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Rock Identification?

  • #1
    Registered Users Posts: 430 ✭✭ emersyn

    Image 1, Image 2, Image 3
    (The coin in the photos is to show the size)
    I found this rock near the sea in Achill Island, Mayo about a year ago and only became curious recently as to what type of rock it is. My knowledge of geology is limited to Junior Cert geography but I did some research and the best I could come up with is rock quartz, but I'm not sure. Can't figure out what the rock at the base of the crystals is either.
    Most of the crystalline columns have six sides. They are just barely translucent and are hard enough to scratch glass.


  • Hi,

    Yes you are correct, the mineral you have is quartz. The habit the crystals form is known as hexagonal.

    Achill is famous for amethyst, the purple variety of quartz, you can still find fragments and crystal points with some patient searching.



  • Is this stone similar in makeup?

  • JJayoo wrote: »
    Is this stone similar in makeup?


    What you have there is dolomite, which is a mineral not a stone. Minerals have a defined chemical composition, in this case for dolomite; calcium magnesium carbonate (CaMg[CO3]2), where as rocks are generally made from minerals.

    It also looks like there are small golden coloured crystals of some sulphide mineral, which is most likely pyrite (FeS2 - Iron sulphide) or chalcopyrite (CuFeS2 - Copper iron sulphide), I'd probably need to see it to give you a more definitive answer.


  • I also would like to identify some rock! It was in some crushed stone I have. I presume the grey rock is a type limestone as it came from a limestone area. But what is the white stuff attached to it? I can make out a crystal structure but have no idea what kind of structure it is. Any ideas?

  • Hi,

    Yes you're correct about the grey coloured rock being limestone (carboniferous limestone to be exact), the white mineral is calcite (calcium carbonate [CaCO3]), the crystal habit is massive (no external crystals present).


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