FMF - Friends of Minerals Forum, discussion and message board

[Sante Celiberti]

[Roger Warin]

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.

[Pete Richards]

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!

[Reef]

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.

[Bob Morgan]

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.

[Dean Allum]

Pete,
After reading this article, I have another theory for you to consider.

Epitaxial marcasite on pyrite from Rensselaer, Indiana -K. J. Brock; L. D. Slater
American Mineralogist (1978) 63 (1-2): 210–212

During secondary growth, perhaps the temperature/pressure went to a borderline marcasite/pyrite mode and the crystals could not figure out which to follow resulting in amorphous bimodal growth which later psuedomorphed to pyrite.

-Dean

[Roger Warin]

What becomes of the crystal lattice?