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Pete Richards
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Joined: 29 Dec 2008
Posts: 828
Location: Northeast Ohio
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Posted: Dec 31, 2020 15:56 Post subject: Pyrite with bubble-like growths |
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For years I have been puzzling over some pyrite crystals found in two quarries in Ohio. They have sharp but unusual morphologies involving major trapezohedral and diploidal faces, but most curious is that rarely there are what look like little drops of water on the faces. These drops are composed of pyrite, and we have determined that it is crystallographically continuous with the surrounding crystal. A few even have little crystal faces on top of the bubble. A few look like a little pyrite circle where the bubble burst, leaving just the rim. Sometimes one of these features occurs at the boundary between two or more crystals, and it forms a saddle shape bridging them, just as water might.
If anyone wants to explain these, that's great, but I have a simpler goal in mind here. Has anyone seen anything like this elsewhere? On pyrite or on any other mineral? Features that look like bubbles, made of the same mineral as they are growing on.
Mineral: | Pyrite |
Locality: | Suever Quarry, Delphos, Allen and Van Wert Counties, Ohio, USA | |
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Dimensions: | crystals are several millimeters, bubbles 0.1 mm or less |
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Mineral: | Pyrite |
Locality: | Suever Quarry, Delphos, Allen and Van Wert Counties, Ohio, USA | |
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Dimensions: | crystals are several millimeters, bubbles 0.1 mm or less |
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Mineral: | Pyrite |
Locality: | Suever Quarry, Delphos, Allen and Van Wert Counties, Ohio, USA | |
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Dimensions: | crystals are several millimeters, bubbles 0.1 mm or less |
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Mineral: | Pyrite |
Locality: | Suever Quarry, Delphos, Allen and Van Wert Counties, Ohio, USA | |
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"Wet" boundary between two crystallographically unrelated crystals. |
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Mineral: | Pyrite |
Locality: | Suever Quarry, Delphos, Allen and Van Wert Counties, Ohio, USA | |
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Dimensions: | crystals are several millimeters, bubbles 0.1 mm or less |
Description: |
A ring and a smaller bubble |
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14253 Time(s) |
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_________________ Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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Peter Megaw
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Joined: 13 Jan 2007
Posts: 963
Location: Tucson, Arizona
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Posted: Dec 31, 2020 19:39 Post subject: Re: Pyrite with bubble-like growths |
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Pete...here are two pyrrhotites from Santa Eulalia which show something similar. I think I have a galena from Naica that has one too
Happy New Year!!
Mineral: | Pyrrhotite |
Locality: | Potosí Mine, level 10, Santo Domingo (Francisco Portillo), Santa Eulalia District, Municipio Aquiles Serdán, Chihuahua, Mexico | |
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Dimensions: | 5cm across pyrrhotite |
Description: |
Peter Megaw specimen and image |
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Mineral: | Pyrrhotite |
Locality: | Potosí Mine, level 10, Santo Domingo (Francisco Portillo), Santa Eulalia District, Municipio Aquiles Serdán, Chihuahua, Mexico | |
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Dimensions: | 5cm across pyrrhotite |
Description: |
shallower bubbles
Peter Megaw specimen and image |
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Philippe Durand
Joined: 10 May 2016
Posts: 628
Location: Normandie
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Posted: Jan 01, 2021 06:21 Post subject: Re: Pyrite with bubble-like growths |
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I am not a crystallographer or scientist in this mater
so be kind if my suggestions are totally wrong :)
Could it be a non-crystallographic structure (I mean amorphous) growing on a crystallographic base ?
Or a growth of pyrite around an impurity, like a pearl in an oyster ?
Or a very fast cooling of a liquid phase at the end of crystallization ?
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Roger Warin
Joined: 23 Jan 2013
Posts: 1177
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Posted: Jan 01, 2021 08:40 Post subject: Re: Pyrite with bubble-like growths |
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Hello Pete, all,
I am thinking of late crystallization (like parallel growth type).
The freshly crystallized micro-pyrite at this time would have been in competition with a dissolution phenomenon.
In this case, at the end of the crystallization process, the kinetics of dissolution favor the disappearance of edges and corners, thus producing a globular shape.
In any case the question is very relevant and deserves reflection.
In Ukrainian beryls, dissolution causes negative cavities.
What do you think ?
Roger.
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Pete Richards
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Joined: 29 Dec 2008
Posts: 828
Location: Northeast Ohio
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Posted: Jan 01, 2021 12:06 Post subject: Re: Pyrite with bubble-like growths |
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Thanks for suggestions. Extra ideas are always welcome.
