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451 Views 1 Reply Last post: Apr 17, 2013 6:24 PM by MikeHardman RSS
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Apr 17, 2013 4:10 PM

Can somebody explain this rock?

rock.JPG

Can somebody explain this rock, found on the island of Kerrera, in western Scotland? The material is like soft slate. There used to be a cube of pyrites in the square hole at the centre of each circular part.

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    Apr 17, 2013 6:24 PM (in response to Palaeotrope)
    Re: Can somebody explain this rock?

    Intriguing specimen!...

     

    Kerrera is composed of: volcanics, the Old Red Sandstone, and metamorphics (mainly slates I would guess).

     

    The slates would be the source of the pyrites.

     

    I am tempted to suggest that maybe your specimen was a lump of old metal mining tailings (settled-out mining waste) that had included cubes of pyrites. But as far as I know, the only mining in the area is/was slate. (I did some of my postgrad research in the area, camping on the Garvellachs; I saw slate quarrying on the island of Seil, if I recall correctly.)

    The circles (sections through spheres/spheroids) are undoubtedly alteration haloes - the pyrites affecting the chemistry/mineralogy nearby. The exact nature of the haloes, I don't know.

     

    The best I can suggest is that it is soft slate (slate can become softened due to alteration of the micas), but I am not confident. I think such soft slate would be unusual, and the haloes are not familiar.

     

    Other opinions welcome!

     

    Map for reference - http://www.snh.org.uk/pdfs/publications/geology/argyll.pdf

     

    Regards,

    Mike

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