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1460 Views 8 Replies Last post: Sep 7, 2014 2:19 PM by pdan RSS
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Aug 17, 2014 3:42 PM

shiny dark green basalt-like rock

I found this rock on the beach in Culebra, Puerto Rico, i wonder what it is. It looks like basalt but it has a dark green sheen and a resinous lustre. The rock is about 5cm long. Thank you!DSC01163.JPG

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    Aug 17, 2014 9:21 PM (in response to pdan)
    Re: shiny dark green basalt-like rock

    As hand specimens, fine-grained rocks like this are problematic, especially from photos.

     

    We need to look for various structures to give us clues: scattered larger well-defined crystals could indicate an igneous or metamorphic nature; depositional structures would suggest sedimentary rock. In this case, unfortunately I can't see anything useful.

     

    So I am guessing, but yes, it could be a basalt (some are greenish, though usually darker than yours). It could be green because of a high percentage of, for instance, olivine. But it could also be a metabasalt - metamorphism having changed some of the primary minerals to green metamorphic ones such as chlorite or epidote.

     

    However, location can constrain the range of possibilities.

    And given that we are talking about Culebra, that makes it very likely to be andesite or andesitic tuff.

    (http://www.coralations.org/MPA/Bombs/Culebra_ASR_Part_1.pdf)

     

    To be sure, it would be necessary to know what minerals and textures it contains, and that would really need analysis using a petrological microscope for starters.

     

    Mike

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        Aug 18, 2014 7:43 AM (in response to pdan)
        Re: shiny dark green basalt-like rock

        A couple of afterthoughts:

         

        - In general, one should also consider the possibility of a specimen like this being industrial slag glass (much of which is opaque). It often has swirls in it, which your specimen does not. And In Culebra, I would think sources for such material are absent (though slag can be brought-in just as a form of aggregate for building purposes, loose paths, etc.).

         

        - I'd go see if you can find some nearby outcrops; see if they are made of similar material. If so, you might be able to find other features, which might allow you to confirm or alter the ID.

         

        Mike

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    Sep 4, 2014 2:50 PM (in response to pdan)
    Re: shiny dark green basalt-like rock

    I found a similar rock not shiny but here's photos I took... Also need help identifying.

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    Sep 7, 2014 6:24 AM (in response to pdan)
    Re: shiny dark green basalt-like rock
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      Sep 7, 2014 6:57 AM (in response to LAdanni)
      Re: shiny dark green basalt-like rock

      Well, it does look similar to Dan's specimen.

      Did it come from the same place?

      If not, it doesn't help us with Dan's ID.

       

      In any case, I think it is definitely not slag; I think it is volcanic, and quite possibly andesite. The lack of sedimentary structures and fossils is what leans me away from sedimentary rock, but I can't actually rule that out. As I have written before, it is very difficult to ID fine-grained rocks from hand specimens.

       

      Mike

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