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Help with mineral identification in thin section from Khibiny, Kola peninsula
  
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Daniel Garcia




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PostPosted: May 14, 2022 10:40    Post subject: Help with mineral identification in thin section from Khibiny, Kola peninsula  

Hi

I'm trying to identify some rare minerals in thin sections from urtites collected in Khibiny in 2005. Maybe somebody is familiar with these minerals.

In these pictures, the clear groundmass is nepheline, and the green laths are aegirine.
The poikilitic mineral in the center of the second picture is eudialyte, and the orange laths are (presumably) astrophyllite.

My question is about the (almost isotropic) black mineral with almost the same shape as astrophyllite and sometimes contains nepheline inclusions. Could this be aenigmatite ?

Thanks in advance



KY03C_2LN.jpg
 Mineral: Eudialyte
 Dimensions: 3 mm
 Description:
thin sections in non polarised light
 Viewed:  850 Time(s)

KY03C_2LN.jpg


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Pete Modreski
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PostPosted: May 14, 2022 12:47    Post subject: Re: Help with mineral identification in thin section from Khibiny, Kola peninsula  

Hi Daniel, seems that only one picture made it attached?

I've been there and like these kinds of rocks, but probably don't know enough to offer a sure guess or confirmation tho!

Pete
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Daniel Garcia




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PostPosted: May 14, 2022 15:59    Post subject: Re: Help with mineral identification in thin section from Khibiny, Kola peninsula  

Hi Pete

I'm not familiar with the forum desk, hence my first picture was discarded. Second try.

Like previously, the green laths are aegirine, the clear groundmass is nepheline and the orange laths should be astrophyllite.
No eudialyte in this picture, but the (unknown) black laths again, intergrown with aegirine.



KY03C_1LN.jpg
 Mineral: Aegirine
 Locality:
Khibiny Massif, Kola Peninsula, Murmanskaja Oblast, Northern Region, Russia
 Dimensions: 3 mm
 Description:
thin section
 Viewed:  793 Time(s)

KY03C_1LN.jpg


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Duncan Miller




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PostPosted: May 16, 2022 01:49    Post subject: Re: Help with mineral identification in thin section from Khibiny, Kola peninsula  

Daniel - According to MacKenzie & Guilford (1980) 'Atlas of rock-forming minerals in thin section', aenigmatite may appear opaque but the deep brown colour is pleochroic,. So perhaps you can increase the light to try to see the pleochroism, by rotating the polarizer in PPL.
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Daniel Garcia




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PostPosted: May 16, 2022 03:20    Post subject: Re: Help with mineral identification in thin section from Khibiny, Kola peninsula  

Thanks Duncan, good idea

In my textbooks too, aenigmatite is said to have a strong pleochroism, but often obscured by the color. It is classified as "nearly opaque" by W.E. Troger.
Indeed, with a higher magnification, a higher light power, and where the mineral is thinner than the normal 35 microns in contact with inclusions, I do see the brownish coulour and the pleochroism.
Is that conclusive ?

Otherwise, I don't feel very comfortable with the shape. The crystals look like slightly curved discs, with very acute terminations like here, In the textbook, they say "columnar" instead.



20220516-100409-357.jpg
 Mineral: Aenigmatite?
 Description:
 Viewed:  686 Time(s)

20220516-100409-357.jpg


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Duncan Miller




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PostPosted: May 16, 2022 05:41    Post subject: Re: Help with mineral identification in thin section from Khibiny, Kola peninsula  

I don't know if the colour and pleochroism are diagnostic, but your images certainly "look" like those in MacKenzie & Guilford, including one with an acute termination. What else might the dark laths be? Can you eliminate any likely alternatives? (Remember, you are looking at 2D sections of 3D crystals, so they could be "columnar".)
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Daniel Garcia




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PostPosted: May 16, 2022 08:20    Post subject: Re: Help with mineral identification in thin section from Khibiny, Kola peninsula  

Duncan Miller wrote:
I don't know if the colour and pleochroism are diagnostic, but your images certainly "look" like those in MacKenzie & Guilford, including one with an acute termination. What else might the dark laths be? Can you eliminate any likely alternatives? (Remember, you are looking at 2D sections of 3D crystals, so they could be "columnar".)


Thanks, but I don't have access to this treatise.
I agree that a section is not a 3D view, anyway.
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