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Ronnie Van Dommelen December 06, 2018 12:08PM
Hi All,

I'm doing some reading on the columbite group minerals and have a question.

On MinDat there is a columbite-(Fe) - columbite-(Mn) series, but the formula given for the first of those is FeNb2O6 and the formula for the second is given as (Mn,Fe)(Nb,Ta)2O6. Why is the first not (Fe,Mn)(Nb,Ta)2O6?



Edited 1 time(s). Last edit at 12/06/2018 12:09PM by Ronnie Van Dommelen.
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Johan Kjellman December 06, 2018 01:11PM
I don't know the answer, i.e. what the person thought.
Be certain that these formulas are all simplified expressions of reality.
To me it makes more sense and enough information to list the end-members and note their structure.
You may have heard of the columbite quadri-lateral (google).
a very short and simplified version of it including what i said abovemay look like this:

FeTa2O6 MnTa2O6
(Rutile str.) (columbite str.)

FeNb2O6 MnNb2O6
(col. str.) (columbite str.)

"give me a break - tabs/space does not work" (after fourth ed)

cheers



Edited 5 time(s). Last edit at 12/06/2018 01:21PM by Johan Kjellman.
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Paul Brandes December 06, 2018 01:35PM
More than likely, it's just how it was entered in and not meant to be confusing.
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Reiner Mielke December 06, 2018 02:06PM
Columbite -Mn is Mn2+Nb2O6 on the IMA list, it should conform to that.
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Pavel Kartashov December 06, 2018 03:24PM
Hi Ronnie,
if you doing some reading on the columbite group minerals, you should to pay more attention to articles with real compositions of these minerals then to look at mindat page.
I don't know English analogues, but good selections of such analyses were given in book of Gorzhevskaya and Ginzburg, A.V. Voloshin monographies or at least in handbook "Mineraly" v.3, pt.2 (all in Russian).

Almost pure end member FeNb2O6 (low-manganoan and almost Ta-free) are quite abundant in many localities worldwide (at least pegmatites of Ural, Ukrainian shield, Southern Siberia, Madagascar, Nigeria, Australia etc.).
Real compositions of some manganotantalites you may to see here - https://www.mindat.org/gallery-2490.html?frm_id=pager&cform_is_valid=1&min=&loc=&u=2490&potd=&pco=&d=&showtype=&photoclass=&phototype=0&checkall=&rockmin=&filtmin=2520&filtcountry=0&loctxt=&keywords=&mycol=&orderxby=1&submit_pager=Filter+Search
For all my practice I met composition close to MnNb2O6 (Ta-free and almost without Fe) only once - in carbonatites of Hongcheon in Southern Korea. What is quite explainable.
Most of columbites-Mn are enriched with Ta, sometimes extremely high enriched. Moreover, most of "tantalites" at a market and in collections (even in museum collections) are in reality high-tantaloan columbites-Mn. It also is explainable, because of exactly this compositions are most abundant in so-call "Ta pegmatites" and often form large well formed crystals and huge masses of kgs weight.
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Pavel Kartashov December 06, 2018 03:29PM
Reiner,
nagelschmidtite also has formula Ca7(SiO4)2(PO4)2 on the IMA list. Do you know how much P presents in real nagelschmidtites from TL? Read its the first description, and you'll be VERY surprised. I don't understand at all, from where IMA took their formula.
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Alfredo Petrov December 06, 2018 04:17PM
For most minerals on Mindat we should have two formulae, the theoretical end-member and another showing the usual substitutions.
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Uwe Kolitsch December 06, 2018 05:04PM
All four columbite/tantalite-(Mn)/(Fe) formulae now changed to IMA formulae.
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Pavel Kartashov December 06, 2018 05:31PM
And all became better?
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Uwe Kolitsch December 06, 2018 05:41PM
?
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