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720 Views 2 Replies Last post: Mar 9, 2014 9:14 PM by Tabfish RSS
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Mar 7, 2014 10:34 AM

Found on beach Is this just a rock?

I have a few of these over the last couple of months, any ideas what this is?

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    Mar 7, 2014 11:10 AM (in response to Jessica84)
    Re: Found on beach Is this just a rock?

    Hi Jessica,

     

    I can't tell what the pale fine-grained mineral is (comprising most of the rock), but I suspect it is mica, probably mainly the white variety - muscovite.

    The larger dark grains are almost certainly garnets (the variety called almandine).

    So, based on your photos, I would call the rock a garnet mica schist (I cannot see much schistosity, but that may just be the photo).

     

    One might argue that because of the fine grain-size of the mica, it is a phyllite instead of a schist. However, phyllite is a low-grade metamorphic rock, and the garnets indicate a higher grade. So, I'm sticking with schist For general reference, though, note that metamorphic rocks can go through multiple metamorphic cycles, prograde and retrograde, so the formation of metamorphic minerals can be complicated. It is a bit like cooking food, letting it cool, cooking it again, etc. several times.

     

    If you Google 'garnet schist' or 'garnet mica schist' you'll find lots of piccies. Here's one, which happens to show the crystal faces on the garnets nicely - http://www.johnbetts-fineminerals.com/jhbnyc/mineralmuseum/picshow.php?id=19112

     

    Mike

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