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GeneralEpitaxy

6th Feb 2017 13:49 UTCThe Evje og Hornnes geomuseum, Fennefoss Expert

I found the following on Wikipedia:

"Some authors[9] consider that overgrowths of a second generation of the same mineral species should also be considered as epitaxy, and this is common terminology for semiconductor scientists who induce epitaxic growth of a film with a different doping level on a semiconductor substrate of the same material. For naturally produced minerals, however, the International Mineralogical Association (IMA) definition requires that the two minerals be of different species.[10]"

I just made a picture of what I believed to be an authentic epitaxy:

But since it is an epitaxy of albite on an older generation albite, it would therefore not be considered to be an epitaxy.

(The orange-brown crust is supposed to be microcline)

Anybody who can comment on this?

Ron Werner

Norway

7th Feb 2017 01:26 UTCMartin Rich Expert

"But since it is an epitaxy of albite on an older generation albite, it would therefore not be considered to be an epitaxy. "

In my opinion, it should named as parallel grown albite of a 2nd generation. Somewehre I heard the term "auto epitaxy" :-S

Martin

7th Feb 2017 13:48 UTCHarold Moritz 🌟 Expert

Rocks and Minerals magazine published an article about this very thing several years ago in their recurring "Let's Get it Right" column, I think the author was John White. In natural minerals, epitaxy must involve two different species. If it is the same mineral, then it is termed an overgrowth. Scepter quartz is a perfect example - these are not referred to as epitaxial quartz on quartz.

7th Feb 2017 14:14 UTCThe Evje og Hornnes geomuseum, Fennefoss Expert

OK, thanks for these replies. I will change epitaxy into overgrowth.

Ron

7th Feb 2017 17:40 UTCGregg Little 🌟

In the petroleum industry where describing quartz as a secondary void filler, often termed "silica cement", it usually regrows in crystallographic continuity on detrital sand (quartz) grains and is termed "quartz overgrowths". Under 10 to 20 x magnification, you can often see the irregular grain boundary under the clear crystalline overgrowth.

7th Feb 2017 17:49 UTCUwe Kolitsch Manager

https://en.wikipedia.org/wiki/Epitaxy

(...)

Homoepitaxy is a kind of epitaxy performed with only one material, in which a crystalline film is grown on a substrate or film of the same material. This technology is used to grow a film which is more pure than the substrate and to fabricate layers having different doping levels. In academic literature, homoepitaxy is often abbreviated to "homoepi".

(...)

[Note that this term doesn't seem to be used very much in mineralogy.]

See also

https://www.mindat.org/mesg-62-332425.html

9th Feb 2017 11:53 UTCThe Evje og Hornnes geomuseum, Fennefoss Expert

What I am still missing is the decisive argument against calling an overgrowth of 2 of the same minerals an epitaxy.

I see that a choice has been made, but I don't see the argumentation behind this choice.

(In the example I showed, the first generation albite belongs to the magmatic stage of the pegmatite, while the second generation albite belongs to the hydrothermal stage. I don't have any numbers, but I would assume the first stage to have a slightly different composition with a higher An content.)

Ron

9th Feb 2017 12:18 UTCJohan Kjellman Expert

without any reference to support it, my intuitive answer is that two of the same are the same structures

epitaxy infers different structures but with one structural property in common where they can "connect"

cheers

9th Feb 2017 14:47 UTCDavid Von Bargen Manager

It comes down to the usefulness of the term. If you allow one mineral overgrowing another to be called epiaxial, then all crystals will become epiaxial (since you can look at the crystal as having grown larger over some period of time - the one constraint you would have is that the two minerals have the same orientation, which usually occurs as crystals do grow).

IUCr and IMA definition

"K. 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.)

Comment: Examples of epitactic overgrowths are NaNO3

on calcite, alkali halides on muscovite, bixbyite on topaz,

arsenolite on fluorite, boehmite on albite, Ni on periclase,

uraninite on columbite, etc. "

http://journals.iucr.org/a/issues/1977/04/00/a14203/a14203.pdf

10th Feb 2017 08:53 UTCThe Evje og Hornnes geomuseum, Fennefoss Expert

"If you allow one mineral overgrowing another to be called epiaxial, then all crystals will become epiaxial"

Not necessarily so. It would again be a matter of definition. The overgrowth I showed on the picture represents two clearly seperate stages of growth, with the second one consisting of many individual crystals.

Ron

10th Feb 2017 13:45 UTCHarold Moritz 🌟 Expert

The Rocks and Minerals article by John S. White (2010 - Volume 85, March-April, 173-176) mentions the JCN ruling and agrees with its simplicity. It gives the reference as Bailey (1977 - American Mineralogist: 62: 411-15), but also includes an argument by R. Peter Richards for including overgrowths of the same mineral in the definition.

