makes even more sense when we look at the crystalline structure of carbonate; they are effectively made of CO3 triangles and not isolate CO2 molecule
. Hence a ratio of 1/3 carbon for 1 oxygen (or 1 C for 3 O). Of course, one could state that we have one CO2 and one atom of O, but to me it makes more sense to simply consider CO3 “as a single anionic molecule”.
The problem of hydrogen is the same! Hydrogen is a cation (that we cannot measure by EMP), but you balance the charges with anion of (OH)-, and you do not consider simply H+, or H2O; again the crystalline structure show H as being OH anion group, not H2O molecule or isolate H cation… For hydrous mineral, you can consider the total positive charges and the total oxygen WITH oxygen from hydroxide group, but you need to “correct” the total amount of oxygen used to balanced the measured cation (i.e., all cations WITHOUT hydrogen). For instance:
Epidote = (Ca2)(Al2Fe3+)(Si2O7)(SiO4)O(OH)
=> 13 oxygens TOTAL = 26 negative charges
……BUT…… There is ONE hydrogen atom (1 positive charge) that is calculated by stoichiometry and charge balance (or by difference - wrong idea here!).
=> actually there are 12.5 oxygen (a number often refer as being ideal for epidote normalization - without taking into account of Fe2+/Fe3+ issue), as the “half-oxygen” is used to balance the charge of the calculated 1 hydrogen atom. However, to my opinion this is wrong to state this, and we should rather consider 13 oxygens INCLUDING one O associated to one H (and correct for this through the normalization process).
BTW, take a look at my website, I have implemented a form to calculate mineral formula, and it does do the trick (well, kind of a black box for user without access to my code) to calculate any H2O or CO2 content based on user input. To make it more “geologically meaningful”, I speak about H2O or CO2 groups, but the calculation does include what I describe above...
http://cub.geoloweb.ch/index.php?page=mineral_formulaJust my $0.02, but I believe, John, you are doing things 200% correctly, although I can understand Karsten's comment about the "user-friendliness" of the input. Maybe I can work with you to implement a solution similar to what I have on my website.
Julien
Edit by John: I'd be pleased to implement any mineral recalculation code in PFE that anyone makes available... it's good to have geologist friends!
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