FMF - Friends of Minerals Forum, discussion and message board

[tfarris6124]

This is from central Texas on a high cliff over a small river. The bulbous form is strange sticking out on both sides of the flatter rock. The round part is about a six inch sphereish shape sticking out about two inches on either side of the flat. What makes the rock form like this? Is there something inside the round part? Thanks for your help.

[tfarris6124]

Other side view.

[tfarris6124]

Internal view.

[Peter Megaw]

What formation is it from? Edwards?

This looks like a textbook concretion...a banded onion-like body that forrms during diagenesis/lithification where something (fossil, organic nucleus etc) acts as a nucleus for deposition of successive layers of silica or carbonate or iron-oxides.Sometimes they're hollow...but not often in your neighborhood.

There are 4 questions

1. Is it chert (flint)? If it is you won't be able to scratch it with a knife or nail. If it's chert, do no more, enjoy it as a yard rock with a cool shape.
2. Is it carbonate? If so, it will scratch easily...and will most likely be fine graimed banded calcite unless it is from one of the strontium rich units in the area and may be celestine.
3. Is it solid? If it's hollow there may be crystals inside. If it is hollow enough you may be able to heft it and tell it's hollow. Find a rock you know is solid and about yhe same size and see if there's a difference. If it's solid leave it alone and enjoy your yard rock
4.if it feels hollow or if you're a gambler willing to exchange a cool complete yard rock for fragments...you can break or cut it open with a chisel or tile saw. If its from one of the Sr bearing units you might find celestine crystals. If not, most likely is calcite crystals...shiny if you're lucky.

I would give you low odds of a void being present...
I think you have a great yard rock with a built in mystery...
You can definitively solve the mystery anytime...

[tfarris6124]

Wow thanks so much for the information.

I'm not sure of the formation it came from. Crews are cutting new roads in the area and digging up a lot of rock so I don't know for sure where it came from. I'm guessing Edwards, but maybe Comanche Peak? I don't know I'm in over my head here.

The flat rock surrounding the concretion is definitely chert. The concretion itself though seems to be different. I think I have scratched down past the outermost sediment and it still seems soft. It also has a sparkle to it that neither the sediment nor chert have making me think it might be calcite.

[SteveB]

Looks like a limestone nodule. Should acid test the round rock as well as the surrounding matrix. Use a drop of hydrochloric acid or white vinegar and note any bubbles forming and how aggressively.. I’m more familiar with iron concretions (this isnt that) but might be other form of concretion.

Are you able to gather more from the location? Maybe find a broken round rock half to show its interior which will help.