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MineralsColumbite Group

6th Dec 2018 12:08 UTCRonnie Van Dommelen 🌟 Manager

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?

6th Dec 2018 13:11 UTCJohan Kjellman Expert

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

6th Dec 2018 13:35 UTCPaul Brandes 🌟 Manager

More than likely, it's just how it was entered in and not meant to be confusing.

6th Dec 2018 14:06 UTCReiner Mielke Expert

Columbite -Mn is Mn2+Nb2O6 on the IMA list, it should conform to that.

6th Dec 2018 15:24 UTCPavel Kartashov Manager

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.

6th Dec 2018 15:29 UTCPavel Kartashov Manager

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.

6th Dec 2018 16:17 UTCAlfredo Petrov Manager

For most minerals on Mindat we should have two formulae, the theoretical end-member and another showing the usual substitutions.

6th Dec 2018 17:04 UTCUwe Kolitsch Manager

All four columbite/tantalite-(Mn)/(Fe) formulae now changed to IMA formulae.

6th Dec 2018 17:31 UTCPavel Kartashov Manager

And all became better?

6th Dec 2018 17:41 UTCUwe Kolitsch Manager

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