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Author: Subject: Presence of traces iron oxide minerals in limestone
Eddygp
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[*] posted on 31-8-2013 at 10:42
Presence of traces iron oxide minerals in limestone


I have been observing near where I live many CaCO3 rocks with what looks like alpha-Fe2O3 strips inside. Breaking a small rock with this pattern at each side of the strip, i have noticed a layer of colourless crystals on each side of this formation. These crystals are insoluble in water, non-reactive with acid and apparently orthogonal. I am thinking about how to test it for sulphates (bearing in mind that a BaCl2 solution can be useless because of the insoluble nature of this crystal), silicates or even oxides, because most wet methods may be inefficient.

So three main questions in mind:

1. - Does anyone know why this α-Fe2O3 formation is present always in the same way in limestone?
2. - Might it be Fe3O4 instead? (doubtful, because of the reddish colour)
3. - Can anyone suggest a test for anions and cations for this crystal layer, or have a guess?

Photos soon.


[Edited on 31-8-2013 by Eddygp]




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[*] posted on 31-8-2013 at 11:08


What you describe sounds to me like <a href="http://www.mindat.org/min-26645.html" target="_blank">manganese dendrites</a> <img src="../scipics/_ext.png" />; commonly found in limestone. A simple test with 3% H<sub>2</sub>O<sub>2</sub>(aq) should distinguish them.

<img src="http://www.mindat.org/photos/0702938001268566787.jpg" width="400" />
(color varies from light brown to black)

I have a couple specimens in my collection; one purchased, one found. I'll try to get some photos on a sunnier day.




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[*] posted on 31-8-2013 at 11:27


No, I'm quite sure that it is not a dendritic mineral. It looks like many iron oxide crystals have formed inside some sort of groove in the rock, or two different iron-coated limestone rocks have crushed together down in the crust.



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[*] posted on 31-8-2013 at 11:58


if these crystals are silicates, they will dissolve in or at least be etched by hot conc. NaOH or KOH. but so will some other stuff like alumina and zinc.

HF is another thing to try, if you're careful.

you might try digging one out and doing a flame test.
HCl will dissolve the calcium carbonate holding it in place.



[Edited on 2013-8-31 by ElectroWin]
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[*] posted on 31-8-2013 at 12:02


Download survival chem here:

http://hclo3.weebly.com/others.html

The bead test rocks, and then the silver-sulphur test is a good choice.

Get some monosodium phosphate and borax.




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[*] posted on 31-8-2013 at 13:16


Excellent work plante. He might read these books:

http://books.google.com/books?id=iF7wAAAAMAAJ&pg=RA1-PA7...

http://books.google.com/books?id=1RwlAQAAIAAJ&pg=SL4-PA4...

Water (bearing minerals) flowing through limestone commonly leaves iron deposits. If I had to guess Fe is most likely what is being seen. Don't overlook Manganese as well.





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[*] posted on 31-8-2013 at 13:41


Thank you! So it can be iron oxide due to some sort of reaction with the limestone (which left those colourless crystals as a result, probably?) while flowing diluted in water. Interesting books there, IrC.
I'm checking that page right now, plante. Let's see what I can do with these crystals once I get them again as I have lost my previous sample. Tomorrow I will begin to analyse those minerals.




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[*] posted on 31-8-2013 at 14:04


Iron does all sorts of interesting geochemical tricks when in solution.

Here are some photos I took of iron staining in a porous calcite-cemented quartz sandstone in Interstate State Park, MN.
<table><tr><td valign="middle">DSCN0856.jpg - 544kB</td><td valign="middle">DSCN0857.jpg - 584kB</td><td>DSCN0860.jpg - 853kB</td></tr></table>I thought I had something more relevant to add, but I just lost my train of thought. Sorry.




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[*] posted on 31-8-2013 at 14:56


Next time you go bfesser take that counter. The best Carnotite rocks I have look nearly identical to the rock in the third pic. In the other thread I forgot to mention sandstone, I think I just used the general term sedimentary rock.




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[*] posted on 31-8-2013 at 17:06


Thanks for the tip, <strong>IrC</strong>. I plan to carry my geiger counter on <em>all</em> future geology excursions&mdash;even if only to map background CPM with GPS. :)

[edit] Being a MN State Park, I couldn't collect specimens, of course. But it would be neat to find them, regardless.

[Edited on 1.9.13 by bfesser]




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[*] posted on 2-9-2013 at 13:51


If I dissolve this sample in HCl and it did contain iron(III) oxide, this would also dissolve yielding a solution of everything... and losing the possibility of getting the compound.

EDIT: Attached a photo to this post. It's just a small sample of a big white rock with that brownish-reddish line from side to the other as well as some other minor ones.


20130902_235231.jpg - 94kB

[Edited on 2-9-2013 by Eddygp]




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[*] posted on 2-9-2013 at 14:44


First try the phosphate/borax bead test, if inconclusive, try wet chemistry thiocyanate test.

Don't forget the silver-sulphur test!

