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What is in this stone please?


ohdearme
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1 hour ago, ohdearme said:

I found this in a London garden I don't know if it came from here or not,

You would likely find many more of these on the beach at the base of a basaltic cliff.

The white veins are quartz that have permeated the basic fine grained black rock after it was formed, but before pieces broke off to form the beach pebbles.

The shape will have resulted from water action 'tumbling' pieces eg on a pebble beach.

Cornwall is a good place to look for such pebbles.

A scale is a good idea to add to a photo such as yours.

Edited by studiot
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Very interesting topic. (+1,+1) Thank you. If you don't mind, I will post a couple more pictures of rocks I collected in Spain and Portugal and be posting them here ASAP.

One of them is very similar. Looks like a sedimentary rock that's been infiltrated by quartz. It's visibly more eroded in the "sedimentary body" and the "quartz" veins have resisted erosion much better, so it has some kind of ridges all around. The other looks to me like obsidian and rougher basalt arranged in layers, but I could be wrong. Is that possible?

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I've attached them here.

The first one with the quartz-like veins was collected hereabouts:

https://www.google.com/maps/@36.9992728,-7.89032,15357m/data=!3m1!1e3

And the obsidian/basalt looking one, hereabouts:

https://www.google.com/maps/@37.9845133,-6.6169228,15156m/data=!3m1!1e3

I could be a little bit off in the second one. I do remember the terrain was ochre-red and the place was littered with many similar obsidian-looking rocks.

 

 

IMG_20200721_140906.jpg

IMG_20200721_140928.jpg

Edited by joigus
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6 hours ago, ohdearme said:

And the "basic" grey?

I'd imagine it's granite and the red vein's in the quartz, possibly iron...

57 minutes ago, joigus said:

I could be a little bit off in the second one. I do remember the terrain was ochre-red and the place was littered with many similar obsidian-looking rocks.

Did you try chipping a piece off?

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11 minutes ago, dimreepr said:

I'd imagine it's granite and the red vein's in the quartz, possibly iron...

Did you try chipping a piece off?

Granite? Really? Would the granite parts be more eroded than the quartz veins?

I've no idea really. I just want to learn more about rocks.

And unfortunately I didn't have any geologist's kit with me.

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What the OP has there is the basic ingredients for prehistoric fire starting - with a few modifications and a bit of imagination.

I'm not confident with describing the geological or chemical processes but flint is a form of quartz, and when flint is struck by iron* it creates sparks and hey presto🔥🍗

 

 

* or variations thereof such as steel

 

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6 hours ago, joigus said:

I've attached them here.

The first one with the quartz-like veins was collected hereabouts:

https://www.google.com/maps/@36.9992728,-7.89032,15357m/data=!3m1!1e3

And the obsidian/basalt looking one, hereabouts:

https://www.google.com/maps/@37.9845133,-6.6169228,15156m/data=!3m1!1e3

I could be a little bit off in the second one. I do remember the terrain was ochre-red and the place was littered with many similar obsidian-looking rocks.

 

 

IMG_20200721_140906.jpg

IMG_20200721_140928.jpg

 

No I don't think the first one is granite or even igneous.

It looks like a fine grained sandstone or siltstone to me, perhaps a grewacke.

https://en.wikipedia.org/wiki/Greywacke

This would be consistent with the Faro peninsula, which is a small sedimentary piece on the edge of the alpine mountain building area in Iberia.

(the Variscan Orogeny

https://en.wikipedia.org/wiki/Variscan_orogeny


The pebble does not look beach rounded but river worn.
Perhaps it was brought down by the Chanca river from the mountains to the north and east.

 

The second pebble definitiely looks like some form of volcanic glass, probably obsidian.
It seems to be layed on duller form of lava, which is very common.
The base layer appears to have bubbles or other structure, characteristic of other forms of lava.
If that was basalt the pebble would be quite heavy.

Again this would have formed during the Variscan.

 

Yes the various discolourations of the veins in the samples will be dues to impurities in the quartz.
Iron compounds give the brown colour.

It should be noted that the quartz veining is quite different from the micro intrusions of liquid rock during vulcanism.
The is no metamorphosis (cooking) of the parent rock along the boundaries.
Quarz is slightly soluble in rainwater of geological timescales so precipitates out as the solution percolates small fissures and the water evaporates.

Edited by studiot
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1 hour ago, studiot said:

 

No I don't think the first one is granite or even igneous.

It looks like a fine grained sandstone or siltstone to me, perhaps a grewacke.

[...]

Thanks, @studiot. That was a pretty impressive informative answer. +1

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