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Photos of epitaxy - (16)
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PostPosted: Dec 23, 2008 03:15    Post subject: Photos of epitaxy - (16)  

Was thinking that it might be interesting to start a thread showing photos of epitaxy. A very simple definition is the condition where one mineral is attached to another in a relationship that is controlled by the mutual crystal structures.

My first contribution to this thread is a photograph of spessartine and apatite in which the 3-fold axis of the spessartine is parallel to the 6--fold axis of the apatite.



Apatite w spessartine - Pakistan.jpg
 Description:
This neat specimen is from Shengus, Haramosh Mtns, Northern Areas, Pakistan and is about 3.5 cm high.
Jeff Fast photo.
 Viewed:  40707 Time(s)

Apatite w spessartine - Pakistan.jpg



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PostPosted: Dec 23, 2008 03:33    Post subject: Re: Photos of epitaxy - (16)  

good idea!
Here are some twins of arseno on pyrite!
Many years ago i had a good cumengite on boleite,with paratacamite...



Arsénopyrite sur pyrite panasqueira 10cm.JPG
 Description:
Epitaxial arsenopyrite on pyrite,on chalcopyrite,calcite.
8cm
Panasqueira beira beixa
Portugal
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Arsénopyrite sur pyrite  panasqueira 10cm.JPG


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PostPosted: Dec 23, 2008 03:39    Post subject: Re: Photos of epitaxy - (16)  

Here is a picture to show ow it grows through the pyrite crystal.
The arséno does the same on other axes,but I can't draw it,because I have not the specimen in the hand!



page de dessin.JPG
 Description:
how it grows through the pyrite crystal!
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page de dessin.JPG


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PostPosted: Dec 23, 2008 10:43    Post subject: Re: Photos of epitaxy - (16)  

This is my contribution to this fascinating topic.

(More info about the Rutile-Ilmenite-Hematite from Zambia here: http://www.mineral-forum.com/message-board/viewtopic.php?t=155 )
(More info about the Rutile-Hematite from Brazil here: http://www.fabreminerals.com/specimens/RSBR-brazil-notable-specimens.php#EJ26F4 )



Rutile epitaxial in Hematite pseudomorph after Ilmenite.jpg
 Description:
Epitaxial growth of red brilliant twinned Rutile on a tabular former Ilmenite crystal pseudomorphed by Rutile and Hematite. The former Ilmenite is showing well-marked geometrical forms.
Mwinilunga, Zambia
Mined in May 2007
Specimen size: 5.6 × 2.9 × 1.5 cm.
Photo: Reference Specimens ( http://www.fabreminerals.com/specimens/RSAFR-africa-notable-specimens.php#MG62K7 )
 Viewed:  40680 Time(s)

Rutile epitaxial in Hematite pseudomorph after Ilmenite.jpg



Rutile and Hematite epitaxial.jpg
 Description:
The crystals of Rutile have grown epitaxially on tabular Hematite.
Novo Horizonte, Bahía, Brazil.
Mined in February 2004
Specimen size: 2.8 × 2.4 × 0.2 cm.
Photo: Reference Specimens ( http://www.fabreminerals.com/specimens/RSBR-brazil-notable-specimens.php#EJ26F4 )
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Rutile and Hematite epitaxial.jpg


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PostPosted: Dec 27, 2008 15:56    Post subject: Re: Photos of epitaxy - (16)  

Hi all,
This is my first contribution to this English forum.
As English is not my native language, please be lenient towards my future mistakes or misspellings.
I'm living in th west of France, not far from Nantes. I'm a microminerals collector and photographer.
Uranium minerals and more widely phosphates are my favorite subjects.
Very interesting thread, following Jordi message about rutile and hematite, a sample from Saint-Chistophe-en-Oisans and a second one from Boucheron mine.
Jean-Marc.



metatorbernite_meta-autunite.jpg
 Description:
Metatorbernite, meta-autunite.
Le Boucheron
FOV: 3mm
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metatorbernite_meta-autunite.jpg



rutile_hematite.jpg
 Description:
Rutile, hématite.
Saint-Christophe-en-Oisans
FOV: 6mm
 Viewed:  40592 Time(s)

rutile_hematite.jpg


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PostPosted: Dec 27, 2008 17:07    Post subject: Re: Photos of epitaxy - (16)  

Wonderful images Jean-Marc, thanks to share them with us!

Jean-Marc Johannet is a well known French photographer. His work is frequently published in French magazines (and others), we are lucky to have him here publishing their photos.

