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Semi-spherical exterior, radiating interior stone
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crowinfocus




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PostPosted: Feb 10, 2023 11:28    Post subject: Semi-spherical exterior, radiating interior stone  

I was a young kid looking for arrowheads just outside the small Mexican border town of Zapata, Texas USA when I found the pictured stone. Now 72, I have carried this stone with me, always curious but never knowing what it is. I've asked several geologists over the years, but no one had an answer.

The stone weighs 1.102 oz.
It measures 3.3 cm (exactly) at its widest point.
It has zero magnetic attraction.

All suggestions welcome!
Thank you.



M. Crow Rock 1 sm.JPG
 Mineral: ?
 Dimensions: 3.3 cm
 Description:
 Viewed:  8763 Time(s)

M. Crow Rock 1 sm.JPG



M. Crow Rock 2 sm.JPG
 Mineral: ?
 Dimensions: 3.3 cm
 Description:
 Viewed:  8757 Time(s)

M. Crow Rock 2 sm.JPG



M. Crow Rock 3 sm.JPG
 Mineral: ?
 Dimensions: 3.3 cm
 Description:
 Viewed:  8756 Time(s)

M. Crow Rock 3 sm.JPG


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Mathias




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PostPosted: Feb 10, 2023 11:38    Post subject: Re: Semi-spherical exterior, radiating interior stone  

Its a fossil shell.

Dohmophyllum involutum, Columnariina.

With google its easy to find similar photos.
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kushmeja




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PostPosted: Feb 10, 2023 11:52    Post subject: Re: Semi-spherical exterior, radiating interior stone  

Looks like a Limonite concretion to me.
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crowinfocus




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PostPosted: Feb 10, 2023 12:11    Post subject: Re: Semi-spherical exterior, radiating interior stone  

Mathias wrote:
Its a fossil shell.

Dohmophyllum involutum, Columnariina.

With google its easy to find similar photos.


That was quick!
Thanks for your keen observation.
Michael
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James Catmur
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PostPosted: Feb 10, 2023 12:40    Post subject: Re: Semi-spherical exterior, radiating interior stone  

I agree

kushmeja wrote:
Looks like a Limonite concretion to me.
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SteveB




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PostPosted: Feb 10, 2023 12:49    Post subject: Re: Semi-spherical exterior, radiating interior stone  

iron concretion was my gut but non magnetic limonite is close enough.
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Linus




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PostPosted: Feb 10, 2023 14:25    Post subject: Re: Semi-spherical exterior, radiating interior stone  

I agree - fossil. Most all concretions form concentric rings, the radial pattern is not at all typical of a concretion.
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Pete Richards
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PostPosted: Feb 10, 2023 16:33    Post subject: Re: Semi-spherical exterior, radiating interior stone  

Some armchair exploration is helpful in choosing between these two proposals.

First, the object is a piece of a sphere. We can get a rough estimate of the density from the information given. A diameter of 3.3 cm produces a sphere with a volume of 18.8 cubic centimeters. This is a partial sphere, somewhat less than 50% of it. I calculated volumes of 30%, 40%, and 50% of the sphere. There is an error here, because if the amount present is less than half the sphere, the true diameter is greater than 3.3 cm. But I plow on. Given the stated weight of the object, converted to grams, we get estimated specific gravities of

50% 3.32
40% 4.15
30% 5.53

Correcting the error in volume estimate would reduce the s.g. estimates for 40% and more so for 30%, but not to lower than the 50% estimate. My guess is that the actual object is less than 40% of the sphere, so the s.g. is perhaps between 4.0 and 4.5 or so.

A fossil should be mostly calcite, with a specific gravity of 2.7.
Pure goethite has a s.g of 4.3; the s.g. of a limonite concretion is likely to be somewhat lower.

The estimated specific gravity favors the interpretation that this is goethite/limonite, and the shape could be expected of a partial concretion.


While the exposed cross section of this object has a radial aspect and a central depression as do many solitary corals, there is none of the detail of septae etc that I would expect of a coral. The bottom has no particular resemblance to the outside of a solitary coral, either in texture or in shape.

