A strange quartz rock...

[Backcountry]

Well maybe not that strange, but I haven't seen a rock like this before. It's about the size of a large water melon, and seems to be made up of tiny bits of quartz. I'm curious how this formed. It seems more like there were a bunch of random quartz bits that got all naturally collected, cemented together and then rolled around a while? Or is it just a regular chunk of pegmatite that has really degraded and weathered? Or maybe something else all together. I tried to get a chip off, thinking it might come apart easily like sandstone, but it's hard as heck. Thoughts?

 

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[Dakota Sioux]

Looks like partially decomposed granite.

 

[Eu_citzen]

Pegmatite has "grains" about the size of your thumbnail or larger - if smaller its granite. So that's the rule of thumb for granite vs pegmatite.

 

[DDancer]

Quartz can form in massive and micro crystalline formations. What you have is a massive formation. The bit shown may have some feldspar in it as well so it might qualify as a pegmatite. During the formation the quartz crystals grew into each other and took all the available space~ no vugs were left so individual crystals will most likely be deformed and rare : contaminates will also have deformed much of the crystals. This type of formation will be pretty tight as the crystals have interlocked as well as fused due mostly to other minerals between the individual crystals and their impaction. These types of formations are good to look over if they impact other material, veins, as there is some probability that vugs may have formed and left good crystalline materials there.

 

[Backcountry]

Thanks for all the great info folks. Decomposed granite does sound like a pretty good descriptor. I believe the decomp would suggest radiation, which I think is good for some types of crystals to form? The quartz pieces for the most part are a little smaller than my thumbnail, although I agree that there does seem to be a bit of feldspar mixed in. So decomposed pegmagranitite

DDancer - great description. Sounds like that chunk isn't so special in of itself, but a good indicator of more exciting things. I believe I found my garnet rock in the same area. Could they have been part of the same formation, or every different?

for reference: rock #5

 

[hvacker]

Here granite decomposes to the point it is mined for driveways. Sparkly!
I don't know for sure but instead of radiation it seems more like freezing water.

 

[DDancer]

The sparkly types of granites are typically feldspar and often mica Driveways and landscaping gravels are good sources for small mineral collections. I've more than a few fossils, crystals and what not from just walking around the parking lots and dividers of hotels, malls and road side stops.

 

[Rockhunter1620]

DDancer's right, decomposing pegmatite. I see mostly quartz an feldspar, not much mica, maybe a little rust stains on the bottom there from some iron minerals breaking down there too, the results of chemical weathering from rain, fairly common.

RH