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I have been observing a nepheline specimen under polarized light and found some interesting places. I have taken several images. The first shows a micro-geode with parallel polarizers, second with crossed polarizers. I wonder.. could these crystals inside be nepheline ? If not, what mineral could it be ? The thin section was labelled "nepheline". Also.. is the bubble real air bubble or anything else ?

Parallel polarizers: Image with parallel polarizers https://www.flickr.com/photos/violetplanet/30883853122/in/album-72157671897003403/

Crossed polarizers: Image with crossed polarizers https://www.flickr.com/photos/violetplanet/30883853562/in/album-72157671897003403/

I have also observed pyroxene crystals in Fe-solution (in parallel nicols could be mistaken with biotite), olivine and there should be nepheline. I have also found secondary quartz filling a cavity near olivine crystals. I believe the original rock was some kind of basalt.

Thanks for help.

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  • $\begingroup$ Hello, and welcome to Stack Exchange. By "nicols", did you mean "polarized filters"? $\endgroup$ – Daniel Griscom Nov 23 '16 at 17:42
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    $\begingroup$ Probably not nepheline, it has much lower birefringence. This rules out most zeolites as well. Most likely a hydrothermal mineral though. Epidote pyrophyllite pumpellyite are possible? Amphibole? Ca-mica? Could be anything really. Can you get the optical properties of it? $\endgroup$ – Gimelist Nov 24 '16 at 23:15
  • $\begingroup$ I have no idea. I am just learning the basic minerals while taking photos of anything, that seems like a good example to put on the internet so people interested in thin sections could learn from it. Reminds me of apatite or pyroxene though. I will give you two more links to similar structures that were in the same thin section. I fear that this is the only way I can help. flickr.com/photos/violetplanet/30368239033/in/… flickr.com/photos/violetplanet/30912236761/in/photostream $\endgroup$ – Petr Hykš Nov 25 '16 at 19:49
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    $\begingroup$ @PetrHykš don't worry - that's not a "basic mineral". To be honest, it could be anything. Even if you get the optical properties getting it with 100% confidence is hard. This is a classic case where you need more advanced methods to do it (such as SEM-EDS for example). $\endgroup$ – Gimelist Nov 26 '16 at 2:21
  • $\begingroup$ Good, thank you. :) Can you help me with one more thing ? I am wondering about the bubble in the micro-geode. Could it be real air bubble ? And how did it form ? I think that there was a bigger air bubble that was secondary fulfilled with solution and crystals. The pressure was rising and formed this almost spherical air bubble. Is it accurate ? $\endgroup$ – Petr Hykš Nov 26 '16 at 14:09

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