Jordi
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PostPosted: Dec 28, 2008 05:06    Post subject: Re: Photos of epitaxy - (16)  

really intersting thing here!
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PostPosted: Dec 28, 2008 08:59    Post subject: Re: Photos of epitaxy - (16)  

Jean-Marc, Thank you for the incredible photographs! I have been in Nantes a few times and love the area where you live. Please continue to share your amazing photographs?
All the best!

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PostPosted: Dec 28, 2008 11:52    Post subject: Re: Photos of epitaxy - (16)  

This specimen consists of marcasite epitaxial on pyrite from the Rensselaer Quarry, in Jasper County, Indiana.


1504 Marcasite.jpg
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1504 Marcasite.jpg


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PostPosted: Dec 28, 2008 14:44    Post subject: Re: Photos of epitaxy - (16)  

Gail, Jordi,
Thank you for your kind comments.
A less described epitaxy growth, smithsonite on dolomite.



smithsonite_dolomite.jpg
 Description:
Smithsonite, dolomite.
Saint-Félix-de-Paillères, Gard, France
FOV: 3mm
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smithsonite_dolomite.jpg


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PostPosted: Dec 28, 2008 14:52    Post subject: Re: Photos of epitaxy - (16)  

These photos are amazing!

- Tracy

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PostPosted: Jan 05, 2009 01:36    Post subject: Re: Photos of epitaxy - (16)  

This is my contribution of photographs of a specimen demonstrating epitaxy.

The specimen shown is two crystals of sphalerite with an epitaxially controlled coating of chalcopyrite crystals from Joplin, Missouri. One photo shows the "front" of the specimen, showing the parallel orientation of the chalcopyrite crystals- by the light reflecting simultaneously off all the chalcopyrite crystals growing on the larger sphalerite crystal on the right. The other photograph is the "back" of the specimen showing the enclosed sphalerite crystals (largest crystal again located on the right) .
Size of specimen is 2 1/4" x 1 3/4" x 1 1/4". I purchased the specimen in Joplin, Missouri, in August of 1971.



DSCN2010MedCr.jpg
 Description:
"Front" of specimen showing epitaxial over growth of chalcopyrite on sphalerite crystals.
 Viewed:  40341 Time(s)

DSCN2010MedCr.jpg



DSCN2008MedCr.jpg
 Description:
"Rear" of specimen showing enclosed sphalerite crystals.
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DSCN2008MedCr.jpg



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PostPosted: Jan 05, 2009 08:03    Post subject: Re: Photos of epitaxy - (16)  

Does epitaxy relate to different minerals growing together and/or the same mineral that changes habit or orientation in the same specimen?
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PostPosted: Jan 05, 2009 09:07    Post subject: Re: Photos of epitaxy - (16)  

Vic, I think you will find that the classic definitiion of epitaxy requires that the two crystals be different species. Habit changes with just one mineral during growth are quite common and if these were to be included it certainly would dilute the meaning of epitaxy. For my purposes I shall limit the definition to one mineral being non-randomly attached to another mineral. I hope everyone agrees.
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PostPosted: Jan 05, 2009 10:41    Post subject: Re: Photos of epitaxy - (16)  

While I think John White's working definition of epitaxy as requiring two phases is useful for guiding this thread, I disagree, John, with your restriction from the larger context of studying epitaxy. I have argued in the past, and continue to believe, that the term can appropriately be extended to include the overgrowth of one mineral on itself. However, this is only useful if the texture of the assemblage provides strong evidence that there were two distinct stages of growth (as opposed to merely a change in the habit). Examples include multiple cube-octahedral galena crystals attached to a first generation galena with cube habit, calcite growing in very distorted shapes from spots along the edges of first-generation crystals, and calcite growing in twinned orientation as many tiny parallel "teeth" on a first generation crystal. I suppose the Mariposa calcites from Mexico, part of another recent thread, would also be an example.
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PostPosted: Jan 05, 2009 11:38    Post subject: Re: Photos of epitaxy - (16)  

Fraid I have to disgree with my friend Pete Richards. There is a notable difference between the oriented attachment of two different species, and extended growth of the same mineral upon itself. In the former case, very special circumstances appear to be required for the structure of mineral A to align itself with a similar structure in mineral B. In most cases the number of orientations in which this can occur appears to be very limited. In the latter case, secondary growth of new material on the same substance can theoretically occur on any surface of the primary crystal. Would you call a hopper crystal epitaxy? I would not, but what is the difference between growth spots along the edges of a primary crystal and a continual ridge along that edge? Just a matter of completeness. Pete's growth spots are just incomplete ridges, and complete ridges become hoppers.

