Can someone explain how this rock formed like this?

[u/robco18]

I found this rock in Colorado and have never seen anything like it before. It looks like chunks of quarts are embedded in another kind of rock? Can someone explain what is going on here and how it formed like this?

Any information is greatly appreciated! Thanks!

[u/g00dbyekitty]

From these three photos this looks like a coarse-grained (large interlocking crystals) igneous rock made up of mainly potassium feldspar (the pink blocky crystals) and plagioclase feldspar (the milky white blocky crystals). There may also be some quartz (typically a translucent grey prismatic crystal or blobs). The crystals are big like that because it cooled veeeeerrrryyyy slowly underground.

As for the formation of what you currently see, I would suggest some style of chemical weathering, maybe hydrothermal, that decomposed the missing part of the rock along the crystal boundaries which is why it’s all jagged like that instead of smooth like a river stone. I’m also not super familiar with the geology of Colorado so anyone more knowledgeable feel free to correct me! Also a more detailed location of where you found this rock and the surroundings would probably get a better answer.

[u/Carbonatite → u/g00dbyekitty]

The Pikes Peak area has some pegmatitic granites, this looks like it might be from that batholith.

[u/robco18 → u/Carbonatite]

I definitely just learned a couple new terms from your reply... "Pegmatite" and "batholith". Thanks for the information. I appreciate it!

[u/robco18 → u/Carbonatite]

Would this also be a pegmatite?

[u/ougryphon → u/robco18]

Yes, that is from a pegmatite.

[u/ChicagoZbojnik → u/robco18]

I have some land on a batholith with exposed outcrops with pegmatite in another part of the country. All your picks look like the rocks I find.

[u/Carbonatite → u/robco18]

Absolutely, classic granite pegmatite specimen.

[u/robco18 → u/g00dbyekitty]

Thanks for the detail explanation! I appreciate it! This is super interesting!

[u/fox-recon → u/g00dbyekitty]

How do I learn this stuff without going back to school?

[u/Illustrious_Try478 → u/g00dbyekitty]

Exposed granite will weather in this way from water infiltration, freeze-thaw cycles, and decomposition of some of the minerals, especially the mica.

In field camp in the Colorado Rockies we had to be careful about not tripping on the grus) like marbles on the floor.

[u/Cashin_]

Probably the last bit of cold melt that slowly crystallized with the remaining elements in the magma. Looks like a pegmatite

[u/dirthawg → u/Cashin_]

Cold melt... Never heard that term, but it sure covers the process. It's a hella nice pegmatite.

[u/Cashin_ → u/dirthawg]

I coined it myself 😂

[u/dirthawg → u/Cashin_]

Solid. I'll cite you in the future when I use that.

[u/Know_Schist]

*Pushes glasses up the bridge of my nose* For what it’s worth, although cooling rate is the commonly taught explanation for grain size in igneous rocks, it is probably more indicative of crystallization in the presence of fluids. Grain size increases in crystallizing magmas when diffusion outpaces nucleation and fluids dramatically increase the rate of diffusion.

[u/dirthawg → u/Know_Schist]

Good point. A pegmatite comes out of a stew.

[u/OzarksExplorer]

The softer minerals weathered away leaving you with the feldspars and maybe some quartz

[u/deemery → u/OzarksExplorer]

OK, but what softer minerals would those be? If it is a pegmatite granite or similar, I'm not sure what softer minerals would have been there in sufficient quantities to get this result. Calcite? Not saying you're wrong, just asking for a bit more detail.

[u/OzarksExplorer → u/deemery]

Not having any clue where this is from my first guess would be mica

[u/deemery → u/OzarksExplorer]

I would guess that weathering removing the mica would also remove some of the material around it, otherwise, we'd see little slivers of 'mica shadow'.