Pyrochlore Supergroup (Var: Betafite (of Hogarth 1977)) : (Ca,Na,U)2(Ti, Nb,Ta)2O6Z(OH)
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According to mindat the current formula for this mineral is UTi2O6 and often contains impurities; an older formula is (U4+,REE,Th,Ca)(Ti,Fe3+,Nb)2(O,OH)6. My attached Si-PIN XRF indeed shows Ti, Fe, and Nb but also Ta. This made me question the identity because what has been reported from this locality is betafite:
https://www.mindat.org/gallery.php?loc=45769&min=649
However, Gunnar is a very knowledgeable mineralogist and provided me the following SEM/EDX data, which is consistent with my XRF results but more quantitative:
Probe 7
4966
Betafo
Hellbraun wachsartig
Wt% At%
O 20,23 64,09
Ti 15,85 16,78
U 44,07 9,38
Nb 11,13 6,07
Ta 7,94 2,22
Al 0,77 1,45
Knut Edvard of mindat explained the id as follows:
We are - for now- sticking with the "betafite (of Hogarth 1977)" as designation for these- in lack of other ways of labeling these correctly. A chemical analysis (SEM-EDS) are not really sufficient alone to fully identified these complex compounds, even together with a XRF. It gives a good idea what it might be though. A XRD, after heating, would also be necessary to exactly identify it.
I have now done a more detailed XRF comparing this specimen to a betafite from Betafo (https://www.mindat.org/ML6-R47). The nice thing about having Gunnar's SEM/EDX data is that it allows me to roughly calibrate my XRF measurements. I have known that my SiPIN detector sensitivity decreases with energy and I have always suspected that the Ti peaks are actually much higher than they look. Gunnar's SEM/EDX At% for Ti compared to U is roughly 2X whereas my XRF Ti and U peaks are roughly 1/5. This says that I can use a rule of thumb of multiplying the Ti peak by 10 when comparing it to the U peak and even more when comparing it to Nb.
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