I came across really weird mineral in marble. It's somewhat metamorphosed: some amphiboles, chlorite, epidote biotite, plagioclase and potassium feldspar has formed. Its composition is (wt%): SiO2 51.5; Al2O3 25.36; CaO 14.14; K2O 0.46; Na2O 5.62; S 0.81, some Cl. Total is 97.89 wt%. It looks a lot like amphibole, but it's composition doesn't mach at all. I feel it's beyond my knowledge, or something that I didn't even consider to be. Thank you, in advance.
Hi Laurynas,
It should be scapolite. Please check the mineral compositions in “Scapolite Stoichiometry：Contr. Mineral. and Petrol. 24, 293--305 (1969)". What are the features of this mineral under optical light microscope?
Rongqing
Dear Mr. Siliauskas,
corresponding to the chemical composition recorded, it is most likely that the unknown mineral is a member of the scapolite s.s.s. (mejonite: Ca-enriched, marialite: Na-enriched). I agree with Dr. Zhang that you should go and see what the mineral looks like under the petrographic microscope. Last but not least can you report on the mineral color, please. These additional comments are to exclude one of the many but very exotic Na-Ca-Al silicates found in skarns and impure marbles. The presence of S and Cl seem to be key in this respect. The same is true for the CO2 content. Maybe you can also report on the lithology which the marble is associated with. Are there any felsic intrusive rocks involved in the thermal or compositional history of the metacarbonates ?
Best regards
H.G.Dill
Thank you all for your help. Scapolite is a good match. High order interference color, some grains have prismatic habit. Unfortunately I am nowhere near microscope, so it's all I can tell from the picture. The annoying thing is, once I googled it and a nice picture popped out, I remembered seeing it in the Uni. Thank you for the article, Rongqing. Dear Mr. Dill, thank you very much for the advises, I will surely try to identify witch variety of scapolite is in my sample.
Dear Mr. Siliauskas,
I think to minimize the room of speculation it would be a reasonable step to try and find somebody who avail himself of an electron microprobe. Through this method you can also definitely rule out that an exotic mineral slips your observations.
Good luck !
H.G.Dill
Hi Laurynasو
َAlthough there isn't microscopic characteristics of this mineral, but I think it should be scapolite.
Kind regards,
Mohammad
Yes, it is scapolite. I just finished calculating the formula unit calculations and it fits really well. Mejonite content changes a bit across the profile (60-50% rim to core). Plus, the rock unit that I currently study (my masters) was exposed to contact metamorphism, so scapolite should have been expected. My incompetence led to this question. Sorry for your time, and thank you, once again.
The skarn mineral is difficult to identify because it is rarely found on earth. How about lepidolite mineral?
There's a great book about metamorphic systems and mineral chemistry by Ralph Kretz that I picked up in a library recently, "Metamorphic Crystallization", published in 1994. It covers all the main phases and major element partitioning between them. There's lots of thermodynamics and it's a bit difficult to read, but once you get a hang on it, it's really great.
Hi
Determining the optical properties could help a lot!
Have you checked your standards?
What is the composition of Pl?
According to your analysis the best fit is Mizzonite. The deficiency in total is due to CO2. Also the cleavage is typical for scapolite.
And by the way, scapolite is common in metamorphosed marls and marbles.
Regards
Dear Laurynas,
The SEM image is not convenient for identification of the mineral, so I agree with colleagues that you should provide us with some optical data and figures. Scapolite group mineral is really the most probable choice. Beside all that was already said, not to repeat it, scapolites are readily altered to secondary minerals (e.g. micas, kaolinite, zeolites, some albite,…). Try to see under high magnification, whether there are some secondary products included in or occurring along cleavage, which can slightly change chemical composition.
It looks like you are presenting electron microprobe data. Is it from just one shot? When %weight is less than 100% it is not considered a trustable result. Try shotting at many different points of the mineral. This type of weird result is commom when you pointed the shot right at the border of two or more different minerals or over some slight inclusion, etc. Cheers!
Marco Antônio Caçador Martins-Ferreira, It doesn't always has to be 100 wight%, if not all elements are measured, for example H2O, CO2, Be, Li. I've checked the sample with optic microscope, checked with my supervisor and she confirmed it to be scapolite. It all fits perfectly. I simply didn't expect to find it in my sample, though I should have. In this link: http://rruff.info/doclib/hom/meionite.pdf there is an example of meionite end member for comparison of what I have posted. Since my sample was coated with carbon, no CO2 was estimated and total is 97 wth%.
Pekka Tuisku, there is no problem with standards. Plagioclase is An30, whereas scapolite is Meionite 60-50, from rim to core.
Thanks to everyone for your response.
Just a nosy question: do you have a microscope? If so, please use it, many people are satisfied with what it can do. Straight extinction and high birefringence would easily confirm scapolite.
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