Can't wait for Pete's response. Be gentle.

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PostPosted: Jan 05, 2009 14:00    Post subject: Re: Photos of epitaxy - (16)  

Well, this will be interesting. And gentle. John correctly refers to "very special circumstances" required for two structures to align well enough to form epitactic relationships, and this is quite true. He says the number of orientations in which it can occur is very limited; also true. But in the case of "auto-epitaxy" - growth of a mineral on itself - there is generally only one orientation in which this can occur, and that is when the lattice of the second generation is parallel to the lattice of the first generation. That's a pretty limiting condition! It is true that the growth of the second generation could in principle occur on any crystal face, but only under the condition of lattice parallelism.

Part of the issue here, presumably, is that epitaxy is perceived as involving specific lattice planes in each mineral which have similar configurations - it is a two-dimensional similarity, not a three-dimensional similarity, as is the case with auto-epitaxy. But even when the similarity is confined to specific lattice planes, it is not a foregone conclusion that the faces with the same orientation must be the faces on which the relationship is expressed in the external morphology. The epitaxy of marcasite on pyrite (and vise versa) involves specific planes and directions in each mineral, but this relationship often develops on other planes as well, either because the edge of the epitactic plane of the host is present in the surface of other faces and that is enough to orient the overgrowths, or because the epitactic plane is present as microfacets in a different face which is not a perfect plane at the atomic level. This relationship is even seen in some cases in which the epitactic plane is not expressed at all as a face on the host crystal, at least macroscopically.

The terminology of epitaxy is somewhat messed up. Epitaxy proper refers to two-dimensional relationships. There is a term, monotaxy I think, for one dimensional relationships, but it's not obvious that anyone has come up with a good example. Syntaxy refers to three-dimensional similarities, but the term is also used in other geological contexts to mean quite different things, related to cementation of grains in sandstone, for example. Topotaxy also refers to three-dimensional similarities (usually, at least) but also carries the connotation of a phase change - for example conversion of ilmenite to oriented rutile mats by leaching of iron and recrystallization of rutile at an atomic scale such that the orientation between rutile and ilmenite is preserved even after the ilmenite is gone. Most folks use epitaxy to refer generally to all of these kinds of oriented relationships; I do, and I think John does too.

Three-dimensional similarities are common in epitaxy involving different phases. For example, overgrowths of different rhombohedral carbonates one on the other are common, sometimes without even a change in morphology. If it's calcite in the middle and siderite on the outside, it's epitaxy. Right? The lattices are quite similar in all directions. But what if probe analysis shows that it grades from pure calcite to iron-bearing calcite to calcium-bearing siderite to pure siderite, with no sharp breaks in concentration and no change in morphology. Is it epitaxy or just chemical evolution of a (chemically) mixed crystal? Is it epitaxy if there is a sharp break in composition, or only if there is a change in morphology?

Someone posted here an image of the classic sphalerite/chalcopyrite epitaxy from the Joplin, Missouri area. The chalcopyrite structure is readily derived from the sphalerite structure by replacing the zinc by copper and iron in a systematic way. The relationship is one of three-dimensional similarity. Is it epitaxy? It can be described in terms of any plane in the structure one chooses, because it is more than a two-dimensional relationship. Many, many classic examples of epitaxy turn out to involve structures that are very similar in all directions. If they are epitaxy, then why isn't calcite on calcite?

An aside: I confess that I consider this auto-epitaxy somewhat uninteresting, because the interesting thing about epitaxy is where are the similarities and how similar are they, and there's nothing to figure out when both structures are identical. But I still consider it to be epitaxy.

John raises the question of hopper crystals. I would not call hopper crystals epitaxy, because they do not result (typically, at least) from interrupted growth. Rather, they occur when growth is too rapid to allow new layers on crystal faces to complete themselves and when, because of rates of diffusion, new layers originate near the edges of the crystal rather than across the whole crystal face. Ichiro Sunagawa treats this very nicely in his book, Crystals: Growth, Morphology, and Perfection (Cambridge Press), which I would recommend to anyone interested in these matters of how crystals grow. The difference between hopper growth and the growth in spots along the edges of crystal that I described (sorry, I don't have a good picture) is that in the latter case the morphology and locations of the second generation crystals, and the presence of a dark coating on the first generation crystals, strongly suggests that growth stopped, other materials were deposited on the surfaces of the first generation crystal, and then the growth of the second generation began, but only in very specific places. It is the two-stage growth (with change of morphology as well in this case) that makes these qualify as epitaxy as far as I am concerned. Hoppers result from continued (but compromised) growth, and thus are not epitaxy.

