FMF - Friends of Minerals Forum, discussion and message board

[MineralParagon]

How can we tell if we have an epitaxy or a post-crystallization?
for e.g., from Italy, I find a quartz crystal with an upper brown shade recovered by a strange crystallization of colorless quartz, like a slight deposit on the tip.

[John S. White]

Here is a very good example of where Peter Richards and I disagree. I would not call this epitaxy, because it is just secondary growth of one mineral upon the same mineral. Peter, on the other hand, considers this epitaxy. I am not hopeful that this disagreement can be resolved, so it appears that there will be two schools of thought unless and until some authoritative body such as the IMA issues an opinion which either includes or excludes secondary growth as epitaxy. I can only hope that they agree with me. Since such issues are seldom resolved by authoritative bodies, it is more likely that the ambiguity will continue, unfortunately.

[MineralParagon]

What is wonderful is that I agree with your two viewings (John or Peter).
The subject needs a real and probant decision.
Is there the same disagreement for the case of pseudoboleite (no Ag) and boleite (including Ag)? And why cumengeite is existing in sixling and single forms?

[parfaitelumiere]

I think the same way as John S White regarding epitaxy and second growth on primary growth of the same mineral.
I would tell of epitaxy when there are different species.
However, we often see chemical variations in the same mineral, that creates ghosts or colors, could we talk about epitaxy.
I don't think so, this is the same crystal, the same crystalline system (good word?!), not like an epitaxial growth, where we can have different systems.

[MARION Claude]

Have you seen "phantom quartz" with a dozen concentric zones underlined by chlorite, liquid inclusions...? The zones outline successive stages of growth. I think that it is not an epitaxy.
I agree with John S White regarding epitaxy.

[chris]

Hi folks,

I've found that sentence in a 1977 IMA report about epitaxy :

"Epitaxy is the phenomenon of mutual orientation of two crystals of different species, with two-dimensional lattice control (mesh in common), usually, though not necessarily,
resulting in an overgrowth. (Vote 7-0 in favor.)"

Here I agree with John (sorry Peter). However in the semiconductor world epitaxy often refers as the deposit of a fine layer of the same material as the substrate...

Christophe

[MineralParagon]

This seems to be the most precise definition. Thanks for your search and definition!

[Linda St-Cyr]

I still think that the term "auto-epitaxy" is clear and serves a purpose, however.

[Pete Richards]

John White and I agreed separately to wind down our discussion, feeling it had produced about as much good as it can. So as my final message on this thread, I will make a couple of comments to respond to more recent contributions prompted by our exchange.

First, I agree with John about the smoky quartz that MineralParagon posted, i.e., that this is not a clear case of auto-epitaxy. For one thing, the smoky color is either a phantom some distance below the surface of the crystal or is a characteristic of the outer (1mm?) layer. It is not a surface coating. Therefore, it would not inhibit growth, and it is not strong evidence of a two-stage growth process. I recognize that establishment of the concept of auto-epitaxy extends the concept into a grey area, but I think it can be useful. But in addition, it must be applied judiciously!

About pseudoboleite and boleite, I think it is well established that this is a good example of classic epitaxy. Same comment about cumengite - usually as "sixlings" epitactic on boleite but can occur alone. These are epitactic groups, not twins, that's why I put sixlings in quotes.

To a couple of posts which raise the question of phantoms or growth layers, I would agree that these are not clear indicators of a two-stage growth process, and don't deserve to be called epitaxy.

To Chris quoting the 1977 IMA report: this is exactly the source I was referring to when I wrote earlier of the various forms of "-taxy", i.e., monotaxy, epitaxy, syntaxy, and topotaxy. This report is, I suppose, the "law" about these terms, but it is also 31 years old, and my experience is that usage among crystallographers has changed, even if the law has not. Or has decided to ignore the "law". Chris refers to the strict version of epitaxy; I use it more loosely to refer to all oriented overgrowths, but with the restriction that "overgrowth" MUST mean two stages of growth. And if one cannot infer that there were two stages of growth from the morphology, then the term epitaxy should not be applied to crystals of only one species.

