FMF - Friends of Minerals Forum, discussion and message board

[yash]

Hi!
I found this stone while digging at 3m depth into a hill between a house and a road (which doesn't rule out being brought from elsewhere) surrounded by forest (part of a limestone mountain range that's rich in iron). I could find only this one. Though it looks as it should be a sort of hematite, some of it's properties do not check out:
Magnetic: no, not even when heated
Conductive: no (checked with electrical meter - maybe i should have tried higher currents?)
Hardness: 5-6 (scratches steel kitchen knife but not a harder steel knife)
Streak: brown
Specific gravity: 5-5.5
Color of scratched surface is the same
Smell: none
Taste: none
Color: as in the pictures - dark dull submetallic brown with reddish subtones
Length of specimen: 2.7cm

Any suggestions/help would be appreciated :)

[SteveB]

An odd one, it has visual characteristics of common aerolites but those characteristics are only found in iron based and so would be magnetic which yours failed. I would guess in that case it's a concretion instead. These are often magnetic but not always and often pebble sized have that dark shiny smooth exterior appearance.often found close to man made activity and construction where dusts and metal oxides find there way into the surrounding environment and dissolved by rains, leach into the ground and form around something like how a pearl forms around a bit of grit in an oyster. Others will offer the opinion its slag of some sort but I don't see anything to indicate that.

[kushmeja]

It looks like Goethite, which is an iron oxide mineral that matches up with the hardness & streak.

[SteveB]

[yash]

Thanks for the reply! Do you suppose I could find out more about it if I break it open?

[kushmeja]

[SteveB]

Thanks, my understanding was opposite, my experience with goethite is poor.

Breaking it open I don't recommend, ever really. Just personally I object to it is all. Its not likely to prove definitively what it is, then you still have no answer and a broken specimen. Nothing externally indicates a broken piece will reveal a fantastic interior. Properly cut in half and polished might be interesting or not.

Is it looking exactly as you found it? Or have you cleaned it much? High iron content usually rusts a lot when exposed to the air. And how you clean it effects that.

It should be asked what sort of magnet did you test with? My iron rich specimens I collected using magnets so I guess my experience is biased against non-magnetic iron variants.

[Bob Carnein]

How accurate is your SG measurement? It's good for hematite but too high for goethite. Is the streak reddish brown or yellowish brown (which?)? If reddish, it's probably hematite; if yellowish, I'd think goethite is most likely and would check the SG again. I don't think magnetism (or a lack thereof) is a reliable property here.

[R Saunders]

You wrote, Hardness: 5-6 (scratches steel kitchen knife but not a harder steel knife). While I have not done this I thought you try to scratch a line on the stone or mineral with a knife blade. Not make a scratch line on a knife blade? Which way is correct? Thanks

[Pete Richards]

[yash]

@SteveB: yes, it is as i found it. I only used a toothbrush and water because all the dirt fell right off. Except for the 2 scratch marks i made to check if the outside color is from patina and to be sure of conductivity. The photos were taken 8 days after discovering and washing but it doesn't seem to have rusted (even the rust-like pores aren't changing in any way). I should try putting it into salty water for a day, see what happens.
I used a speaker magnet of which i'm not sure if it's strong enough, though it works well with other iron objects. I'll try to find a stronger one.

@Bob Carnein: scale estimated 8.4g and my mini measuring cup 1.6ml = SG of 5.25, so i gave them some room for error
The streak is a reddish brown so all points to hematite? How strong of a magnet should i get or how much must i heat the specimen for it to become magnetic?
Is antiferromagnetism also an option for hematite?

@R Saunders: i tried it both ways and the result was the same. It also left a scratch on glass (at least the type i had at hand).

[Pete Richards]

This looks like a piece of a weathering rind from a stone that has sat in the soil where it is repeatedly wet with rainwater filtering down and then dried. I can't give details about the kind of stone, but I have seen such pebbles (or parts of them) with a powdery zone inside and then more or less altered rock in the center. My guess is that repeated wetting and drying has led to the deposition of iron oxides sourced both from the rock and from the surrounding deposit.

If this is anywhere near correct, we are not really dealing with a single mineral. It is likely to be composed of several iron oxides and hydroxides, maybe some manganese minerals thrown in for good measure, and some grains of sand or silt of unknown composition frozen into the chemically-deposited rind. Standard tests such as have been tried would not be very reliable because they are designed for identifying single minerals (mostly single crystals), whereas this is more like a rock with different constituents with different properties.

The process that forms such materials is quite different from concretion formation, as suggested earlier, in that the latter occurs early in diagenesis of soft sediment that will become a sedimentary rock, while the former occurs during the weathering of rock fragments in a terrestrial environment. For us, this is usually in thick glacial deposits like valley fills and moraines.

[yash]