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41586
MineralsColumbite Group
6th Dec 2018 12:08 UTCRonnie Van Dommelen 🌟 Manager
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
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
6th Dec 2018 14:06 UTCReiner Mielke Expert
6th Dec 2018 15:24 UTCPavel Kartashov Manager
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
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
6th Dec 2018 17:04 UTCUwe Kolitsch Manager
6th Dec 2018 17:31 UTCPavel Kartashov Manager
6th Dec 2018 17:41 UTCUwe Kolitsch Manager
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