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I found this beautiful euhedral pyroxene crystal in thin section today. There is one thing that I can´t explain though.

enter image description here

In crossed polarizers, the crystal shows two different interference colors, which do not change as I rotate the table (no color shifting, no extinction).

enter image description here

As far as I know, this happens only in basal sections. But then why are there two different interference colors ? Could it be pyroxene twinning ?

Thanks for all your answers.

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    $\begingroup$ Yes, this is a textbook example of pyroxene twinning. $\endgroup$
    – Gimelist
    Commented Dec 6, 2016 at 20:28
  • $\begingroup$ for what it's worth there is a slight difference in colour in plane polarized light $\endgroup$
    – haresfur
    Commented Dec 6, 2016 at 21:51

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