Sciencemadness Discussion Board - Presence of traces iron oxide minerals in limestone

Presence of traces iron oxide minerals in limestone

Eddygp - 31-8-2013 at 10:42

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]

bfesser - 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% H2O2(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.

Eddygp - 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.

ElectroWin - 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]

plante1999 - 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.

IrC - 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.

Eddygp - 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.

bfesser - 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">

</td><td valign="middle">

</td><td>

</td></tr></table>I thought I had something more relevant to add, but I just lost my train of thought. Sorry.

IrC - 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.

bfesser - 31-8-2013 at 17:06

Thanks for the tip, IrC. I plan to carry my geiger counter on all future geology excursions—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]

Eddygp - 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.

[Edited on 2-9-2013 by Eddygp]

plante1999 - 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.

bfesser - 2-9-2013 at 14:50

Eddygp, 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?

[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...

[Edited on 2.9.13 by bfesser]

Eddygp - 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]

bfesser - 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—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]

Eddygp - 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—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.

Eddygp - 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.

bfesser - 3-9-2013 at 12:28

I wouldn't say that it "almost proves" that it's calcite, merely that it could be 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?

Eddygp - 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 could be 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]

bfesser - 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]

Eddygp - 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]

bfesser - 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?

Eddygp - 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.

12AX7 - 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.

Tim

Eddygp - 7-9-2013 at 12:17

So here they come, three photos from different angles.

12AX7 - 7-9-2013 at 14:24

Can you take any bigger photos or zooms of the texture of the dark stuff?

Eddygp - 8-9-2013 at 00:13

Yes. I will attach them to this post. I advance that tiny transparent crystals are within this dark massive mineral (iron-impurity vaterite? don't think so, too rare

) and those might be the clue to find out what this really is.

bfesser - 8-9-2013 at 05:49

Quote: Originally posted by Eddygp

Yes. I will attach them to this post.

Did you forget?

Anyway, that's a damn interesting specimen. Could you describe the geology of the area or even the formation where you found it?

[edit]I keep forgetting to ask: Is it limestone or dolostone? An acid test with dil. HCl would differentiate the two.

[Edited on 8.9.13 by bfesser]

Eddygp - 8-9-2013 at 10:57

I did forget, sorry. The rock dissolves even in acetic acid, so I suppose that it will dissolve in HCl too. So well, I've attached the photo here instead. Hope it's useful. Regarding the place, it is a sedimentary rock area near a K-T clay deposit. The soil is mostly clay, in a warm-to-hot, dry area.

EDIT: That photograph is apparently the most interesting part of this rock, as you can see the transparent crystals and the barrier between the coloured and white parts of the specimen.

[Edited on 8-9-2013 by Eddygp]

bfesser - 8-9-2013 at 17:59

Don't you have HCl to do tests with? The photo's not as useful as I'd hoped (compression & lighting), but it's still neat. Thanks for uploading it. Forgive my ignorance, but I've never heard of "K-T clay." Are you referring to what used to be called the <a href="http://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_boundary" target="_blank">K–T boundary</a> <img src="../scipics/_wiki.png" />, by chance?

Also, if you don't mind (anonymity), could you provide a specific location for where you collected the specimen? It's difficult to impossible to do amateur arm-chair geology without knowing where exactly virtual specimens are from. Without knowing the geology of the area, it's like playing a game of <a href="http://en.wikipedia.org/wiki/Twenty_Questions" target="_blank">Twenty Questions</a> <img src="../scipics/_wiki.png" />...

Eddygp - 9-9-2013 at 06:33

Quote: Originally posted by bfesser

Don't you have HCl to do tests with? The photo's not as useful as I'd hoped (compression & lighting), but it's still neat. Thanks for uploading it. Forgive my ignorance, but I've never heard of "K-T clay." Are you referring to what used to be called the <a href="http://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_boundary" target="_blank">K–T boundary</a> <img src="../scipics/_wiki.png" />, by chance?

Also, if you don't mind (anonymity), could you provide a specific location for where you collected the specimen? It's difficult to impossible to do amateur arm-chair geology without knowing where exactly virtual specimens are from. Without knowing the geology of the area, it's like playing a game of <a href="http://en.wikipedia.org/wiki/Twenty_Questions" target="_blank">Twenty Questions</a> <img src="../scipics/_wiki.png" />...

Yes, I was referring to the K-T boundary, a clay deposit from that time (according to some webpages). Sorry for the mix up. I'll send you a PM with the Mindat location.

Eddygp - 13-9-2013 at 04:07

Mini-bump. Not really a bump, just a mini-bump.

bfesser - 13-9-2013 at 04:21

Sorry, I've been busy with other projects. Have you made any progress on the chemical tests or to determine the suspended solid?

Eddygp - 15-9-2013 at 09:59

When reacting with HCl, it becomes orange in solution (probably Fe ions?) and effervesces totally. Actually, there is a light brown precipitate, so maybe the colour is not just in solution. Nothing unexpected. I will edit this post if there are any new things going on after a while.

