FMF - Friends of Minerals Forum, discussion and message board

[Pete Richards]

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.

[Peter Megaw]

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!!

[Philippe Durand]

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 ?

[Roger Warin]

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.

[Pete Richards]

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!

[Philippe Durand]

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 :)

[alfredo]

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.

[Roger Warin]

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.

[Mathias]

I always thought that this was a subsurface oxidation process going on, pushing up some of the material above it.

[John S. White]

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.

[Dean Allum]

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

[Sante Celiberti]

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

[Mathias]

I thought that pyrite won't melt, upon heating it will decompose to Iron and Sulphur at a certain temperature.

[Bob Morgan]

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.

[Sante Celiberti]