I was having a discussion with a guy who said he "dabbled" in geology when he worked for a mine. I don't fully agree with his explanation. It related to the rocks in my garden that have now been exposed to reveal something the caught my interest.

Here is a pic of the area. I removed about 30 tons of rock from the area to make a fire pit feature. How I managed is another story but it is now complete and we have had many wonderful nights sitting around socialising.

You will notice that halfway up the large rock at the back is a "water line". This is the old level of the ground before the rocks were removed.

The exposed area had an interesting formation that become visible.

Imbedded in the bulk of the rock are many small white stones. We identified them as quartz. Notice that below the old level, the quartz has been dissolved. There are neat little holes where the quartz once was. I am guessing that rain water seeped down along the rock face and slowly ate the quartz away. The main part of the rock that was also below the ground level is darker and softer than the rest of the rock. More like a sand stone feel to it.

My guess that water dissolved the quartz is based on the pic below which shows a small cavity, almost like a "fairy house" with shiny crystal formations. You can clearly see that the edges of the cavities are composed of what was a large quartz stone.


Here comes the disagreement between the geologist and myself. I say that the quartz pieces that we can see embedded in the main bulk of the rock were collected as the molten rock flowed over an area that had small stone lying on the ground. Much like raisins in a bun. He says that they grew in place from water flowing through cracks in the rocks. Another option that I can accept is that this is sedimentary rock that had quartz deposited in some unknown way.

I disagree because I don't see any cracks where the water could have flowed. It is too homogenous. The quartz stones are tightly packed. There would have been gaps of some sorts where all this water was supposed to have run. Also, the cavities are formed by water dissolving the quartz. If water can dissolve the quartz, how is it going to deposit it so tightly into the rock in a growth method. The holes where the quartz used to be, below the ground level have been totally dissolved. Like someone picked the raisins out of the bun.

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?

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?

I am not around here much these days so forgive the tardiness of my reply...

The rock looks porphyritic to me:-
"Porphyry deposits are formed when a column of rising magma is cooled in two stages. In the first, the magma is cooled slowly deep in the crust, creating the large crystal grains with a diameter of 2 mm or more. In the second and final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, creating small grains that are usually invisible to the unaided eye." https://en.wikipedia.org/wiki/Porphyry_(geology)
So, neither of your above explanations may be correct.

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.

If the background crystals are small enough to need a magnifying glass to see them and the phenocrysts are much larger then it may be a aphanitic rock, otherwise if all the crystals are discernible by the naked eye it may be a phaneritic rock. Without knowing the composition of the crystals it would be hard to tell if it were mafic like a porphyritic basalt or felsic like a porphyritic granite.

The phenocrysts may not be quartz, if you can scratch them with a steel knife or steel file they will be a feldspar not quartz.
Feldspathic minerals are much more prone to chemical weathering than quartz - they can be "dissolved" by hydrolysis to make clay minerals so the dissolution of them may indicate that the phenocrysts are not quartz.

It is also possible that some of the minerals arose by subsequent crystallisation within joints or voids in the rock. The larger ones with holes in them that look to you like they have been dissolved are probably formed this way.

My guess would be a porphyritic basalt. Basalt is very durable, however in warm and wet conditions the plagioclase phenocrysts will decay to clay minerals leaving behind the more durable minerals and I guess that this is what you see at the previous soil level.


Note the pinkish orthoclase alkali feldspar phenocrysts in a matrix of discernible background crysatals in the above granite rock.



Note the whitish plagioclase feldspar phenocrysts in a dark matrix of indiscernible crystals in the above basalt rock.

If you can find out about the geology of your area and what other like outcrops are composed of it would help with the identification.

Reviving this thread because I've had a chance to post some pics of wet rock.

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.

Newly exposed rock. Still blue. About 5 years old. Not long enough to oxodise. You can see the older rock on the upper part of the pic. Don't know how old but it has "rusted" already.

In the center is an area where the type of "fairy house" is forming. The grandkids call it that!

Now, is that section growing or is it dissolving? There lots of those "fairy house" formations all over the rocks. Some are large enough to put your hand in.


On this picture you can see the old exposed level (with the blue rock and the white quartz chips), then the ground level(the dark soil stained section), and the the area that was below the surface. Note the hollowed out pieces. I'm guessing those missing round pieces were once white quartz chips that have dissolved in rain water that has penetrated the surface and along with other minerals, has eaten the white pieces away.

It looks as if water was trapped on the right side of the vertical crack as that is where hollows have formed. Perhaps the drainage was better on the left side and water wasn't retained as much.

