Naked Science Forum

Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: Anastasia.fr.1 on 07/08/2009 16:10:19

Title: Why has this rock got these lines and colours in it?
Post by: Anastasia.fr.1 on 07/08/2009 16:10:19

I found this rock at the beach. It feels rough like sandpaper and looks a bit like a pumice stone. It has light and dark brown stripes and dark lines running through it.

What is it, and how did the different colours get there?

Thank you.

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Title: Why has this rock got these lines and colours in it?
Post by: Anastasia.fr.1 on 07/08/2009 16:14:04

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Title: Why has this rock got these lines and colours in it?
Post by: RD on 07/08/2009 16:27:53

The process of sedimentation (http://en.wikipedia.org/wiki/File:Horseshoe_Canyon.jpg) can cause rocks to have layers.

Either the different coloured layers were caused by different coloured sediments being laid down at different times, (a bit like summer and winter rings in a tree trunk cross-section), or some layers are more permeable to the brown (Iron?) stain which was added much later.

Title: Why has this rock got these lines and colours in it?
Post by: Bass on 07/08/2009 17:32:40

Great pictures!

I suspect the rock is sandstone- a sedimentary rock made from compressed sand grains.  Some sort of iron mineral (hematite, magnetite, or pyrite) was deposited either along bedding or in fractures, and weather to produce the brown stripes. The black irregular looking blebs are the remnant of the iron mineral.

Title: Why has this rock got these lines and colours in it?
Post by: frethack on 07/08/2009 20:51:50

I may be wrong, but it looks like leisegang banding.  If it is, then the banding is caused by mineral staining (in this case iron) as water moves through the rock.

Title: Why has this rock got these lines and colours in it?
Post by: JimBob on 08/08/2009 00:17:01

Anastasia, is this rock really blue?

I do believe RD and frethac are correct in suspecting post-sedimentary iron staining.

Title: Why has this rock got these lines and colours in it?
Post by: Anastasia.fr.1 on 08/08/2009 08:01:56

no it is white, it just looks blue on the picture.

Title: Why has this rock got these lines and colours in it?
Post by: JimBob on 08/08/2009 14:49:41

Then it is most likely iron staining due to water rich in iron moving through the rock.

Title: Why has this rock got these lines and colours in it?
Post by: frethack on 08/08/2009 15:08:57

Is the rock porous and light like pumice, or can you visually see sand grains?

Title: Why has this rock got these lines and colours in it?
Post by: RD on 08/08/2009 15:38:05

Whiter you say ...

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[could the blue tint be due to fluorescence (http://en.wikipedia.org/wiki/File:Fluorescent_minerals_hg.jpg) ?]

Title: Why has this rock got these lines and colours in it?
Post by: Anastasia.fr.1 on 08/08/2009 20:58:43

Quote from: frethack on 08/08/2009 15:08:57

Is the rock porous and light like pumice, or can you visually see sand grains?

No. It looks like pumice but is heavy and solid.

Title: Why has this rock got these lines and colours in it?
Post by: Anastasia.fr.1 on 08/08/2009 21:00:14

Quote from: RD on 08/08/2009 15:38:05

Whiter you say ...

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[could the blue tint be due to fluorescence (http://en.wikipedia.org/wiki/File:Fluorescent_minerals_hg.jpg) ?]

Closer to your second image RD. But it does not flouresce.