Hi All
I would appreciate any suggestion about mineral with this EMPA chemical composition. EDS is attached
SiO2 37.23
Al2O3 25.29
Cr2O3 0.00
TiO2 0.03
FeO 27.62
MgO 5.68
MnO 0.91
CaO 0.03
Na2O 1.93
K2O 0.00
Is it magnetic in nature?
If it natural mineral, presence of FeO is having least possibility,
As per EDS, it may be Iron based alumino-silicate. You can confirm it by XRD analysis.
Do you have photograph of mineral?
Please share a geological location of mineral from where you obtained it.
Dear Ali Akbar,
it would be very useful to know the amount of water (if present) and to see the SEM image of this mineral. Are they available?
Cheers
Michele
Dear Sobhi,
do you mean "Amphibole Group"? In this case the Si/Al ratio is hard to explain (too much Al), as well as the anomalous cation relative ratios.
The mineral chemistry appears to be that of tourmaline
you can check with the available EPMA data of different tourmaline from net..
Thanks for all answers
Dear Yakshil
It is natural mineral but its photo-micrograph is not good, scince they are very small.
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Dear Sobhi
First assumption before analysis, was Ortho-amphibole, because of paragenetic relations. But as Mattioli noticed, high Al vs lower Si in analysis, almost do not match to any amphiboles.
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Dear Mattioli
H2O is not available, but since total is about 98.5, some water is possible. I attached a SEM image, showing paragenesis and un-known (?) mineral.
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Dear Asoori
Could be Tourmaline. I will check it, thanks
Dear Ali Akbar,
I think the suggestion of Asoori is right, the mineralogical composition could represent a tourmaline group mineral. This is also supported by the associated minerals that can be seen in the SEM image: spinel + cordierite + sillimanite is a typical assemblage of metapelites and gneiss, and it is often associated with tourmaline and other phases.
See Darling et al. 2004, GSA Memoir 197: 325-336 as example.
Best regards
Michele
I think it has too much Fe for a tourmaline, and normal tourmaline analyses close at ca. 87% wt%. Also, I don't believe that it is a tourmaline from the habit you show in the figure. Did you try new measurements? Is this the only attempt that you have made in this grain?
Dear Mattioli
Thanks. I am thinking about Asoori's suggestion. Thanks for the chapter you mentioned.
But as Marta mentioned, the problem is high Fe and high total, comparing to normal tourmaline. As you see in Darling et al. 2004, about 10 % of B2O detected in their tourmaline. I do not have SIEMS analysis, but at least I think, total could be less than 90.
Of course, I am thinking about B-free tourmaline. But as you know, most of the tourmalines in metapelites, are metasomatic, there are rare metamorphic reactions discussing tourmaline. Proposed reaction in Darling et al. 2004, almost is impossible in this sample,
In this case, based on paragenesis, tourmaline in this sample could form during the reaction:
Spl+Crd= Tour + St
I think this reaction is possible, but needs more work.
So if this is really a tourmaline, then I should find a reaction. Of course, SIEMS could help, but for me is not available.
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Dear Marta
Thanks. I mentioned your comment in above part. Yes, could be problematic.
There are more than 10 analysis from different grains in the samples with more or less same composition. The analysis in my question, is average of 12 analysis and is not simple one.
Dear Ali,
I cannot access to the reference you're talking about, so I cannot give my own opinion about it. I can say for sure that you need 10-11 wt% B2O3 (3 apfu of B) to have tourmaline. Furthermore, I don't believe that there is such a thing as B-free tourmaline! If the mineral is B- free is not a tourmaline, because tourmaline is a boron-silicate.
Best regards,
Marta
It is probably an aluminous gedrite. Paragenesis is correct and the analysis is a little aluminous, but not the highest Al ever found by any means. The presence of a little Na is also typical.
Dear Beard
Thanks. Do you know any literature with more or less same analysis on Gedrite?
Look at the analytical tables in Deer, Howie and Zussman for a start. If you google gedrite, you should get some other sources..
Please check the attached file, calculated at the ends of possible Fe2+/Fe3+ ratios. I would bet on all Fe divalent. Withal, I'm so nostalgic of the times they used to make thin sections!
Dear Ali Akbar,
XRD will identify this mineral more precisely.
Regards,
I think the same as Yakshil Chokshi a DRX analysis would specify precisely what mineral it is.
It is gedrite. XRD is not needed. If you do a stoichiometric calculatoin, the ampoibole formula is clearly indicated.
Thanks for all comments and answers.
First I should notice why thin section is not appropriate. the minerals are very thin and in association with other minerals like spinel and cordierite, form tiny symplectites. So their optical properties is not clear. and for the same reason, XRD doesn't work.
As I mentioned in previous notes, first guess was gedrite (Ortho-Amphibole). But Al was higher and, Si lower than normal. In Rock Forming Minerals (Chain Silicates) among many analysis, just one or two have more or less same composition.
Thanks For answers from Beard and Săbău.
I checked stoichiometry and almost all are Ferro-gedrite.
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