FMF - Friends of Minerals Forum, discussion and message board

[crowinfocus]

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.

[Amir Akhavan]

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.

[crowinfocus]

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.

[Linus]

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...

[Bob Carnein]

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.

[Jordi Fabre]

[crowinfocus]

Thanks for joining in. I really appreciate your time and education.

Iron is not a possibility in that there is zero magnetic attraction. That is the first thing I tried.

So the quest continues.

Happy hunting!
Michael

[Bob Carnein]

[crowinfocus]

Thank you for the suggested ways of determining the hardness. I'm not good with math in figuring densities, but I'll watch the tutorial and give it a try. I have a better understanding of knives.

I eliminated the possibility of any iron composite in that there is zero magnetic attraction. Unless there is something I missed in my high school chemistry class...

Since whatever it is has survived this long mostly intact, I will not desecrate it in the name of human curiosity. It is a joy to behold just as it is. But a small scratch isn't too much to ask!

[crowinfocus]

Well that tid bit of magnetic information is a real help.

That leaves me totally lost as to what it is--again!

[Amir Akhavan]

Sorry, I've sent you an alamy linkin German, not English.
Here it is:
http://www.alamy.com/stock-photo-iron-pyrite-nodule-or-concretion-fes2-from-beach-at-east-wear-bay-113378446.html?imageid=AC80D43F-6E9E-48D1-922A-F8BC8C1E0606&p=13110&pn=1&searchId=fa898e3b36a349efde5f0df131b75b68&searchtype=0
(link normalized by FMF)

The substance of what you got is pretty obvious. It is either goethite (limonite, a mixture of iron oxides-hydroxides, yellowish brown streak) or hematite (iron oxide, red streak) at the surface, and maybe there is some pyrite (iron sulfide) left inside.
They all have approximately the same density of 5-6, so they are noticably heavier than a quartz pebble of the same size.

>> If this is an iron-oxide concretion, it is not at all likely to have survived transport for any significant distance.

Maybe I misunderstand the sentence, but limonite and hematite concretions are actually quite tough and frequently found in gravel pits. I've once found a largely undamaged 5cm wide pyrite nodule in a gravel-covered dry river bed in Italy.

[alfredo]

I agree with the several members who think this is an oxidized pyrite concretion, which often form such spheres with radial internal structure, hosted in sedimentary rocks.

[crowinfocus]

Much reading and viewing images has left me with an information overload (at 72) that I am not sure what to do with. My uneducated assessment is that as a result of a streak test, which yielded a dark reddish brown and no hint of yellow-brown, this specimen is a hematite with a compact mass which is found in west Texas, and none indicated in south Texas. Transportation to this location would explain why even the interior radiating ridges are quite smooth. This is entirely possible considering Zapata is in the immediate Rio Grande River basin.
The Generalized Geological Map of Texas places the original development of this stone in the Eocene Tertiary period, now known as the Paleogene Period, between 55.8 to 33.9 ma.
However, everything I think may be wrong!
The iron pyrite concretion image you provided does appear closest to my rock than anything I have viewed.
Beyond all of that, my question is, why is it shaped as it is? Being spherical tends to eliminate the possibility of being fossilized coral such as the suggested dohmophyllum Rugosa Cnidaria which is found no closer than northern Canada.
Suggestions? Am I even close to being on the right track?!

[James Catmur]

It does seem that the answer is a Hematite nodule.

We cannot be certain how it formed but it was a long slow process.

This might be what happened: Organic material contains Sulfur and as it rots this combines with iron in the water to create the mineral Pyrite. This starts to grow from the center and attracts more iron and sulfur as it grows to form a roughly spherical Pyrite nodule. Over time the nodule oxidised from Pyrite to Hematite and got broken (it is possible that the Pyrite nodule was subject to 'Pyrite rot' so split - yes rocks can rot and we worry about this as it destroys our specimens!). The nodule then weathered out from the rock where it formed and transported to where you found it.

[crowinfocus]

Thank you very much for that explanation. There will always be mysteries I will never solve: why this shape, and why here? Not as definitive an ending as I had hoped for after all these years, but Life is like that, and isn't that the answer after all?
I remain open to any other information, and SINCERELY APPRECIATE all who generously contributed to this thread.
Michael