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Unusual Circular Pattern on Goethite
  
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Joe Haigh




Joined: 29 Dec 2016
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PostPosted: Dec 29, 2016 17:46    Post subject: Unusual Circular Pattern on Goethite  

Hi all,
I am a geology enthusiast from Yorkshire, uk, and have a particular interest in the minerals of the North of England, the Isle of Skye in Scotland and Cornwall.
Whilst visiting Botallack mine, near St Just, Cornwall, this summer, I collected a piece of geothite which I split into two pieces. On each piece there are tiny - and to me inexplicable - sets of circular marks on the freshly broken faces. The marks on the two pieces are matching. Could anybody suggest an explanation for the marks? I have attached a variety of images of different sizes, I apologize for the poor quality of the close-ups; they were taken with a digital camera through a binocular viewer.
Many thanks, Joe Haigh.



DSCN0001.JPG
 Mineral: Goethite
 Locality:
Botallack Mine, West Wheal Owles, St Just District, Cornwall, England, United Kingdom
 Dimensions: 35x30x27(mm)
 Description:
one of the halves of the specimen.
 Viewed:  1191 Time(s)

DSCN0001.JPG



Capture.JPG
 Mineral: Goethite
 Locality:
Botallack Mine, West Wheal Owles, St Just District, Cornwall, England, United Kingdom
 Dimensions: 35x30x27(mm)
 Description:
specimen with one set of the marks highlighted.
 Viewed:  1194 Time(s)

Capture.JPG



DSCN0006.JPG
 Mineral: Goethite
 Locality:
Botallack Mine, West Wheal Owles, St Just District, Cornwall, England, United Kingdom
 Dimensions: Field of View: 10mm
 Description:
1st set of marks under binocular viewer.
 Viewed:  1195 Time(s)

DSCN0006.JPG



DSCN0009.JPG
 Mineral: Goethite
 Locality:
Botallack Mine, West Wheal Owles, St Just District, Cornwall, England, United Kingdom
 Dimensions: Field of View: 10mm
 Description:
2nd set of marks under binocular viewer. There are more sets of marks although all are very similar.
 Viewed:  1189 Time(s)

DSCN0009.JPG


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Bob Harman




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PostPosted: Dec 29, 2016 19:35    Post subject: Re: Unusual Circular Pattern on Geothite  

Frankly, I have no idea what those circular areas on the crystal surfaces represent, but often times when I field collect calcite crystals I see small somewhat similar crystal surface disturbances.

The calcite crystals seem to have localized surface disturbances secondary to the physical action of the collection. They are less than very superficial dings or chips, just being a "surface disturbance". Could this be what they represent???? BOB
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kushmeja




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PostPosted: Dec 29, 2016 20:46    Post subject: Re: Unusual Circular Pattern on Geothite  

It's probably from a nodule of mineralization that was spherical and was grown over at a later point and encapsulated. Probably just a bit of goethite that formed early on in the process, but could be another similar mineral.
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Pete Richards
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PostPosted: Dec 29, 2016 21:12    Post subject: Re: Unusual Circular Pattern on Geothite  

Goethite often forms as "cave formations" - stalactites and stalagmites - which have a concentric banding structure that is often visible in cross section, even after the individual formations get merged together into a more massive deposit. This may be what you are seeing, in cross section. The more-or-less round faceted shapes also suggest breakage across a cylindrical structure.
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Collecting and studying crystals with intersting habits, twinning, and epitaxy
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Joe Haigh




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PostPosted: Dec 30, 2016 05:16    Post subject: Re: Unusual Circular Pattern on Goethite  

Thank you all for the replies, this has been a great help.
Joe.
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Riccardo Modanesi




Joined: 07 Nov 2011
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PostPosted: Dec 30, 2016 12:04    Post subject: Re: Unusual Circular Pattern on Goethite  

Hi to everybody!
May I suggest one of my "adventures"? It could be an inclusion of a radioactive mineral too, thus creating some tension circular surfaces! Any possibility is open, just a careful analysis could solve it!
Greetings from Italy by Riccardo.

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Hi! I'm a collector of minerals since 1973 and a gemmologist. On Summer I always visit mines and quarries all over Europe looking for minerals! Ok, there is time to tell you much much more! Greetings from Italy by Riccardo.
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Dale Hallmark




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PostPosted: Dec 30, 2016 15:12    Post subject: Re: Unusual Circular Pattern on Goethite  

So I guess that would make it a pleochroic halo....didn't think of them in the macro sense but I guess it could be...maybe the beginning of metamictation.

Interesting whatever it is.
Dale




Riccardo Modanesi wrote:
Hi to everybody!
May I suggest one of my "adventures"? It could be an inclusion of a radioactive mineral too, thus creating some tension circular surfaces! Any possibility is open, just a careful analysis could solve it!
Greetings from Italy by Riccardo.
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alfredo
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PostPosted: Dec 31, 2016 14:53    Post subject: Re: Unusual Circular Pattern on Goethite  

If this this rock was broken where there was once a very narrow crack (and when you hammer on a rock it usually breaks first some place where there was a very narrow or incipient crack) then the other side of the crack is now gone, so we're looking at a surface that had no open space for anything like a hemisphere growing on it. IF that was the case here, then the rings could represent an advancing precipitation front, similar to Liesegang rings: http://irna.lautre.net/Tout-ce-que-la-nature-ne-peut-pas,150.html
Mostly you see these in porous material like sandstone, but the same chemical reactions can happen in very narrow cracks too, and the resulting precipitates can be iron oxides, clay minerals, or others.
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