I have to agree with the JCN. Its a matter of recognition and of degree. The problem with calling overgrowths of the same mineral epitaxy is first having to recognize it has occurred (not a problem with two different minerals) and then having to decide on where to draw the line between what could be partial to complete overgrowth. If a scepter quartz continued to grow it would possibly cover the "stem" crystal entirely, leaving perhaps a phantom inside. Where does one stop calling it epitaxy in this continuum? Is every crystal with a phantom inside to be considered epitaxial?

10th Feb 2017 14:02 UTCThe Evje og Hornnes geomuseum, Fennefoss Expert

I see your point.

Maybe the case you describe is exactly where you need to draw the line.

But when there is a clear transition from one stage of crystallisation to another, like in the example I presented, I would be inclined to call it epitaxy.

In your example it is a proces with a pause in between two stages, while in my case there are two completely different types of processes.

Just brain-storming...

Ron

10th Feb 2017 14:22 UTCHarold Moritz 🌟 Expert

"while in my case there are two completely different types of processes."

But can that difference in process always be recognized by anyone? It is always obvious when two minerals are involved. Perhaps think of the definition as describing a result rather than a process.

In any case, nature does what it does and doesnt care what we call it! ;-)

10th Feb 2017 14:26 UTCThe Evje og Hornnes geomuseum, Fennefoss Expert

Exactly! My fascination and respect for nature stay exactly the same, however you call it!

10th Feb 2017 14:34 UTCAlfred L. Ostrander

I think David's quote says it all. The accepted definition by two internationally recognized groups is clear. Should a working definition for oriented crystals of same species that clearly are seperate stages be adopted? I think so. But I will stay with the given definition of epitaxy. One solid part of the foundation of science is accepted and observed definitions. Changing or expanding definitions to fit our own thinking or purposes is not good science. That is exactly why the IMA was created. Maybe the system could improve (or what ever) but it should be respected.

So, even if this photo is an oriented growth of albite over albite during two seperate stages, it does not entirely fit the definition of epitaxy and should not be considered an authentic epitaxy.

My take on what David said runs along the line that once a definition gets compromised, any thing that someone thinks is epitaxial can be called that regardless of the definition. The definition is then rendered nearly useless. So, the definition itself becomes the decisive argument against calling this specimen epitaxial. That is what definitions do. They clearly define the situation at hand.

10th Feb 2017 20:14 UTCGregg Little 🌟

I am also in agreement with Alfred and David. Definitions have a strict criteria for naming the process or mineral/rock at hand. If one cannot assign a definition to the sample then one is obligated to describe as much as possible its characteristics for others to consider. At this point, the debate can then ensue to fit the mineral or rock to the appropriate definition rather than debating the definition to fit the specimen in question.

11th Feb 2017 13:29 UTCJohan Kjellman Expert

Hi all,

Since, from my understanding, everybody are OK with the definition of point K in the JCN document, just a comment on the adjectival form of epitaxy from under point M in the same document (my italics/fat/underlining):

M. Adjectival forms of the terms defined under I through L

above may end in -tactic or -taxic, but not in -axial. For

example, epitactic and epitaxic are acceptable, but epitaxial

is not acceptable.* (Vote 7-0 in favor.)

* Editorial note: Although -tactic has been used throughout this

report, the Editors of Acta Crystallographiea and Journal of Applied

Crystallography prefer -taxic for etymological reasons (Schneider,

1963).

cheers

11th Feb 2017 17:23 UTCAlfred L. Ostrander

Johan, thanks for the clarity!

13th Feb 2017 10:59 UTCNiels Brouwer

Johan, interesting point on the etymology of the adjective. Curious that they would say that 'epitaxial' would be unacceptable.

When comparing the literary usage of the three different versions, it seems that -tactic is used roughly three times as often. Also, -tactic comes into use much earlier (1840s) than -taxic (1960s). However, if we compare it with the 'unacceptable' -taxial variant (first used in the 1950s), it seems to completely dwarf the other two.

I know it's just some simple statistics, but it appears that very few authors have taken the journal's etymological advice to heart. ;)

13th Feb 2017 13:33 UTCAlfred L. Ostrander

Niels, fascinating graph! After Johan posted I made a note to double check the usage as I had double checked on line about homoepitaxy and heteroepitaxy. Articles relating to epitaxic layers in the electronics industry use epitaxial all the time. Maybe it is a thing between two different branches of science. For the average person using a quick Wikipedia check, epitaxial would appear to be quite acceptable.

Language can be funny: if the plural of mouse is mice why isn't the plural of house hice? What are you gonna do!

13th Feb 2017 16:25 UTCTimothy Greenland

Also Alfred, if you eat a bowl of rice, how many grains of rouse did you ingest???

Cheers

Tim

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