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Author: Subject: Two strange rocks I found outside
Draeger
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[*] posted on 9-2-2020 at 04:20
Two strange rocks I found outside


So, I've collected some rocks from outside a while ago. I'm pretty unsure of their composition and I was wondering which general tests I could do?

The first rock seems to have some strange yellow-orange powder encapsulated in it. It also easily falls out if you scrape at it. Not really much to say other than that.

The second rock is much weirder. It consistently smelt like rotten eggs; so I'm guessing it was gassing off H2S. I wasn't really interested in chemistry when I found that rock, so I luckily googled once and found a general description of a gas that's toxic and smells like that, so I wrapped it into a bunch of towels and stuffed it into a box which I closed tight. Not really sure if I should test this one.

I can provide pictures of the first rock if you want.
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Metacelsus
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[*] posted on 9-2-2020 at 05:44


Yes, pictures would be helpful.

The second rock could be some kind of sulfide mineral (for example https://en.wikipedia.org/wiki/Siegenite which is found in a mine near you)

[Edited on 2020-2-9 by Metacelsus]




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Draeger
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[*] posted on 9-2-2020 at 06:20


Okay. Here's a picture of the first rock.


Before I take a picture of the other rock, I will probably either buy or make lead acetate strips in case it's somehow still venting off H2S or whatever it was that I smelt.

[Edited on 9-2-2020 by Draeger]
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Ubya
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[*] posted on 9-2-2020 at 12:37


if it smells of hydrogen sulfide, it is a sulfide. you don't really need to worry about the smell, even sulfur smells of something (sulphur is odorless) because it reacts with gases in the air to make H2S and SO2.

it can smell bad, but if it was producing toxic amounts of H2S, by now that rock should be totally outgassed. you are safe.

the first rock could be realgar, arsenic sulphide, or maybe a led mineral. it could also be uranium, see if it fluoresces green under uv light.





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Draeger
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[*] posted on 9-2-2020 at 13:17


Quote: Originally posted by Ubya  
if it smells of hydrogen sulfide, it is a sulfide. you don't really need to worry about the smell, even sulfur smells of something (sulphur is odorless) because it reacts with gases in the air to make H2S and SO2.

it can smell bad, but if it was producing toxic amounts of H2S, by now that rock should be totally outgassed. you are safe.

the first rock could be realgar, arsenic sulphide, or maybe a led mineral. it could also be uranium, see if it fluoresces green under uv light.

I managed to find a huge UV light just randomly laying around, so I tested the uranium theory. (Sadly) it didn't fluoresce green.

How could I further determine it's composition?
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Xenoid
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[*] posted on 9-2-2020 at 14:49


Looks like a bit of quartz/quartzite with some Fe-oxide/hydroxide staining on the fractured surface!
Not sure why you are getting so excited!
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diddi
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[*] posted on 9-2-2020 at 17:36


the one pictured does look like iron stained quartz as xenoid suggests. i would like to see a closer pic. just in case but it is likely that the gritty part at one end is just everyday hard sandstone or marl or similar. i would be excited to place this one in the garden :)



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Draeger
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[*] posted on 10-2-2020 at 05:26


Quote: Originally posted by diddi  
the one pictured does look like iron stained quartz as xenoid suggests. i would like to see a closer pic. just in case but it is likely that the gritty part at one end is just everyday hard sandstone or marl or similar. i would be excited to place this one in the garden :)

My camera doesn't really allow for any more zoom than that without losing the ability to focus, but I could scrape off some of the yellow stuff and put it in a petri dish to look at it under my microscope, if that helps.
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diddi
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[*] posted on 10-2-2020 at 12:43


the presence of iron in quartz is extremely common and it looks just like one would expect, so i don't think there is really need to investigate much further on that rock imo. it looked like there was some grainy material on one edge of the specimen and that was more the part i would possibly have looked at further. that said, examination under 20-50X magnification is very useful for ID of many minerals. i reserve the microscope for photographing nice crystals.