To all, thanks for your interest in this discussion. I hope we have all gained some thoughts worth pondering, and some advance in our thinking about the fascinating question of how crystals grow.

[chris]

Hi Pete,

Thanks for your comments and for posting the thread. I really enjoyed reading it.

Christophe

[parfaitelumiere]

see that epitaxial growth begins with growths of two species at the same time, so secondary growth of quartz on first generation quartz cannot be called epitaxy!
What makes me smile is that I wanted to tell the same thing yesterday (about first stage of epitaxial growth, especially visible on quartz and feldspath "cuneiforme"), to tell those two species grow together at the same time, and I have seen your answer...

[marvsT/Nminerals]

John, Very interesting photo of four generations of quartz. It took me a little while to "see" the various stages of growth. But I have a question generated by the flat little "window" quartz's: Aren't the flat little crystals actually several composite twinned crystals? Are they Brazil law twinned?

[John S. White]

Interesting question that I have never considered. My first reaction was that they are not the product of Brazil twinning, because some of them are more or less centered on rhombohedron faces, while others appear on the edges between two rhombohedron faces. This may not, however, exclude them from being Brazil twins. All of them are limited to rhombohedron faces, none are seen on prism faces. I think I shall have to defer to Pete Richards on this one as he has a much better understanding of this type of twinning in quartz than I do.

[Pete Richards]

John White has invited my opinion again, in the thread including his pictures of complex overgrowths of quartz on quartz from China. Perhaps after awhile the limits of my understanding will become clear to all! That said, I don't see anything in those little clear overgrowths that suggests twinning, Brazil or otherwise. Perhaps Marv can clarify what he's seeing in this image that suggests twinning.

Brazil and dauphiné twinning are common in quartz. In fact, it may be rare to find a crystal that is lacking these twins. But they are often not visible, because they do not usually change the morphology of the crystal, except when certain modifying faces (e.g. the triangular "x" face) are repeated in an order inappropriate for an untwinned crystal. In my experience, and I'm not really an expert in this regard, dauphiné twinning is more common than Brazil twinning. Both can be detected on naturally etched crystals because the two twinned individuals etch differently, but without etching, they may be undetectable. Dauphiné twins usually have individuals that are separated by irregular, jig-saw-puzzle-like boundaries, whereas Brazil twins usually have individuals that are separated by boundaries that are polygonal, and one individual is usually dominant and the other is present as small regions within the dominant crystal.

This is probably very confusing; I wish I could offer some diagrams or photos. Check out the third volume of Dana's System, 7th edition, or Rudolph Rykart's Quartzmonographie if you have access to them - they will give more information,

At any rate, for now I don't see indications of twinning in the images John shared with us.

[cgoings]

Post moved to: A mineralogical trip through the states of USA - Iowa

[jibby]

Wonderful images

[prcantos]

Small epidote crystals covering pink feldspar and a single smoky quartz.

Warning!! This is NOT epitaxy, it is just an arbitrary crystal covering, as it is explained below.

[Michael Shaw]

Pablo,

That is an interesting specimen of epidote, feldspar and quartz. I certainly don't consider myself much of a crystallographer, but the more I look at this specimen, I wonder whether it is really epitaxy. To me, the epidote doesn't seem to have any specifically defined orientation relative to the faces of the quartz crystal. Rather, it appears to to have "settled" on selective parts of the specimen as a result of the orientation of the specimen in the pocket. This is sometimes referred to as a "snow on the mountain" effect.

Michael

[Pete Richards]

[prcantos]

Thank you, Michael and Peter, for your correction. I realize I haven't understood the concept of epitaxy with its restriction concerning directions.

Maybe my picture can be used as a counter-example to compare to real epitaxial specimens. (I will add a warning in my previous post to avoid confusion).