Name This Llano County Mineral!

[griffinfaxon]

Hey everyone!

New to the community and glad to be here...haven't quite written my hello yet, but couldn't wait to post either. I have this little unique specimen found in Llano County, TX in the hill country and while I've identified the makeup of the specimen, I haven't put a name to it yet, although I'm leaning toward beta spodumene (lithium) ore. I'd like to get everyone's input. The vast majority of the rock is feldspar with the black parts being roughly 90% biotite mica and 10% hornfels. At first I thought some type of granite until looking under scope at the formation of the feldspar. They're monoclinic vertical striations and crystalline in nature. Here is the photo, the best I could get without my scope:

Thanks in advance and glad to be a part of The Fossil Forum!

Brance

Edited May 24, 2013 by griffinfaxon

[lissa318]

Can't help with an ID but want to say welcome to the forum!

[2ynpigo]

I can't see the photo very well, but I suspect the "striations" are polysynthetic twinning commonly seen in plagioclase feldspar.

[griffinfaxon]

Thanks y'all for the welcome (lissa318) and the help(2ynpigo)! I really appreciate it. I'll try taking a better photo under microscope here in a bit once the kids are finally asleep. I agree, 2ynpigo, it's definitely a feldspar although I was thinking it was alkali up to now. I'll know more when I get those microscope photos posted.

[griffinfaxon]

Ok, upon further study under microscope...new pics added to original post.

[bdevey]

potassium feldspar?......LINK .....notes "Proterozoic Llano Uplift of central Texas"

Edited May 24, 2013 by bdevey

[griffinfaxon]

bdevey, thanks for the information! That map is spot on. We were in the Sixmile region north of Enchanted Rock when when we found this particular piece. It was so different than all the other granites I had to pick it up. Although no one thought it was anything special at the time, no one could name it either. We're planning a little trip back to that region in July which should yield some great mineral specimens and hopefully fossils as well. We're actaully going to pan for a little gold this time along one of the storied or "lost" creeks in the region. I'm headed to Northeast Texas this weekend to an old salt dome/coal bed to see what I can come across. I'll probably just take some ore samples from the old operation that used to run there years ago.

[bdevey]

bdevey, thanks for the information! That map is spot on. We were in the Sixmile region north of Enchanted Rock when when we found this particular piece. It was so different than all the other granites I had to pick it up. Although no one thought it was anything special at the time, no one could name it either. We're planning a little trip back to that region in July which should yield some great mineral specimens and hopefully fossils as well. We're actaully going to pan for a little gold this time along one of the storied or "lost" creeks in the region. I'm headed to Northeast Texas this weekend to an old salt dome/coal bed to see what I can come across. I'll probably just take some ore samples from the old operation that used to run there years ago.

Cool, glad that helped you out.