Tell us the results please.




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[*] posted on 2-9-2013 at 14:50


<strong>Eddygp</strong>, it's difficult to make out anything in that photo. Try getting stronger illumination and setting your camera to <a href="https://en.wikipedia.org/wiki/Macro_photography" target="_blank">macro</a> <img src="../scipics/_wiki.png" /> mode.

[edit] I tried tweaking the levels a bit in Photoshop. How's this in terms of color reproduction? Is it close to actual?

tweaked.jpg - 226kB

[second edit] Strictly from the photo; it looks a lot like iron-stained calcite (see second attached photo) to me. Have you tried an acid test on the mineralized zone? Macro photos would be extremely helpful, though...

2vpagodacalcite61213.JPG - 50kB

[Edited on 2.9.13 by bfesser]




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[*] posted on 3-9-2013 at 01:51


That colour is exactly the same one I see. I'll take a photograph of the actual mineral face, looks like it broke off the rock on that part. I was taking photos with my mobile phone, so well... I guess I'll get the camera out.

EDIT: I found something that might be useful: http://en.wikipedia.org/wiki/Vein_(geology) It probably is what I'm looking for, and calcite does that too. My bad, it actually looks like iron-stained calcite... I guess I thought it was too easy to be true and discarded it as a possibility.

[Edited on 3-9-2013 by Eddygp]




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[*] posted on 3-9-2013 at 06:08


Don't be discouraged, it could be something else; wait until you've conducted some conclusive tests/analyses. I was merely hypothesizing based on a simple visual observation. However, calcite is going to be the most common mineralization you find in limestone/dolostone, and Fe-staining is also quite common. A close-up photo of the crystals would help&mdash;and pretty pictures are always appreciated. Do you have other specimens (enough to ship off one or two for independent analyses)?

Relevant Terms:
<a href="https://en.wikipedia.org/wiki/Calcite" target="_blank">Calcite</a> <img src="../scipics/_wiki.png" />
<a href="https://en.wikipedia.org/wiki/Dolostone" target="_blank">Dolostone</a> <img src="../scipics/_wiki.png" />
<a href="https://en.wikipedia.org/wiki/Druse_(geology)" target="_blank">Druse</a> <img src="../scipics/_wiki.png" />
<a href="https://en.wikipedia.org/wiki/Limestone" target="_blank">Limestone</a> <img src="../scipics/_wiki.png" />
<a href="https://en.wikipedia.org/wiki/Mineralization_(geology)" target="_blank">Mineralization</a> <img src="../scipics/_wiki.png" />
<a href="https://en.wikipedia.org/wiki/Vein_(geology)" target="_blank">Vein</a> <img src="../scipics/_wiki.png" />
<a href="https://en.wikipedia.org/wiki/Vug" target="_blank">Vug</a> <img src="../scipics/_wiki.png" />

[edit: added terms]

[Edited on 3.9.13 by bfesser]




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[*] posted on 3-9-2013 at 07:13


Quote: Originally posted by bfesser  
Don't be discouraged, it could be something else; wait until you've conducted some conclusive tests/analyses. I was merely hypothesizing based on a simple visual observation. However, calcite is going to be the most common mineralization you find in limestone/dolostone, and Fe-staining is also quite common. A close-up photo of the crystals would help&mdash;and pretty pictures are always appreciated. Do you have other specimens (enough to ship off one or two for independent analyses)?

Relevant Terms:
<a href="https://en.wikipedia.org/wiki/Calcite" target="_blank">Calcite</a> <img src="../scipics/_wiki.png" />
<a href="https://en.wikipedia.org/wiki/Dolostone" target="_blank">Dolostone</a> <img src="../scipics/_wiki.png" />
<a href="https://en.wikipedia.org/wiki/Druse_(geology)" target="_blank">Druse</a> <img src="../scipics/_wiki.png" />
<a href="https://en.wikipedia.org/wiki/Limestone" target="_blank">Limestone</a> <img src="../scipics/_wiki.png" />
<a href="https://en.wikipedia.org/wiki/Mineralization_(geology)" target="_blank">Mineralization</a> <img src="../scipics/_wiki.png" />
<a href="https://en.wikipedia.org/wiki/Vein_(geology)" target="_blank">Vein</a> <img src="../scipics/_wiki.png" />
<a href="https://en.wikipedia.org/wiki/Vug" target="_blank">Vug</a> <img src="../scipics/_wiki.png" />

[edit: added terms]

[Edited on 3.9.13 by bfesser]


Yes, it probably is a druse, certainly a vein. I'm not sure if it's worth it to ship it, especially if we live far apart. However, I can get some more specimens, maybe even larger size.
I'll carry on some analysis this afternoon to see if it is a carbonate, which would hopefully discard other minor possibilities. I do not have too many chemicals at home, so I'll try to do some tests with it sacrificing some decigrams of my sample :) I'll post the results, if any, later on.




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[*] posted on 3-9-2013 at 12:24


The mysterious mineral dissolved in 9% acetic acid, with a very very thin iron(III) oxide suspension. This almost proves that it was just high-[Fe] calcite.



