FMF - Friends of Minerals Forum, discussion and message board

[Daniel Garcia]

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

[Pete Modreski]

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

[Daniel Garcia]

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.

[Duncan Miller]

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.

[Daniel Garcia]

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.

[Duncan Miller]

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".)

[Daniel Garcia]