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Techniques for CollectorsWhat do you do when XRD & EDS indicate different minerals?
26th Jun 2009 03:46 UTCFred E. Davis
Now consider an EDS analysis that returned the following Wt % oxides:
Y2O3 9.51%, Nb2O5 16.47%, ThO2 2.38%, UO2 3.91%, CaO 5.21%, TiO2 18.02%, Nd2O3 3.66%, Ce2O3 1.79%, La2O3 1.20%, Fe2O3 2.59%, Ta2O5 35.25%.
From this, I derived structural formulas, with the best fit based on six oxygen atoms:
(Y+REE0.447 Ca0.366 Fe0.142 U0.057 Ce0.043 Th0.036){1.091} (Ti0.889 Ta0.629 Nb0.488){2.007} O6
I will become rapidly clear that if it's betafite, there is a problem. For one, betafite has two atoms at the A site, while my best fit provides only one. The next problem is that my specimen is Ti>Ta>Nb, compared to Ti>Nb>Ta for betafite. In fact, I could find only one Ti>Ta>Nb mineral poking around Strunz & Dana (it's not even close *and* it was discredited in 1977).
The A site is dominated by Y, so let's consider for a moment betafite-(Y). Just in terms of elemental composition, my sample is fairly close to yttrobetafite-(Y) *except* for Ta>Nb, two A site atoms vs. one, and the XRD pattern for yttrobetafite-(Y) is a very poor fit.
Suggestions? Ideas?
26th Jun 2009 05:35 UTCDonald Vaughn
26th Jun 2009 08:37 UTCjacques jedwab
26th Jun 2009 08:49 UTCPavel Kartashov Manager
What about some difference in XRD patterns of your mineral and yttrobetafite from ASTM card, it may greatly depend on annealing conditions in both cases. During a heating quite different daughter phases able to form together with recrystallization of the main metamict phase. Their nature and quontity can be quite different in both cases. So you obtain different difractograms.
What about Ti>Ta>Nb succession in the B site. Relations of the second and the third elemens don't effect on position in systematic even in Hogath's nomenclature. In coming new nomenclature this relations will means nothing. Only the first/main element able to produce mineral specie name.
By the way most of hatchettolites from carbonatites are strong enriched with Ta and may to have Nb~Ta~Ti. But subgroup name (pyrochlore-microlite or betafite) depends from ratios 2Ti and Nb+Ta (according to Hogarth) or Ti and Nb+Ta (according to new nomenclature). So maximum what you able to obtain from this composition even in the network of Hogarth nomenclature is only adjective "tantaloan".
What about total population of A site. Compare both formulas of my betafite http://www.mindat.org/photo-230522.html with one from textbooks - they also are quite different. :)
Kind regards,
Pavel
26th Jun 2009 13:16 UTCFred E. Davis
Yes, annealing can cause problems, especially with excessive temperatures (introduces new phases). Annealing was first performed by my friend in Basel (plot attached, SlocumXRDSwc). The three scans show before annealing (blue), and at two stages of heating (600C red, then 800C black). The reference lines are betafite. Also attached (Betaf-13-0197) is one of my own patterns, with ICDD 13-0197 as the reference. I heated it at 750C for about 2 hours.
26th Jun 2009 15:33 UTCUwe Kolitsch Manager
Was any chemical inhomogeneity or zonation observed?
26th Jun 2009 16:19 UTCFred E. Davis
26th Jul 2009 15:48 UTCChris Stefano Expert
26th Jul 2009 17:56 UTCRob Woodside 🌟 Manager
26th Jul 2009 19:15 UTCChris Stefano Expert
26th Jul 2009 19:48 UTCFred E. Davis
26th Jul 2009 22:39 UTCRob Woodside 🌟 Manager
27th Jul 2009 00:27 UTCPavel Kartashov Manager
Only presence in XRD of lines belonging to TiO2 polymorphs may be fatal for this identification. In such case it able turn out yttropyrochlore.
