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263 Views 2 Replies Last post: Jul 10, 2014 1:24 AM by Kittykittyvii RSS
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Jul 9, 2014 1:51 AM

Please help us identify these rocks

My husband found these rocks while he was digging around the garden. The first one has tiny crystal like specks in it, the specks are a purplish-red. The second rock has black shiny specks in it.

 

Thanks :3

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    Jul 9, 2014 11:16 AM (in response to Kittykittyvii)
    Re: Please help us identify these rocks

    Kitty,

     

    The black and white one is, roughly speaking, a granite.

    The white minerals will be quartz and feldspars (there are multiple types); the dark mineral(s) will be a dark mica called biotite and/or an amphibole such as hornblende and/or other minerals.

     

    The other one is probably a garnet mica schist; I can't be sure of the schist part without seeing photos of other sides (where I'd be looking for a metamorhpic foliation called schistosity). The chemistry in terms of elements is probably not much different from the granite, but this is a metamorphic rock - one that has been affected by heat and pressure. That made some of the original minerals change to different ones - the new ones being stable in the altered physical conditions. One of those minerals is garnet. The reddish-purple lumps you see are crystals of garnet, probably one called almandine; you'll see a nice example in the the photo here - http://en.wikipedia.org/wiki/Almandine. The black shiny specs will be biotie.

     

    Mike

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