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793 Views 4 Replies Last post: Mar 15, 2013 5:19 PM by Ivan RSS
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Mar 12, 2013 12:22 PM

What's up with this stone?

Hello.

 

Just wondering what this cool stone might be.  It was found laying around in a field.

 

Thanks.

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  • Currently Being Moderated
    Mar 12, 2013 2:04 PM (in response to Ivan)
    Re: What's up with this stone?

    Hello Ivan,

     

    Still busy collecting, I see

     

    You have found a lump of feldspar porphyry.

     

    It is a type of igneous rock. It is composed of light-coloured crystals of feldspar in a dark matrix of much finer-grained igneous rock. In this setting, the large crystals are called phenocrysts. There are other types of feldspar, distinguished by the mineral comprising the phenocrysts. I can tell the phenocrysts in your specimen are feldspar from their shapes.

     

    Porphyry forms when a magma that has begun to crystallize (the large crystals) quickly gets cooled (for instance through being ejected from a volcano or dyke) - the chilling causing the remaining liquid rock to crystallize quickly, which results in small crystals.

     

    Porphyry can be described in terms of its phenocrysts and/or its matrix, eg.:

    - andesite porphyry (the matrix is andesite)

    - quartz porphyry (the phenocrysts are quartz)

    - olivine dolerite porpyhry (olivine phenocrysts in dolerite matrix).

    Such names are a matter of available information and choice.

     

    Ref.

    - introduction - http://www.google.co.uk/url?sa=t&rct=j&q=porphyry&source=web&cd=1&cad=rja&ved=0CDIQFjAA&url=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FPorphyry_(geology)&ei=3jE_UbT6LMjxOrjxgdAL&usg=AFQjCNH_sSbhT1TJgWIOKwdxJ3_04Uk5XQ&bvm=bv.43287494,d.ZWU

    - phenocryst gallery - http://geology.about.com/od/more_igrocks/ig/phenocrysts/

     

    Regards,

    Mike

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      • Currently Being Moderated
        Mar 12, 2013 4:27 PM (in response to Ivan)
        Re: What's up with this stone?

        Ivan,

         

        ...Well, they are not actually circular; we'd call them equant (as opposed to elongate).

        In 3-dimensions, the phenocrysts are oriented somewhat in one direction, and the side of the specimen that shows them as equant cross-sections happens to be across that direction (as you thought). Such non-random orientation can be referred to as a fabric. Fabrics can have various causes. In this case it might reflect flow in the melt just before it solidified.

         

        Great that you have a sub-contractor fossil hunting with you

         

        Cheers,

        Mike

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