Two strange rocks I found outside

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]

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.

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

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

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!!

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!!