The particular genus proposed, Dohmophyllum, is a Devonian (Paleozoic) fossil. But the area where it was found is part of the Gulf of Mexico coastal plain, with sedimentary rocks of the Eocene Period (Cenozoic), hundreds of millions of years later. According to the map I consulted, no Devonian rocks crop out in the entire state of Texas.

Mathias' suggestion is creative and useful in reminding us not to overlook biological origins for mystery objects like this one, but in this case the evidence favors the concretion. Until someone comes up with another possibility! (No, it's not a meteorite).

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Linus




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PostPosted: Feb 10, 2023 17:15    Post subject: Re: Semi-spherical exterior, radiating interior stone  

oh come now, fossils can easily be preserved by silica (corals in the Horquilla Formation in Southern Arizona, petrified wood; and brachiopods on the Colorado Plateau; I have seen them preserved as well by baryte; and smithsonite (Bisbee).
see on the web: "safossils. com/rugosecoral1010small. jpg" of a silicified horn coral cross section.
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Pete Richards
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PostPosted: Feb 10, 2023 17:35    Post subject: Re: Semi-spherical exterior, radiating interior stone  

No question that fossils can be preserved by replacement by all kinds of minerals, silicification being the most common, with the possible exception of pyritization (which would be pretty obvious). Silica, however, has a specific gravity of 2.67, lower than calcite.

Replacement by exotic substances like baryte certainly does occur, but rarely. I have seen fossil molds in sphalerite. I found a brachiopod at Mont Saint-Hilaire that was replaced by pectolite and a few other minerals, well enough preserved that it could probably have been identified to genus or at least family by a Silurian brach expert. But I hardly think these oddities are at play here.

The age discrepancy also remains. If this is not a Devonian solitary coral, what is it?

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Linus




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PostPosted: Feb 10, 2023 17:54    Post subject: Re: Semi-spherical exterior, radiating interior stone  

I guess the real question is, "was this transported? or found in outcrop"? judging from all the eocene sediments surrounding zapata, tx and the pronounced rounding and abrasion of the item I would suggest that although found there it may have been transported a significant distance. Perhaps crowinfocus can comment exactly on the nature of his find?
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Amir Akhavan




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PostPosted: Feb 10, 2023 18:54    Post subject: Re: Semi-spherical exterior, radiating interior stone  

I agree with those who say very likely limonit(ized) concretion.
I used to find many pyrite (often called marcasite) concretions in a limestone quarry that looked like these.
Interestingly they had a grainy, soft multicrystalline core, and radially grown, hard shell, pretty much in the proportions seen in the specimen on the photo.
When you split them, the core would disintegrate very quickly and leave the shell.
If something like that gets limonitized, it would look much like the specimen in question.

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Linus




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PostPosted: Feb 10, 2023 20:16    Post subject: Re: Semi-spherical exterior, radiating interior stone  

Cretaceous age fossil corals are known from the San Antonio area of south Texas, and from north of Nuevo Laaredo, both approximately 200 kms from Zapata.I have enclosed a cross section drawing of a Cretaceous coral from Australia - note the amorphous center and the outwards radiating septa ...


th-26707518.jpg
 Mineral: fossil coral
 Locality:
Australia
 Description:
thanks to: Jell, Cook, and Jell (2011): Australian Cretaceous Cnidaria and Porifera
 Viewed:  8474 Time(s)

th-26707518.jpg


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Mathias




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PostPosted: Feb 11, 2023 00:09    Post subject: Re: Semi-spherical exterior, radiating interior stone  

Hi doesn't this fossil shelll look similar?


Screenshot_20230211_060600_Samsung Internet.jpg
 Description:
 Viewed:  8428 Time(s)

Screenshot_20230211_060600_Samsung Internet.jpg


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crowinfocus




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PostPosted: Feb 11, 2023 00:14    Post subject: Re: Semi-spherical exterior, radiating interior stone  

To All: I am delighted in your fervent explorations of the possibilities.

