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Mineralogical Classification"Struverites" from Ialamitana.
18th Apr 2021 21:30 UTCPavel Kartashov Manager
Photos of huge crystals of Struverite from Ialamitana pegmatites are well represented in the Struverite photogallery.
But the first sight to the mineral list of these pegmatites
https://www.mindat.org/loc-73010.html bring witness, that these pegmatites aren't evolved enough for Struverite formation. They are simply insufficiently rare-metal.
https://www.mindat.org/loc-73010.html bring witness, that these pegmatites aren't evolved enough for Struverite formation. They are simply insufficiently rare-metal.
If we'll look at analyses of such material, we'll without surprising find, that Ta predominate (not always) over Nb in these "Struverites" only in mas.%. In at.% Nb always predominate over Ta in this material. So all them are Ilmenorutiles.
I am afraid, that many other photos of "Struverite" from the gallery
https://www.mindat.org/gallery.php?min=3812 in reality also are Ilmenorutiles.
https://www.mindat.org/gallery.php?min=3812 in reality also are Ilmenorutiles.
Edit: corrected locality name from Lalamitana to Ialamitana.
15th Sep 2021 01:20 UTCPavel Kartashov Manager
13th Dec 2022 17:46 UTCPavel Kartashov Manager
Copyright © * Unknown User *
This time I analyzed "struverite" from Tsaramanga pegmatite - https://www.mindat.org/loc-27803.html
The same story: matrix of ilmenorutile of composition (Ti0.66Fe3+0.16Nb0.13Ta0.04Fe2+0.03W0.01)1.00O2.00 (the upper spectrum) with very minor inclusions of ilmenorutile (Ti0.59Fe3+0.19Nb0.13Ta0.08Fe2+0.01)1.00O2.00 (the lower spectrum). 
13th Dec 2022 18:19 UTCFrank Mersch
Ti_0.66 O_1,32 plus Fe(iii)_0.16 O_0,24 plus (Nb,Ta)_0,17 O_0,42 and forget the rest ;-)
Know I will add the oxygen in that mixture
O = 1,32+0,24+0,42 is pretty close to 2 not to 4.00
Do you see my problem? Can you help me?
BR
Frank
13th Dec 2022 21:07 UTCPavel Kartashov Manager
I have a problem to understand the formula
This is not surprising, because according to Černý ilmenorutile is a solid solution of columbite FeNb2O6 or/and ixiolite-like phase (Nb,Ta,Fe,Ti,Ta)2O8 in rutile. I don't remember which of them is ilmenorutile-I and which -II. No magnetite or Nb2O5 in ilmenorutiles.
Schemes of isomorphic replacement are 2Ti4+ <-> Fe3+ + Nb5+ and 3Ti4+ <-> Fe2+ + 2Nb5+.
13th Dec 2022 21:25 UTCFrank K. Mazdab 🌟 Manager
"ixiolite-like phase" wouldn't be (Nb,Ta,Fe,Ti,Ta)2O8, but would be (Nb,Ta,Fe,Ti,Ta)4O8.
So your two reported formulas would be (Ti0.66Fe3+0.16Nb0.13Ta0.04Fe2+0.03W0.01)∑1.00O2.00 (the upper spectrum) with very minor inclusions of ilmenorutile (Ti0.59Fe3+0.19Nb0.13Ta0.08Fe2+0.01)∑1.00O2.00.
As originally written, they can't be charge balanced.
13th Dec 2022 23:09 UTCPavel Kartashov Manager
Yes, I made typo, 4.00- is charge, what means O2-2.
If a person undertook recalculation the charge of cations and found, that the charge of anions is exactly twice as large, then it wouldn't be difficult for him to understand, that this is a typo, and not an accidental error. Actually there was one mistake, I just copypasted the first formula to write the second one.
14th Dec 2022 08:11 UTCFrank Mersch
Frank
14th Dec 2022 08:42 UTCPavel Kartashov Manager
Of course all this is very important and interesting, formulae must to be electrochemicaly balanced. But in fact, if anyone did not pay attention, this thread is dedicated to the tragic shortage of tantalum in Madagascar "struverites".
May be somebody will show here real, confirmed struverite from Madagascar!?
17th Dec 2022 15:58 UTCKnut Edvard Larsen 🌟 Manager
confirmed struverite from Madagascar!?
The only analysis I have found is this:
Alfred Lacroix (1922) in his monograph Minéralogie de Madagascar, presents a chemical analysis made by Zambonini of a Strüverite from Tongafeno ( = Tsaramanga, https://www.mindat.org/loc-27803.html) on p 232-233 :
TiO2: 30.68; Ta2O5: ca 46.5; Nb2O5: 6.0; SnO2: 0,52; FeO: 15,48, MnO: 0,41, SiO2: 0,20. Total 99,80:
He calculated the approximate formula as Fe (Ta,Nb)2*(3TiO2, 0.74FeO) with Ta2O5: Nb2O5= 0.83: 0.17. The density of the sample was measured as 5.87.
He also states that when there is predominace of Ta over Nb the density will increase as well as the elongation of the vertical axe of the crystal ("La predominace du tantale sur le niobium entraine une augmentation de la densité et da la longeur d l’axe vertical"): I suppose the latter statement is the reason for the labelling some of the ilmenorutiles from Ialamitana as strüverites without any analysis.
TiO2: 30.68; Ta2O5: ca 46.5; Nb2O5: 6.0; SnO2: 0,52; FeO: 15,48, MnO: 0,41, SiO2: 0,20. Total 99,80:
He calculated the approximate formula as Fe (Ta,Nb)2*(3TiO2, 0.74FeO) with Ta2O5: Nb2O5= 0.83: 0.17. The density of the sample was measured as 5.87.
He also states that when there is predominace of Ta over Nb the density will increase as well as the elongation of the vertical axe of the crystal ("La predominace du tantale sur le niobium entraine une augmentation de la densité et da la longeur d l’axe vertical"): I suppose the latter statement is the reason for the labelling some of the ilmenorutiles from Ialamitana as strüverites without any analysis.
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Tsaramanga pegmatite, Mahaiza, Betafo District, Vakinankaratra, Madagascar