It still looks like Porphyritic basalt to me - the grey matrix and white plagioclase feldspar phenocrysts are very clear in the wet image.
Here's another geological term for you 'amygdaloidal'.
Amygdaloidal basalt occurs when pockets of gas which are captured as the lava cools are subsequently filled with crystals as the porous matrix allows liquids containing minerals to percolate through the rock. The crystals are sometimes composed of quartz or otherwise are calcite or zeolites. So, therefore the explanation for some of these crystals, especially the ones with voids - the fairy houses, that they grew in place from water flowing through cracks in the rocks filling voids would be right, but only for some of them.
There certainly are many different processes going on in the formation of some rocks.

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.

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.

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.

Goddit when he was burying those fossils.

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.

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.

Between the two of us, Adco, you're the one who can hit the rock with a hammer until you get a fresh piece. If you don't want to wreck your rock garden, you should be able to find an exposure of those rocks by a nearby roadside, since you're still mulling this several years on. If the rocks are boulders from some unknown location, it's a waste of time to try to determine their history. If there are no fresh pieces to be had, your commentary about the weathering is by the way. If you did get a fresh piece, you could illuminate it properly using fluorescent light and take a picture of it dry, striving to avoid reflections. This may be more work than it's worth to you. Photos such as you have taken are not anywhere near informative enough, given the criteria I've offered you, and that's the source of my frustration with this exercise as a waste of everyone's time. Hallucigenia may know what he's talking about (and I've also seen fine grained volcanic rocks that had large phenocrysts in them), but those phenocrysts (if that is what they are) need to be identified mineralogically, and in a basaltic rock, they would not be quartz, as hallucigenia undoubtedly knows.

You can clearly see that the edges of the cavities are composed of what was a large quartz stone.

I seriously doubt that. What you're taking to be the rim of what was once a homogeneous "stone" could just as well be fractures that have been mineralized with quartz, which is one of the most weathering-resistant of minerals. That's why beach sand comes to be dominated by grains of quartz. You haven't even tested the hardness of the material, and you could easily distinguish quartz from calcite by its hardness. You're the one with his boots on the ground, there, and there are lots of conjectures you can rule out just by doing a little work. Anything that's quartz, you won't be able to scratch with the blade of a pen knife.

Too much effort to reply to all the points. I'm going to leave it here. Just want to mention that this hasn't been several years of discussion. I was looking through old threads, saw this one, realised I now have a new camera, replied to halucigenia's request for some pics, took a few unremarkable shots and posted them. And then my little bubble got burst.

Thanks halucigenia, you have given me something to think about. I see that my thoughts difficult to answer because you would need to be here and not rely on photos alone. This is much more complex than what I thought.

Jesus, Cito, wind yer bloody neck in.

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.

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.

Very complex. Lucky some of us engage constructively. Thanks.

My location is a koppie. Very rocky with trees suited to the rocks such as kiepersol or mountain cabbage and resident dashes, or hyrax. Dessie's are my pet hates. They munch my plants and breed like rabbits. Their call is very eerie. Sounds like Chuckie and Freddy on a date.

Figured.
So you have to sort the original basalt geology/chemistry and very long weathering. I doubt the crystals are quartz as it's a very hard mineral but I'm no rockhound.

You'd want to take a solid hunk off one of the basalt boulders and take up close photos. Then your weathered bits.

The white inclusions on the upper left are the main items of discussion. They are in an old exposed area. The rock has a blue tinge to it because it hasn't had enough time to weather.

The dark brown marking through the center is the old ground level. I excavated the rocks and soil away to turn the area into a fireplace.

The lower right section of the pic shows pit holes where the white inclusions used to be but are now missing. This thread is about these missing white pieces. I made an assumption that because they were below the water line, the retained water was dissolving the white inclusions over a long period. My mistake was calling the white pieces quartz.

In the first pic I posted, which shows the entire fireplace area, down at the new ground level, these white inclusions are still there. It seemed as if the water only penetrated about 1m before finding a new path, perhaps through a crack in the rocks.

I know this all seems trivial and consuming but it is fascinating to see the geology of the area that was only visible after it got exposed. I had to remove at least 20 tons of rock from that area! A huge mission but we get many hours of enjoyment from it and it was worth the effort.

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/

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/

Same or similar answer to what halucigenia gave earlier on.

I guess that's the answer. Not complicated at all. Thanks for the input. At least the pain will go away now....

Every time I see a koppie I think of Tsavo.
https://en.wikipedia.org/wiki/Tsavo_Man-Eaters

One scary film ....



I've been nervous around long grass ever since.