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[*] posted on 3-9-2013 at 12:28


I wouldn't say that it "almost proves" that it's calcite, merely that it <em>could be</em> calcite. Did you observe effervescence? What leads you to believe that the suspended material was indeed iron(III) oxide? You wouldn't happen to have some <a href="https://en.wikipedia.org/wiki/Eriochrome_Black_T" target="_blank">Eriochrome Black T</a> <img src="../scipics/_wiki.png" />, would you?



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[*] posted on 3-9-2013 at 13:41


Quote: Originally posted by bfesser  
I wouldn't say that it "almost proves" that it's calcite, merely that it <em>could be</em> calcite. Did you observe effervescence? What leads you to believe that the suspended material was indeed iron(III) oxide? You wouldn't happen to have some <a href="https://en.wikipedia.org/wiki/Eriochrome_Black_T" target="_blank">Eriochrome Black T</a> <img src="../scipics/_wiki.png" />, would you?


I did observe effervescence. I do not have Eriochrome Black T... but well, I think that calcite is the most probable mineral, due to the location, formation type and everything. If I can do anything else with my primitive equipment, tell me. I still have the suspension, so I could dissolve it in dilute HCl to yield CaCl2 and FeCl2 too, if it was the case.

Is there any other possible mineral with the same characteristics? Should I do a flame test (bearing in mind that just a little sodium would ruin the whole test)?

[Edited on 3-9-2013 by Eddygp]




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[*] posted on 3-9-2013 at 17:33


Does it exhibit the expected <a href="https://en.wikipedia.org/wiki/Mineral_cleavage" target="_blank">rhombohedral cleavage</a> <img src="../scipics/_wiki.png" /> and <a href="https://en.wikipedia.org/wiki/Birefringence" target="_blank">double refraction</a> <img src="../scipics/_wiki.png" />? How about the <a href="https://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness" target="_blank">Mohs hardness</a> <img src="../scipics/_wiki.png" />? There's no need to resort to more complex tests if you can confirm the identity of a mineral by trivial means. Also, if you have a suitable balance or scale, try determining the <a href="https://en.wikipedia.org/wiki/Specific_gravity" target="_blank">specific gravity</a> <img src="../scipics/_wiki.png" />.

[Edited on 4.9.13 by bfesser]




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[*] posted on 4-9-2013 at 01:38


Quote: Originally posted by bfesser  
Does it exhibit the expected <a href="https://en.wikipedia.org/wiki/Mineral_cleavage" target="_blank">rhombohedral cleavage</a> <img src="../scipics/_wiki.png" /> and <a href="https://en.wikipedia.org/wiki/Birefringence" target="_blank">double refraction</a> <img src="../scipics/_wiki.png" />? How about the <a href="https://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness" target="_blank">Mohs hardness</a> <img src="../scipics/_wiki.png" />? There's no need to resort to more complex tests if you can confirm the identity of a mineral by trivial means. Also, if you have a suitable balance or scale, try determining the <a href="https://en.wikipedia.org/wiki/Specific_gravity" target="_blank">specific gravity</a> <img src="../scipics/_wiki.png" />.

[Edited on 4.9.13 by bfesser]


I'm afraid that the crystals are way too small for the refraction test, as well as near-opaque, one of the most useful to determine calcite. However, I tested its Mohs hardness and to my surprise, it's around 2-3, as I was able to scratch it with a fingernail and with fluorite too. Rhombohedral cleavage.

[Edited on 4-9-2013 by Eddygp]




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[*] posted on 4-9-2013 at 04:18


It's a safe bet that you've got calcite (dolomite would be harder, at Mohs 3.5-4). You shouldn't be able to scratch it with a fingernail, though. Unless you're pressing hard enough to actually cause cleavage in the cyrstal. It would be interesting to continue on and identify your suspension, if possible. Thoughts on how to proceed? Also, do you plan to take more detailed photos?



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[*] posted on 4-9-2013 at 08:49


Yes, I'll take two more photos tomorrow, because I do not have time today. I will see what to do with my suspension, first get whatever the metal is into solution and then do some wet ionic tests.



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[*] posted on 5-9-2013 at 10:55


Pyrite in shale and slate comes to mind; I don't recall that pyrite is very common in limestone, but shale layers are common. Such iron-bearing beds could be oxidized into rust (or staining other minerals forming at the same time) quite easily.

The marked difference between layers in your specimen makes me think water infiltration along a bedding layer; this would be typical of midwestern US limestones. If this is the case, then an iron (and other stuff) bearing shale layer is unlikely, and the filler could be anything the water brought along for the ride.

I've seen aragonite in that color. Don't know how common it is in limestones, but it does occur as an alteration of calcium carbonate.

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[*] posted on 7-9-2013 at 12:17


So here they come, three photos from different angles.


Piedra 004.JPG - 258kBPiedra 007.JPG - 256kBPiedra 009.JPG - 269kB




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