feldspar phenocryst under POL

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feldspar phenocryst under POL

#1 Post by geo_man » Sat Aug 17, 2019 7:02 pm

My friends brought me a few petrographic thin sections to experiment with. This one has the big feldspar crystals in a finer grained volcanic matrix of early tertiary age. Enjoying the effects...
IMG_0001.jpg (164.58 KiB) Viewed 625 times
IMG_0002.jpg (216.79 KiB) Viewed 625 times
IMG_0003.jpg (223.36 KiB) Viewed 625 times

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Re: feldspar phenocryst under POL

#2 Post by 75RR » Sun Aug 18, 2019 2:06 pm

Hi geo_man,

I take it that as you have maintained the view point in all 3 images, the changes between them are due to pol filter rotation?

What is the difference between each image telling us?

Many Pol images are very attractive, but I always felt there should be more to them than just being pretty.
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Re: feldspar phenocryst under POL

#3 Post by geo_man » Sun Aug 18, 2019 8:35 pm

Hi 75RR, thanks.

Yes, i rotated one of the polarizers as I don't have a real petrographic microscope with a rotating stage. I haven't done petrography in ~45 years so I am quite rusty. Other than being attractive, colors on the feldspar crystal, when the polarizers are crossed, indicate that it is anisotropic, where light travels at different velocity depending on orientation through the mineral. Also, the different colors and bands on the mineral indicate slight variations in composition while the crystal was forming/cooling (zoning).

There is lots more to decipher, i'm sure, but that will have to wait till I scan my optical mineralogy text and maybe someday get a nice petrographic scope.

Comments from others are welcome!

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