FMF - Friends of Minerals Forum, discussion and message board

[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

[James Catmur]

I am glad we could help.

The shape is fairly typical, but how it got to where you found it is hard to say. Things move due to erosion or human intervention (collectors are known to dump their unwanted material in the countryside, causing all sort of confusion).

On a farm in Landenberg, PA there is a large lump of Mexican obsidian my parents left there when they moved (I collected it in Mexico in about 1973)

[crowinfocus]

May your rocks never rot!
m