Help with this very sparkly rock?

[Lola]

Hi again! I found this rock in south central Wisconsin and can't identify it... the pictures don't do the sparkliness justice--this is a very difficult rock to photograph accurately!

I tried to get some pics of the texture (like little sparky grains of salt) and to show that these little spots aren't (all) mica, as someone on Reddit suggested. All of these shiny spots/tiny grains are fairly clear with some color fro. The rock itself coming through.

Any idea what this could be?

 

[Geologyhound]

I am thinking it is a piece of gneiss. The mica is a holdover from a lower grade of metamorphism (mica schist). It looks like you maybe had one layer which was a little more mica rich. The rock split along that layer since mica is weaker and cleaves easily. This explains why you have mica on one surface and less elsewhere. The clear crystals are likely quartz, and the whitish crystals may be feldspar. The rest of the sparkle is from either tiny flat crystal faces or broken quartz surfaces. The “sparkly grains of salt“ are the individual mineral crystals.

 

[Johnho]

My first thought was a mica schist but I didn't see that much schist quality except on that one photo of the dark side of the specimen. It's a tough one just from the pics but it sure seems metamorphic for sure.

 

[Lola]

My first thought was a mica schist but I didn't see that much schist quality except on that one photo of the dark side of the specimen. It's a tough one just from the pics but it sure seems metamorphic for sure.

Agh, I know, it’s so hard to photograph accurately because it just doesn’t look the same, no matter what lighting I used. If it makes any difference, all of the pics are of the same surface of the rock. There are maybe some flecks of mica that contribute to the sparkle, but there really isn’t much of it. And in real life the rock isn’t as grey as it appears in the darker pic in my original post.

Would you happen to have any tips to get a better picture? Here are some more of that same part of the rock (the rest of it is a little bit the same but mostly unremarkable)

 

[Johnho]

I do not see any schist structure now. But now think what you have is a coarse grained quartzite. Look here; https://www.sfu.ca/archaeology/research/research/etap/use-wear-raw-material.html But, if you look here https://wgnhs.wisc.edu/pubshare/M067.pdf you will see there is only one small spot for quartzite in SC WI. If you found it loose it could have been brought down by a glacier, if it's quartzite.

 

[Odanscoils]

Agh, I know, it’s so hard to photograph accurately because it just doesn’t look the same, no matter what lighting I used. If it makes any difference, all of the pics are of the same surface of the rock. There are maybe some flecks of mica that contribute to the sparkle, but there really isn’t much of it. And in real life the rock isn’t as grey as it appears in the darker pic in my original post.

Would you happen to have any tips to get a better picture? Here are some more of that same part of the rock (the rest of it is a little bit the same but mostly unremarkable)

Try sunlight

 

[Lola]

I do not see any schist structure now. But now think what you have is a coarse grained quartzite. Look here; https://www.sfu.ca/archaeology/research/research/etap/use-wear-raw-material.html But, if you look here https://wgnhs.wisc.edu/pubshare/M067.pdf you will see there is only one small spot for quartzite in SC WI. If you found it loose it could have been brought down by a glacier, if it's quartzite.

Thanks! The coarse grained quartzite is probably correct, since the area in that map (“one small spot for quartzite in SC WI”) is exactly where I found it. It’s interesting (no probably nobody but me, hah)… I grew up there and have been around that quartzite my entire life, but have never seen one quite like this!