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MrHomeScientist
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[*] posted on 11-2-2020 at 09:06


I agree with quartz. The yellow could be iron-staining, or maybe just dirt trapped in some fractures since you said it powders easily. Iron staining is more of a thin coating. I clean my quartz by soaking in an oxalic acid solution for several days, which dissolves all of the iron.

Pictures would be very helpful. The second rock sounds like a sulfide, but we need more information. Color, luster, hardness, etc. Sulfide minerals generally look metallic.

A general idea of hardness would be easy:
1. Can you scratch the rock with your fingernail?
2. Can you scratch the rock with a penny?
3. Can you scratch the rock with a knife?
4. Can you scratch the rock with a nail?
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barbs09
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[*] posted on 11-2-2020 at 17:27


Excellent post! Given such photos and 10 different geologists, you will get 10 different opinions :)

I’m surprised only one smells as they look like they are part of the same rock. Are they roughly cylindrical-they look like they could be broken pieces of diamond drill core??

As others have stated the surfaces exposed are predominantly quartz, and the dusty material looks like limonite, a common yellow iron oxy-hydroxide mineral. Being colloidal, it is easily deposited by meteoric/ground water along fractures therefore secondary (not formed insitu). The deep red mineral in the quartz is more interesting. I suspect it is haematite and looks to be primary i.e. co-formed with the quartz viening under likely hydrothermal conditions.

The quartz and the dusting hides the original rock type, but as others have pointed out there appears to be a clastic texture at two of the edges. Could hint at being of sedimentary origin, or a breccia or just a bit of crud that has become cemented onto the original rock more recently.

Not much to go on, but to me, the light-grey colouration of the non quartz portion looks like clay alteration. Having worked in epithermal mineralised systems, This rock looks consistent with forming in such an environment. However any system with a heat source and resulting hyrothermal fluid generation e.g. magmatic or metamorphic, could form this occurrence.

The sulphurous smell could be due to marcasite, an unstable form of iron sulphide (same formula as pyrite FeS) that breaks down under moist oxidising conditions. The acidic reaction product could react with other sulphides to form H2S?.

Therefore as always the context of where you found the rock is important! If it looks like it came from nearby, look at a local geological map. If they were bought in by someone, they obvious thought it worth their while, and it could be ore. The presence of quartz veining, sulphides and potential clay alteration would get many an exploration geologists excited!!
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[*] posted on 14-2-2020 at 15:24


Quote: Originally posted by barbs09  
Excellent post! Given such photos and 10 different geologists, you will get 10 different opinions :)

I’m surprised only one smells as they look like they are part of the same rock. Are they roughly cylindrical-they look like they could be broken pieces of diamond drill core??

As others have stated the surfaces exposed are predominantly quartz, and the dusty material looks like limonite, a common yellow iron oxy-hydroxide mineral. Being colloidal, it is easily deposited by meteoric/ground water along fractures therefore secondary (not formed insitu). The deep red mineral in the quartz is more interesting. I suspect it is haematite and looks to be primary i.e. co-formed with the quartz viening under likely hydrothermal conditions.

The quartz and the dusting hides the original rock type, but as others have pointed out there appears to be a clastic texture at two of the edges. Could hint at being of sedimentary origin, or a breccia or just a bit of crud that has become cemented onto the original rock more recently.

Not much to go on, but to me, the light-grey colouration of the non quartz portion looks like clay alteration. Having worked in epithermal mineralised systems, This rock looks consistent with forming in such an environment. However any system with a heat source and resulting hyrothermal fluid generation e.g. magmatic or metamorphic, could form this occurrence.

The sulphurous smell could be due to marcasite, an unstable form of iron sulphide (same formula as pyrite FeS) that breaks down under moist oxidising conditions. The acidic reaction product could react with other sulphides to form H2S?.

Therefore as always the context of where you found the rock is important! If it looks like it came from nearby, look at a local geological map. If they were bought in by someone, they obvious thought it worth their while, and it could be ore. The presence of quartz veining, sulphides and potential clay alteration would get many an exploration geologists excited!!


It is the same rock.
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