I know nothing. And the Charlie Brown peanut gallery can only offer that it was found in desert sand with nothing remotely resembling it lying about, or I would have drug that home too. My pockets were always full of rocks. I have never seen an outcropping of this nature in this area, nor have I encountered anything like it, though there are several hundred square miles of desert I have not explored.

My best observation is that it has been tumbled a lot, as in transported a great distance. All surfaces are incredibly smooth, yet densely strong enough to maintain the ridges. I have picked at it with a steel tool and never left a mark.

I do hope together you can solve this mystery.
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crowinfocus




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PostPosted: Feb 11, 2023 00:19    Post subject: Re: Semi-spherical exterior, radiating interior stone  

Yes. I found this very image during my research. BUT, the major difference is mine is completely rounded, and all surfaces, as I just posted, are absolutely smooth. Your image indicates a protrusion on the top that apparently was never on my specimen.

Thank you.
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Amir Akhavan




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PostPosted: Feb 11, 2023 06:31    Post subject: Re: Semi-spherical exterior, radiating interior stone  

https://www.mineralienatlas.de/VIEWmaxFULL.php/param/1500751758-Pyrit-strahlig-Pyrite.jpg

https://www.mineralienatlas.de/VIEWmaxFULL.php/param/1500751680-Pyrit-strahlig-in-Kalkstein-Pyrite.jpg

https://www.alamy.de/stockfoto-eisen-pyrit-knotchen-oder-konkretion-fes2-vom-strand-bei-verschleiss-ostbucht-folkestone-kent-uk-113378446.html?imageid=AC80D43F-6E9E-48D1-922A-F8BC8C1E0606&p=13110&pn=1&searchId=17fc2776fb895fa858419ee300f46b69&searchtype=0


Here are a few examples from Mineralienatlas, and one from alamy.

My own samples did all fall apart, unfortunately.
The core is grainy and tends to weather more quickly.

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crowinfocus




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PostPosted: Feb 11, 2023 10:45    Post subject: Re: Semi-spherical exterior, radiating interior stone  

Thank you for these images.

The last one from alamy strikes me as the closest to my specimen as anything I have seen, though the mineral content is not the same. My uneducated guess is that the mineral content would have everything to do with the location of origin.

Forgive my N. American inability to speak more than one language (besides a little pigeon Spanish). Could you give me more details about what the alamy image is as a classification, where it came from, what it is composed of, when it might have been formed and how in geological's name did it find its way to south Texas? If not, any clue would be of value.
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Linus




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PostPosted: Feb 11, 2023 12:26    Post subject: Re: Semi-spherical exterior, radiating interior stone  

Hi, I do think your image of mcrowrock3 shows a centralized protrusion - thus shell is a better explanation for the spherical bottom. shell or coral, either way, I still am leaning towards fossil. The limonite nature (actually probably silica + limonite due to your inability to scratch it with a steel probe) is not incompatible with fossil replacement, especially considering fluids percolating through dirty limestones and siltstones typical of southwest texas might well contain elevated iron content.
Anyway thanks to all for your comments and input, alas I have to move on to other things...
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Bob Carnein




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PostPosted: Feb 11, 2023 12:34    Post subject: Re: Semi-spherical exterior, radiating interior stone  

There have been a really amazing variety of responses to this thread. I would suggest the following: (1) measure the specific gravity (not at all difficult if you have a simple scale accurate to tenths of a gram) (see method at John Betts' website ); (2) test the streak (if any), using an unglazed porcelain tile or the unglazed ring on the bottom of a porcelain plate or teacup); (3) check the hardness again. If the streak is colored (most likely rusty yellow or rusty red), the hardness should be lass than that of a steel knife blade (but remember that knives are made of different kinds of steel, so their hardnesses vary). Always try the hardness "both ways"--try scratching the mineral with a knife and turn the test around and try to scratch the knife with the mineral.

That said, this looks to me like an iron-oxide concrettion replacing pyrite. It would really help to break it in half and see what the material looks like on a fresh surface. Radial pyrite concretions are not rare; they often form around organic material in sediments. The organic material doesn't always leave a trace (it could be rotting soft parts buried in the sediment). If this is an iron-oxide concretion, it is not at all liiely to have survived transport for any significant distance.
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