Eddygp - 16-9-2013 at 11:22

Changes:
The precipitate has completely settled down. There is a fine suspension of this precipitate in the solution, but apart from that, the solution's colour is just off-white, not creamy though, just a little coloured. This might be because of a different impurity.
Fe₂O₃ is highly soluble (reacts) in HCl, so the ppt. is not iron(III) oxide. However, what else might it be, a light brown to orange-ish ppt. which is insoluble in HCl? I do not have any flocculants available so I cannot get the suspension to settle at the bottom. However, the nature of this ppt. is highly intriguing. Any guesses?

12AX7 - 16-9-2013 at 17:56

What pH are we talking? Fe(OH)3 drops out above 3 or so; it's quite acidic itself. It also dissolves slowly if it's much bigger than colloidial.

Drop in a whole bunch more, give it more calcite to suck on? Then see if anything drops out from the raised pH?

Tim

Eddygp - 17-9-2013 at 07:14

Well I added 21% HCl in this beaker with about 1.2 grams of the mineral, so... well.

Eddygp - 19-9-2013 at 09:54

Observation after 4 days:
All the impurities have precipitated off solution giving a colourless solution and a powdery orange (not brownish, pure orange) precipitate. I've no idea... what is it? I'll read up a bit on transition metals' oxides and halides to see what it is.

12AX7 - 20-9-2013 at 14:59

Cu2O and Fe(OH)3 would fit the bill; can't think of any others? (V2O5 maybe, but that obviously would behave differently. And Cu2O itself would at least partly oxidize in solution, giving something green-stained, not orange.)

Tim

bfesser - 20-9-2013 at 15:10

Cu2O is red, not orange. I agree that it's likely an iron oxide, hydroxide, or oxide/hydroxide. Determining exactly which might prove difficult.

<a href="https://en.wikipedia.org/wiki/Iron_hydroxide" target="_blank">Iron oxide</a> <img src="../scipics/_wiki.png" />

Eddygp - 21-9-2013 at 00:22

Quote: Originally posted by bfesser

Cu2O is red, not orange. I agree that it's likely an iron oxide, hydroxide, or oxide/hydroxide. Determining exactly which might prove difficult.

<a href="https://en.wikipedia.org/wiki/Iron_hydroxide" target="_blank">Iron oxide</a> <img src="../scipics/_wiki.png" />

These would dissolve in the HCl, AFAIK.

12AX7 - 22-9-2013 at 21:40

Quote: Originally posted by bfesser

Cu2O is red, not orange.

Actually it can be as bright as yellow -- this is seen electrolyzing a copper anode in a concentrated NaCl solution. Initially, nearly colloidial Cu2O precipitates, with a refreshingly yellow to golden color (there's probably some "quantum dot" action at work). So it depends on precipitation method and rate: the classic method with Fehling's solution yields a dull brick red, for example. But I don't think normal precipitation methods (i.e., quenching a Cu(I) solution in base) yield anything brighter than dull orange, so you are right for the most part.

Tim

Eddygp - 29-9-2013 at 02:49

Cu oxides would also dissolve in acid.

Eddygp - 25-1-2014 at 08:00

Reason for the necroing: I have found iron minerals (not trace impurities, minerals by themselves) from the exact same area.

They are reddish brown with orange tinges and when dissolved in HCl, they form a yellow solution, indicating Fe (unless I have found an awesome mining site, which I don't think so). Their crystal habit is massive. They are (locally) very common.
Here is the problem. I looked at possible iron oxide minerals and saw a possible candidate: maghemite. However, maghemite is strongly ferromagnetic and this mineral... was not. Actually, it probably is diamagnetic.

Eddygp - 30-1-2014 at 14:06

EDIT: similar massive formations of this mineral have been sighted attached to large sedimentary rocks (calcium carbonate and other calcium rocks). Brownish to dark orangeish, just like rust (couldn't edit, doubleposting, read last post for this to make sense), which led me to think about that mineral. Locally very common.

bfesser - 30-1-2014 at 14:35

Quote: Originally posted by Eddygp

Reason for the necroing: I have found iron minerals (not trace impurities, minerals by themselves) from the exact same area.

Have you collected any specimens? Also, photos of both the specimens and the outcrops where they've been collected from would be great.

Eddygp - 9-2-2014 at 11:13

Photos 1&2. THIRD attempt to post.
In a RAR.

Attachment: MineralPhotos1.rar (1.8MB)
This file has been downloaded 313 times

[Edited on 9-2-2014 by Eddygp]

Eddygp - 9-2-2014 at 11:14

Photos 3&4.
In a RAR.

The fifth photo is too large to upload. I'll crop it within the next few hours.
Attachment: MineralPhotos2.rar (1.5MB)
This file has been downloaded 341 times

[Edited on 9-2-2014 by Eddygp]

Eddygp - 10-2-2014 at 12:23

The photos are available in this science madness FTP server.
http://www.scimad.org/users/Eddygp/Agost%20047.JPG

bfesser - 10-2-2014 at 13:18

Quote: Originally posted by Eddygp

Photos 1&2.

Quote: Originally posted by Eddygp

Photos 3&4. … The fifth photo…

Eddygp - 13-2-2014 at 12:24

Thank you, bfesser!
@Everyone else; you have the photographs there, reduced to a suitable size.

Eddygp - 21-2-2014 at 14:21

Can anyone help me out? Any tests that I could perform? The colour of the products when the mineral and HCl are mixed is yellow, indicating a probable Fe origin.

Eddygp - 1-3-2014 at 07:27

Mini-bump.