While performing petrographic analysis in a rock composed mainly of plagioclase, pyroxene and olivine, I came across the mineral shown in the photo. Do you have any clues of what it could be?

The images of the rock in thin sections were taken with a polarising microscope. The top photo is the image with the crossed polarizers and the bottom photo is the image with one polarizer.

The biggest mineral (shown in photos) is about 800μm but there are also smaller ones.



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    $\begingroup$ What is the scale? $\endgroup$ Commented Oct 12, 2022 at 14:01
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    $\begingroup$ I'm voting to keep this question open because it is not a typical "rock" identification question. It is a mineral identification request that is part of what appears to be a genuine academic study. The pictures are not pictures of a rock but appear to be pictures of a specifically prepared slide showing the slide under two different types of lighting conditions - presumably one is UV light. $\endgroup$
    – Fred
    Commented Oct 12, 2022 at 18:19
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    $\begingroup$ Hi everyone and thanks for showing interest. I have added more detailed information about the sample, including the scale. $\endgroup$ Commented Oct 13, 2022 at 9:03

1 Answer 1


It would help to be able to see photos with the stage at different points of rotation. However, this looks like a mafic volcanic rock with olivine being replaced by iddingsite in the top left; so there has been some post-crystallization alteration.

Two ideas: it may just be an altered phenocryst that has been replaced by a fine-grained clay mineral; or (and more likely in my opinion), it may be a piece of devitrified glass - the radial fibrous texture in XPL resembles a spherulite, which is a common devitrification texture. In felsic igneous rocks these are commonly composed of quartz and K-feldspar; I do not know what the equivalent constituents would be in a mafic rock.


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