Post subject: Globular brownish and blueish "stones" ?
Posted: Mon May 11, 2020 5:35 pm
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Joined: Wed Jan 23, 2013 5:29 pm Posts: 1047 Location: Paris
I know of course that these are definitely no gemstones... Not even sure these are stones actually...A friend living in the Guadeloupe asked for some help in identifying strange pebbles he got in his garden. I am no mineralogist... Maybe someone among you have already seen such things ?
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Always a good idea (when dealing with nongems). If they're concretions tap lightly. They'll tend to have hollow areas and be crumblier than you'd expect
Post subject: Re: Globular brownish and blueish "stones" ?
Posted: Tue May 12, 2020 8:29 am
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Joined: Wed Jan 23, 2013 5:29 pm Posts: 1047 Location: Paris
So, I asked the question and, as you nailed it, they are definitely crumbly. Some just under the finger and the biggest ones with the help of a tool. Concretions, then. I don't know about the frequency nor the reason of these formations, but nothing very unusual I guess !
Probably the same mechanism as the formation of liesegang banding--precipitation of iron-rich minerals out of solution in pore space tends to organize into concentric bands because of local nucleation depleting the surrounding solution (this is probably also an important process in granites, per my undergraduate advisor's research). Gemologically this process is responsible for some spectacular patterning in picture jaspers (particularly the greatest of all picture jaspers, Biggs from Oregon)
Post subject: Re: Globular brownish and blueish "stones" ?
Posted: Tue May 12, 2020 6:14 pm
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Joined: Wed Jan 23, 2013 5:29 pm Posts: 1047 Location: Paris
Nice landscape !
Can we suppose that the 2 different colorations, blueish-brownish, as well in your picture as in mine, could be due to the state more or less oxydized of iron ?
The blue in blue biggs jasper is from reduced iron, yes. On the concretions it could also be hydration of iron oxides to limonite kor some recrystallization of hematite, thin films can have a blueish look.
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