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407 Views 1 Reply Last post: Jan 17, 2013 2:07 PM by MikeHardman RSS
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Jan 17, 2013 1:02 PM

Can anyone tell me what type of rock this is please? Thanks in advance!

This rock was brought to me recently, hope you can help!

Many thanks

 

MP

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  • Fine-grained, massive (no obvious layering), dark purplish... Could be volcanic, but can't rule out sedimentary, especially a metamorphosed sedimentary rock. The lighter veins don't help much.

    You'd really need to look at the structure under a microscope, though you might just see enough with a hand lens. If you can see distinct grains with a cement holding them together, that suggests sedimentary, or lightly-metamorphosed sedimentary. If you can't, you're really none the wiser - because it could be sedimentary but with cement and grains made of similar stuff. Under a microscope, viewing a thin section of the rock, you'd be able to see the crystals (and identify them) and how they are arranged (with or without a cement, with or without minerals indicating alteration or metamorphosis, and microstructures such as twinning or cleavage).

    It is all about petrography (http://en.wikipedia.org/wiki/Petrography).

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