I think that this is a complex matter with a lot of grey area separating cases which are clearly epitaxy and cases which are clearly not. If this is true, what is important is not that John and I agree, but that our discussion leads us to a better understanding of this very interesting and special case of crystal growth.

John, it's your turn!

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PostPosted: Jan 06, 2009 06:27    Post subject: Re: Photos of epitaxy - (16)  

Well, I have to say that I feel a little overpowered here, as I am not nearly the student of crystal structures that Pete is and am already in way over my head.

I would say, however, that the example Pete gave of a gradual chemistry change from calcite to siderite versus a sharp change is a perfect reason to exclude such substances from admission to the epitaxy club. I would suggest that they are no more entitled to admission than are the alternate layers of albite and orthoclase in perthite, due to exsolutiion. Inasmuch as there are already very effective and useful terms to describe both the calcite/siderite relationship, as well as the albite/orthoclase relationship, then I feel it is not necessary to bring them under the umbrella of epitaxy.

If one wants to explain spots of second generation growth along crystal edges as the result of an interrupting coating over the first generation, then one has to ask how was there any second generation growth at all. Why did not the coating cease all growth altogether?

And, while speaking of coatings interrupting growth I feel I must attach these images of a quartz specimen I have from China. I have written an article about this piece that was published in Rocks & Minerals. There are four generations of quartz crystallizaiton on this specimen. First came the large individual crystals which appear dull gray, then came the clear little windows on pyramid faces, then a coating of chalcedony over the large crystals, but not the little windows, and finally a fine druze of tiny quartz crystals sprinkled over the whole piece. Anyone looking at this just casually would think that steps two and three were reversed, but if that were true then translucent chalcedony would have covered the large crystals entirely and the littel windows, if later, would show a film of chalcedony beneath them.

The specimen is from Jinlong, Guangdong Province and the photo were taken by Terry Huizing.



12-9-1 Quartz China.jpg
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12-9-1 Quartz China.jpg



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PostPosted: Jan 06, 2009 06:29    Post subject: Re: Photos of epitaxy - (16)  

There were supposed to be two photos, here is the other one.


12-9-1 Quartz - China b.jpg
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12-9-1 Quartz - China b.jpg



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PostPosted: Jan 07, 2009 09:58    Post subject: Re: Photos of epitaxy - (16)  

In our dialog on epitaxy involving two generations of the same species, John White wrote "If one wants to explain spots of second generation growth along crystal edges as the result of an interrupting coating over the first generation, then one has to ask how was there any second generation growth at all. Why did not the coating cease all growth altogether?" A good question, as usual.

I think the answer is, first, that sometimes the coating does indeed prevent further growth anywhere on the crystal, in which case the crystal is not favored over any other site on the matrix, and if there is a second generation growth, crystals that form on the first generation crystal are not oriented. But, second, in some cases edges or corners clearly are preferred sites for second generation growth in an oriented fashion. In other cases, certain faces are favored over others. For example, overgrowths may occur only on octahedral faces but not on cube faces on the same crystal. In these cases, I think the reason is that edges and corners have higher surface energy than faces, and are consequently more likely to etch slightly, causing a new clean surface to be exposed to nucleate second generation growth. In the case of growth selectively on certain faces, it may be more a question of the texture of the faces - growth occurring on smooth faces but not on rougher ones or vise versa. It may even involve the chemistry of the faces, since faces in different orientations expose different combinations of atoms.

The attached image shows a second generation calcite crystal growing on the tip of a first generation crystal; several other examples are present in the image but are not easy to see clearly. About half of the first generation crystals on this specimen have similarly located second generation crystals. All have the same orientation relative to the first generation crystals ("auto-epitaxy"). No second generation crystals are found in other locations. The brown color of the first generation is due to a surface coating of unknown composition; this coating is thinner on the small faces that are at the tips of crystals that lack second generation overgrowths. Presumably it is thinner on the ones with second generation overgrowths as well, and this is why the second generation formed at the tips.

Whatever the reason, these selective overgrowths are not uncommon and are very interesting.



Calcite.jpg
 Description:
Second generation calcite growing selectively at the tip of first generation crystals. Penske Quarry, near Raymond, Iowa
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Calcite.jpg



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