about quartz in rocks
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Adco wrote:I am not too sure what type of rock this is. My guess is basalt. The above is assumption only. Who knows about this stuff?
halucigenia wrote:
It is difficult to tell exactly what rock type it is from the pictures but if you have a sample that you could crack open and wet with water and get a close up image it might be easier to tell.
Adco wrote:In the 3rd picture, the one with the hollows: were the hollows formed through water erosion? I can see clearly that those hollows only formed below the old water line, i.e. where soil retained water. There are no hollows above ground. Also, at depths greater than where the water would sit, there are no hollows and it looks the same as the rock above the ground line. That tells me that water sitting in soil against the rock face would react with the white chips and erode them through some chemical process.
I see exactly what you mean and I'm sorry it pains you that I haven't been able to show you anything of worth. That's exactly the reason I started this thread. I don't know. I'm not a geologist. I was merely looking at these formations in my garden and wanted to discus and hopefully get some answers from more learned people. I tried looking on the net but there is too much to learn casually. I see from your answer that you know more than me about rocks and things but still not enough to give a proper answer either. I posted the second set of pictures after a discussion with halucigenia who knows what he is talking about. Very helpful I must say.Cito di Pense wrote:
It's painful to see you keep puzzling about this three years on with nothing better to show than a few crude photographs. You haven't been able to identify whether this is an igneous, sedimentary, or metamorphic texture. You haven't reassured me that the white inclusions are quartz. although the fracturing is suggestive. For all we know, this could be a metamorphosed sedimentary rock that had irregularly-shaped quartz pebbles in it. You can't even show which way is "up" in the rock as it was originally deposited. If it is a metamorphic texture, good luck with finding "up" if all you have is that. You don't know which way was up when the differential weathering was taking place that removed some of the inclusions. There are no questions about this rock that you've been able to answer definitively. How dense is the matrix, which you say is 'blue". If it's quite dense, odds-on this is a metamorphic rock. It's possible that the clasts or pebbles that are missing did not dissolve, but were simply washed away when the matrix around them weathered. See what I mean?
If the pebbles are quartz, it's unlikely they dissoved. Quartz is pretty resistant to weathering in a water-saturated environment, but it's not even clear from the photos that these are crystals of quartz and not chunks of polycrystalline quartz or some other light-colored mineral.
Adco wrote:Cito di Pense wrote:
It's painful to see you keep puzzling about this three years on with nothing better to show than a few crude photographs. You haven't been able to identify whether this is an igneous, sedimentary, or metamorphic texture. You haven't reassured me that the white inclusions are quartz. although the fracturing is suggestive. For all we know, this could be a metamorphosed sedimentary rock that had irregularly-shaped quartz pebbles in it. You can't even show which way is "up" in the rock as it was originally deposited. If it is a metamorphic texture, good luck with finding "up" if all you have is that. You don't know which way was up when the differential weathering was taking place that removed some of the inclusions. There are no questions about this rock that you've been able to answer definitively. How dense is the matrix, which you say is 'blue". If it's quite dense, odds-on this is a metamorphic rock. It's possible that the clasts or pebbles that are missing did not dissolve, but were simply washed away when the matrix around them weathered. See what I mean?
If the pebbles are quartz, it's unlikely they dissoved. Quartz is pretty resistant to weathering in a water-saturated environment, but it's not even clear from the photos that these are crystals of quartz and not chunks of polycrystalline quartz or some other light-colored mineral.
I see exactly what you mean and I'm sorry it pains you that I haven't been able to show you anything of worth. That's exactly the reason I started this thread. I don't know. I'm not a geologist. I was merely looking at these formations in my garden and wanted to discus and hopefully get some answers from more learned people. I tried looking on the net but there is too much to learn casually. I see from your answer that you know more than me about rocks and things but still not enough to give a proper answer either. I posted the second set of pictures after a discussion with halucigenia who knows what he is talking about. Very helpful I must say.
You can clearly see that the edges of the cavities are composed of what was a large quartz stone.
An inselberg or monadnock (/məˈnædnɒk/) is an isolated rock hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. In southern and south-central Africa, a similar formation of granite is known as a koppie, an Afrikaans word ("little head") from the Dutch word kopje.[1] If the inselberg is dome-shaped and formed from granite or gneiss, it can also be called a bornhardt, though not all bornhardts are inselbergs.
Macdoc wrote:Adco your rock formation looks like kopje
They are volcanic in nature and then eroded to form the very fertile plains.
Also called an Inselberg which is a new word for me tho the meaning is obvious.An inselberg or monadnock (/məˈnædnɒk/) is an isolated rock hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. In southern and south-central Africa, a similar formation of granite is known as a koppie, an Afrikaans word ("little head") from the Dutch word kopje.[1] If the inselberg is dome-shaped and formed from granite or gneiss, it can also be called a bornhardt, though not all bornhardts are inselbergs.
https://en.wikipedia.org/wiki/Inselberg
Pretty sure the geology is fairly complex but fun to chase down.
Same or similar answer to what halucigenia gave earlier on.Macdoc wrote:I'd say some sort of precipitate - can you test the white with something acidic. If it's limestone it will bubble.
I'm getting past my knowledge but I think the white has collected and then washed out
Seems your answer is here
https://www.sandatlas.org/porphyry/
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