Philippe, your ideas are good ones, but I don't think they apply in this case. We studied these crystals and found that the bubbles are part of the same single crystal. This finding proves that they are not amorphous, or crystallized in layers like a pearl, a rapidly cooled liquid. Any of these would either contain many crystals in different orientations (or no crystals at all, in the case of amorphous materials), but this is not what the x-ray studies showed.
Roger, dissolution at the end of growth or after the growth period definitely attacks edges and corners first, as well as defects and cracks in the main body of the crystal. But the edges of these crystals are perfectly sharp with no rounding. Only the bubbles are rounded. And they generally have a reversed curvature where they meet the main crystal, which would be harder to explain as a result of dissolution. I attach a sketch of the cross-section of what I think of as the typical bubble.
Thanks again for thinking about these curious misbehaving crystals!
Description: |
Schematic cross section of a "typical" bubble. |
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Philippe Durand
Joined: 10 May 2016
Posts: 628
Location: Normandie
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Posted: Jan 01, 2021 13:07 Post subject: Re: Pyrite with bubble-like growths |
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Pete, another idea:
Could you have deep inside the crystal a default in the crystal lattice (an inclusion of something) which is locally deforming the crystal; and your bubble is just the outside outcome of the phenomenon ?
like an object under the carpet :)
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alfredo
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Joined: 30 Jan 2008
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Posted: Jan 01, 2021 13:37 Post subject: Re: Pyrite with bubble-like growths |
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This phenomenon is not limited to metallic minerals; I saw the same type of perfectly smooth (to the naked eye) dome on the surface of a Herkimer quartz once. I cannot speculate about a cause - No idea at all.
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Roger Warin
Joined: 23 Jan 2013
Posts: 1177
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Posted: Jan 01, 2021 15:15 Post subject: Re: Pyrite with bubble-like growths |
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I think this is not a problem. I see in this crystal a big difference in the time scale.
I therefore suppose that "long" after the first perfect crystallization and a long resting time, the initial crystal underwent a last micro supply of pyrite in parallel growth in a new supply of FeS2, under uncertain equilibrium conditions, since the redissolution was able to do its work, without resetting an attack on the edges of the large crystal already stabilized. Because it was a superficial mini-episode.
In any crystallization, crystal growth is the result of two reactions in opposite directions:
1) material deposit, 2) material removal. It is for this reason that the thermodynamically most stable sites are the seat of regular growth.
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Mathias
Joined: 01 Apr 2018
Posts: 146
Location: Vlodrop
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Posted: Jan 03, 2021 01:54 Post subject: Re: Pyrite with bubble-like growths |
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I always thought that this was a subsurface oxidation process going on, pushing up some of the material above it.
Mineral: | Galena on Fluorite |
Locality: | West Pastures Mine, Stanhope, Weardale, North Pennines Orefield, County Durham, England / United Kingdom | |
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Dimensions: | 10mm x 10mm x 5 mm |
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John S. White
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Location: Stewartstown, Pennsylvania, USA
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Posted: Jan 03, 2021 06:06 Post subject: Re: Pyrite with bubble-like growths |
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This may not be the same phenomenon but viewers may find it interesting just the same. The termination of a fluorapatite crystal from Panasqueira, Portugal. 4 cm across.
Mineral: | Fluorapatite |
Locality: | Minas da Panasqueira, Aldeia de São Francisco de Assis, Covilhã, Castelo Branco, Cova da Beira, Centro, Portugal | |
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Dimensions: | 4 cm |
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Dean Allum
Joined: 27 Jan 2010
Posts: 17
Location: Colorado
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Posted: Jan 05, 2021 22:03 Post subject: Re: Pyrite with bubble-like growths |
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Pete,
Thank you for posting this fun puzzle for us to think about over the holidays.
1) I own many spherical pyrite concretions formed in a limestone environment, nucleating on organic matter. Is there any way that these bumps could have formed from iron rich clay which later filled a cavity?
2) These bumps might be due to a complex series of events:
a) Outer surface of pyrite oxidizes to a thin layer of hematite.
b) Pinholes and popped bubble residue are formed in some locations.
c) Later source of iron and sulfide ions preferentially deposit epitaxially at a rapid rate at locations of b)
I am not happy with either of these explanations, so I hope you receive more suggestions.