Also i have a question. Form this mineral any crystals typical for cubic niobates - octahedrons, cubooctahedrons or it was met as massive grains? Very often cubic niobates replace (during albitization process) more former rhombic precursors - euxenite-aeschynite, samarskite, fergusonite. In such case resulting pyrochlore mineral is enriched with Y+HREE and with exeption of fergusonite as precursor by Ti. By the way, very often during such pyrochlorisation U and Th form own uraninite-thorianite phase.
When you make EDS from unpolished grain, it is too simple to analyze later pyrochlore and rombic precursor together. In polished preparate these phases has quite different reflectance and are well visible in reflected light. Also they should be well distinguishable in reflected electrons on microprobe.
27th Jul 2009 12:47 UTCFred E. Davis
27th Jul 2009 16:49 UTCPavel Kartashov Manager
By the way yellow powder on sides of the specimen and on muscovite is kasolite or uranophane.
My adwice is to make polished section from large (5 or more mm) fragment of this aggregate in direction perpendicular to "linear structure" and look on it under reflected light or in BSE on SEM.
27th Jul 2009 19:52 UTCFred E. Davis
There is no kasolite or uranophane present, only golden muscovite. Compressing the image for MinDat may have blurred such details.
Two additional photos show close-ups of quasi-linear surface details attached. #4247 width ~ 12 mm. #4253 width ~25 mm.
29th Jul 2009 20:29 UTCFred E. Davis
I've been studying an interesting article: T. Scott Ercit, "Identification and alteration trends of granitic-pegmatite-hosted (Y,REE,U,Th)-(Nb,Ta,Ti) oxide minerals: a statistical approach," CanMin Vol. 43, pp. 1291-1303. This seemed to be quite appropriate to my situation. Accepting the fact that I don't have great EDS data to work from, I decided to plug in my values into the author's three-group model just to see what fell out. The plot is attached. This suggests my unknown falls in the euxenite-aeschynite groups. To further refine that, the author gives another comparison, and that result suggests euxenite. Curiouser and curiouser. . .
As an easy reality check, I put in data for a known samarskite-(Y) which appears right where it should. I recently added pyrochlore, fergusonite-(Y), and two tanteuxenite-(Y) points from reference data sheets.
29th Jul 2009 23:29 UTCJohan Kjellman Expert
I also agree with a lot of Pavels comments, these minerals become "pyrochlorized", mainly due to Ca-exchange. BUT remember Ca can also be a primary component within the AB2O6 framework as a "fersmite" component (Ca(Nb,Ta)2O6).
The pyrochlore politics aren't even settled yet - so why would you like to pursue this further?
cheers
30th Jul 2009 00:08 UTCPavel Kartashov Manager
Now try to recalculate it by cation method on basis Ti+Ta+Nb+Fe3+=2.
Aparently you have only slightly "pyrochlorised", completely metamict euxenite. After heating, pyrochlore phase was recrystallized, but euxenite one hadn't. So you obtain pyrochlore pattern from mixture of pyrochlore+glass+REE2O3+TiO2. Pyrochlore phase in this mixture faster of all has yttropyrochlore-(Y) composition (with low Ti content).
You need in further microprobe investigation of polished section of the mineral to find out compositional heterogeneity of it.
Look at http://www.mindat.org/photo-138676.html - such "stibiomicrolite" also will give PXRD patterns of usual microlite with some additional/exessive lines... ;)
30th Jul 2009 01:13 UTCFred E. Davis
Inquiring minds want to know! I've learned a great deal tilting at this windmill, and hope to learn more.
Pavel - Yes, I hope to get some SEM time in the near future. Polished cross section & element mapping should be interesting. A description of tanteuxenite-(Y) crystals mention striations along {100} and {110}; interesting.
I truly appreciate all of your thoughts and suggestions.
30th Jul 2009 13:13 UTCJohan Kjellman Expert
cheers
2nd Aug 2009 17:02 UTCFred E. Davis
I read in another thread (Bogus formula accounting), I found a description of the method used by Excalibur (Tony Nikischer):
"In our EDS/SEM lab, when we run unpreped, single grain samples, we do so at least three times, with each grain either harvested from three areas of the original sample, or analyzed at three different areas of a single larger grain. This helps us evaluate homogeneity while rapidly pointing out any potential inconsistencies in the material and the data we have acquired."
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