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6599 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?



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



  • 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.



    - introduction -,d.ZWU

    - phenocryst gallery -




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



        ...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




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