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ID rock from Dallas Texas


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I found this when i was a kid around the Chalk Hill -Eagle Ford area of Dallas, which is somewhat famous for that amount of fossils and rock found there. The area was once home to an enormous TDI cement company, so there are many large piles of dirt and rock that have been dug up from hundreds of feet down. Anyway, i just wanted to see what this rock is and if it is anything special. I remember that i broke it open w another rock and found the crystal inside. There are many round rocks like this around the area but ive never seen one w this type of crystal inside. Also you can go 5 feet without seeing a nautilus fossil in this area. Thanks for any help. Jonathan.

DSC_6335.jpg

DSC_6331.jpg

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Looks like calcite. There is a heck of a lot of that stuff in Austin chalk. Nice looking rock!

I thought I might add that you mentioned that the property was a big cement company. Well, what you have is the "ore" of cement. Calcite and limestone rich in calcite are the feedstock for portland cement.

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this is a serpintine nodule. It was a clay ball being gently rolled around on a mud flat by wave action.

the mud ball then drys out slowly and cracks internally from the volume loss of the water. Then the dried mud ball is deeply buried and the cracked inside is filled with Calcite crystals. The one that I have has a brown calcite druze with creamy yello dog tooth calcit crystals sitting up on top if the druze.

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I disagree with the serpintine (sic) nodule. I think you mean septarian.

http://en.wikipedia....erpentine_group

http://en.wikipedia....ian_concretions

Really nice find though regardless.

Steve

Septarian nodule. I dont think so. Neither is it sepentine from my understanding of the word. Septarians are a whole lot like my Moqui Marbles. I have a few with fossils. I will post some photos this evening.

Calcite for sure. But what you would call the "nodule" I could not say. It looks exactly like the deposits of calcite in S. New Mexico. We have them 20 feet in diameter filled up with that dog tooth calcite and banded in all colors. A lot like geodes but huge.

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I just spoke to a friend that runs a concrete plant. He calls that nodule a "dogger" or "calcrete". He says that these nodules were formed as calcite leached through a calcium/magnesium rich strata. They are found in and below layers of old sea bed where the soluable minerals concentrate. He says that calcite bedded in layers is generally called "calcrete" and that calcite formed in nodules is called a "dogger". It is the common formaiton of calcite in a bedded formation.

Septarians are very different and it is a mystery how they were formed, although they can have calcite crystals. Serpentine is an ore of asbestos and looks completely different...

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