Magnetite? Meteorite?

Hey everyone,

I found this rock on a limestone outcropping approximately five miles north of Marble Falls where a path had been bulldozed through small trees to build a fence. It's very heavy and somewhat shiny, has brownish dots mostly, but it is uniform blackish in some spots. I've never found anything other than dolomitic limestone out here, which contains silica and iron impurities (usually less than 1% for silica, around 0.01% for iron). Any ideas on what this rock is and where it came from would be helpful.

Thanks

Izaak

Hi Izaak,

It looks like a waterworn cobble, possibly Magnetite. Have you checked it's streak color? Is it ferromagnetic and strongly attracted to a magnet? If so, you may be correct, it certainly could be Magnetite. Good luck with it Izaak.

Ron

Based on it's weight and appearance I would've put money on it being highly magnetic. However, it appears to show no attraction to any of the magnets on my fridge. This being said, it may be attracted to a more powerful magnet. I don't currently have a tile to streak it on, but I'll try to get that when I get a chance. It should be noted that the geology in the area consists primarily of limestone and granite. That's the only rocks I've ever seen, and I only picked up this rock cause it was so different.

Looks like a waterworn fragment of a mafic intusive rock. The weak magnetism may be due to small amounts of finely disseminated magnetite.

Hi Izaak,

Since no magnetic attraction, I agree with this being some sort of rock rather than a single mineral and it could be mafic as Reiner suggests. Since it appears waterworn and polished it's difficult to say definitively just what type it is without breaking it to examine it properly. Perhaps you can do that and repost a close-up, in focus image and we can try to help further. As you say, it looks blackish, the brown spots and higher density might seem to suggest some iron content and I notice a bit of green in the image. At this point it's just speculation but some of the mineral constituents may be things like hornblende, epidote, etc. but again this is just speculation from remote viewing. Is this locality, Marble Falls, Burnet Co., Texas, USA or Marble Falls Mine, Markleton, Dodd City District, Marion Co., Arkansas, USA? I was assuming Texas, is that correct?

Ron

It was found atop a limestone bed (approximately 100 to 200 feet of high magnesium content) that is the only rock found on the property. There didn't appear to be any water action in the immediate area, or any other water worn rocks, although I'm not ruling past water action out. Tectonic activity and caving has fractured and altered the bedding of the limestone somewhat, but other than that the geology of the area is somewhat unremarkable. Mining of the limestone has not uncovered anything unusual. This rock appears to a geologic anomaly.

Marble Falls, Texas

Hi Isaak,

Here's somewhere for you to start. page 365, fig.7, POST METAMORPHIC IGNEOUS ROCKS, Town Mountain Granite
[rockfractureandstress.com] Your specimen could be from this geologic unit or one of the geologic units adjacent to Town Mountain Granite. After looking at your specimen again, I think it could also be a rock type called serpentinite. The serpentinite nearby there consists mostly of lizardite with minor magnetite, chromite, anthophyllite, and tremolite. Well anyway, have fun and good luck with it.

Ron

I should mention that I'm a mining engineer and the geology is well defined in the area through core holes and other geologic sampling methods. I can say that this rock is not something that can be found anywhere in the immediate area. The rock in question was uncovered and brought to the surface with a bulldozer and road grader. Considering this equipment usually stays and is used primarily on the property, I doubt they brought the rock in. That leaves water action as the most likely source. There is no evidence of any river or streambed in the surrounding area. Glacier deposition maybe? I haven't researched whether or not there was ever glacial activity in the area.

Thanks Ronald! I'll have to research the various igneous rocks of the area and see what I can come up with.

Let us know how it works out. It's an interesting area in a geologic sense and we'd all like to hear your take on this after your research. Good luck with it and all the best.

Ron

Hello all the only iron on that piece is on its surface caused by whatever its been in contact with in the past, it looks to me to be a compact limestone smoothed by water, none of them rare earth fridge magnets would stick eh !

Izaak

You can use the underside of your toilet tank lid to do a streak test in lack of a tile.

Wayne

It is almost certainly not a meteorite. No fusion crust and even if that was eroded away then there would still be chondrules in the structure. I think it may just a pebble of the local rock* it is doubtful but it may have low amounts of iron or possibly copper inclusions.
*Personally i think in is some sort of serpentinite which originated in the mantle so it has traces of many metals including Fe and Cu.
Hope this helps