Posted: Nov 05, 2018 10:55 am
by Cito di Pense
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.