-Dean
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Sante Celiberti
Joined: 04 Oct 2019
Posts: 699
Location: Tuscany
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Posted: Jan 06, 2021 08:06 Post subject: Re: Pyrite with bubble-like growths |
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Hi, Pete. How are you?
As a pyrite fan, I'm curious to know your explanation.
"Very naively" I assume a melting that affected only the sub-millimeter (autotactic?) crystals, where the degree or duration of temperature could easier work. This could also explain the occasional residual facets (when the melting was not complete).
However, I don't understand the role of crystallographic continuity which you have certainly specified for a valid reason.
Just for fun, I attach a pyrite from Niccioleta mine where the feature is "only apparently" similar to your specimen.
In fact, although the drops are the same mineral (melnikovite FeS2) of the underlying pyrite, being amorphous they cannot be in a crystallographic continuity with it.
A very warm greeting.
Sante
Mineral: | Pyrite, melnikovite |
Locality: | Niccioleta Mine, Massa Marittima, Grosseto Province, Tuscany, Italy | |
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Dimensions: | 71 x 57 mm |
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13659 Time(s) |
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Mineral: | Pyrite, melnikovite |
Locality: | Niccioleta Mine, Massa Marittima, Grosseto Province, Tuscany, Italy | |
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Dimensions: | 71 x 57 mm |
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13649 Time(s) |
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Mathias
Joined: 01 Apr 2018
Posts: 146
Location: Vlodrop
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Posted: Jan 06, 2021 16:16 Post subject: Re: Pyrite with bubble-like growths |
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I thought that pyrite won't melt, upon heating it will decompose to Iron and Sulphur at a certain temperature.
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Bob Morgan
Joined: 18 Jan 2018
Posts: 228
Location: Savannah, Georgia
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Posted: Jan 06, 2021 19:58 Post subject: Re: Pyrite with bubble-like growths |
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Could some of these structures be spiral growth hillocks with rounded edges? Such tend to develop on small cracks or irregular boundaries on the underlying surface.
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Sante Celiberti
Joined: 04 Oct 2019
Posts: 699
Location: Tuscany
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Posted: Jan 07, 2021 02:47 Post subject: Re: Pyrite with bubble-like growths |
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Mathias wrote: | I thought that pyrite won't melt, upon heating it will decompose to Iron and Sulphur at a certain temperature. |
Hi, Mathias. How are you?
You are right!
In the Scarlino plant (a few kilometers from Gavorrano mine), the largest in Europe for the production of sulfuric acid, pyrite was roasted at 750°C to obtain sulfur dioxide.
But I must admit that I don't know what can happen in a natural geological context, with different conditions of temperature, pressure and mineral fluids present.
As I said, my conjecture is "very naive". :-)
Warm greetings from Gavorrano.
Sante
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Sante Celiberti
Joined: 04 Oct 2019
Posts: 699
Location: Tuscany
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Posted: Jan 07, 2021 02:53 Post subject: Re: Pyrite with bubble-like growths |
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Bob Morgan wrote: | Could some of these structures be spiral growth hillocks with rounded edges? Such tend to develop on small cracks or irregular boundaries on the underlying surface. |
Hi Bob. I hope you are fine.
I suppose you're referring to Pete's specimen and not to mine.
Warm greetings.
Sante
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Roger Warin
Joined: 23 Jan 2013
Posts: 1177
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Posted: Jan 07, 2021 14:48 Post subject: Re: Pyrite with bubble-like growths |
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Hi, Sante, all,
The answer is complex.
In industry, sulfuric acid can be prepared by the pyrite roasting process.
In rotary kilns, pyrites are burnt in the presence of air at high temperature. SO2 (sulfur dioxide) is obtained.
After SO2 is oxidized to SO3 (sulfuric oxide), to obtain sulfuric acid with water.
In rocks (ores),
Due to the lack of oxygen, other chemical and biological processes take place (acid runoff in mines).
Attacks by bacteria are also the cause.
This bacterial activity is very old on Earth (origins of marine life near black smokers). It has been discovered on ocean ridges where it appears on manganese nodules.
These bacteria are called:
Ferrobacillus ferrooxidans (Fe2 +)
Acidithiobacillus ferrooxidans (S, Fe2 +) And many others.
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Pete Richards
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Joined: 29 Dec 2008
Posts: 828
Location: Northeast Ohio
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Posted: Jan 07, 2021 14:57 Post subject: Re: Pyrite with bubble-like growths |
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This thread has produced a lot more comments than I had expected! I feel I should respond briefly to the ones since my last comment. In order in which they were posted:
Philippe: There could be something inside the crystal, as you suggest, but usually the crystal would just grow around it, like water rising over a pebble. It would not be like "a rug over an object" at least in the sense that there would not be atomic layers of pyrite wrapped over the impurity. Had that been the case, our x-ray crystallography studies would have detected it.
Alfredo: Thanks for the comment. I did not mean to imply that this phenomenon would be confined to pyrite or other metallic minerals. I've never seen it on Herkimer diamonds; I'd love to see that some time!
Roger: yours is a complex hypothesis, but I suppose a possible explanation. I do think the "bubbles" are late-stage growths, maybe even in a second separate stage of growth. I find it hard to understand the recurved, concave outer edge of the profile as resulting from dissolution, though.
Mathias: your galena is very interesting! Looks like a slightly depressed cube face with the bumps on it. I would think that if something happened inside to "push them up", the expansion would have caused them to crack. I suspect they grew that way, and are generally similar to my bubbles.
John: Fantastic fluorapatite! To me, the surface ornamentation is suggestive of growth hillocks of the sort that Bob mentioned later, or maybe renewed crystallization at specific points. In that sense the bumps might be comparable to my bubbles, except of course that they are bounded by crystal faces, not continuous "liquid" surfaces.
Dean: I think a concretion-like process would probably produce polycrystalline bumps. Your second idea is similar to mine in that it says "something special happened in specific places to make this happen", which is clearly true! And like mine it does not completely explain what happened (and why it does not happen more often). The faces of the pyrite around the bubble are mirror sharp, which would seem unlikely if they had altered enough to form even a thin coat of hematite or goethite.
Sante: Hi! These crystals occur in an environment that has never been above 100°C, probably not above 60°. There was no melting! The role of crystallographic continuity: it is part of the enigma - a surface feature that apparently is not reflected at all in the underlying structure! As you say, your melnikovite has only an apparent similarity - it is amorphic and not crystallographically related to the pyrite - it could equally well form on a quartz crystal, or directly on a rock matrix.
Mathias: Yes, you are right. Pyrite decomposes to pyrrhotite and sulfur at about 540°C; the sulfur is presumably molten at this temperature. According to one phase diagram I saw, this becomes liquid at about 1080°. This diagram says nothing about pressure, so presumably it is for surface pressures. The phase diagram presumably assumes a closed system, so there would not be any air to "roast" the sulfur, in the manner that Sante writes of.
Bob: Growth hillocks usually form around screw defects. The ones I have seen are generally not as smooth in texture, and they are much flatter than these. The merit of the idea is that it produces a raised structure without disturbing the crystal structure correspondingly. It would not explain the examples where the bubble-like surface occurs between two crystals, looking like a film of water.
This reply contains a lot of statements saying "No, that's not it"! And I think I am correct in saying so. I hope I don't offend anyone; I really appreciate the suggestions, even if I think they are not the answer. I have been puzzling about these little bubbles for 30 years or more (they were collected in the late 1980s) and my best ideas are still not totally satisfying, even to me! Reading and considering all these responses has reminded me of some of the weak spots in my own hypotheses, so I have some work to do! Thanks to all!
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Reef
Joined: 13 Dec 2016
Posts: 36
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Posted: Jan 08, 2021 16:25 Post subject: Re: Pyrite with bubble-like growths |
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Hello everybody!
This is probably due to anti skeletal growth. Crystals of a cubic system during anti skeletal growth under ideal conditions grow in the form of a ball.
The area of each subsequent layer decreases as the growth conditions deteriorate. Accordingly, the concave outer edge of the profile is the result of growth changes.
(imho) Roger Warin correctly described gist.
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Bob Morgan
Joined: 18 Jan 2018
Posts: 228
Location: Savannah, Georgia
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Posted: Jan 09, 2021 09:21 Post subject: Re: Pyrite with bubble-like growths |
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Pete,
The 'bubbles' aren't overgrowths. Pyrite is too rigorously isometric to do something like that on the surface of sharp crystals. They have nothing to do with dissolution. The crystals would show dissolution effects.
The 'bubbles' are best explained as prior structures with the sharp crystals as overgrowths, and the 'bubbles' being what of those structures has not been overgrown.
What they were? Who knows? They remind me of goethite or hematite? Can such pseudomorph into pyrite? Perhaps the circular feature is the result of iron removed to contribute to the overgrowing pyrite crystal.
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