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EducationEpitaxial Overgrowth

19th Aug 2014 20:21 UTCOlivier L.

I'm looking to learn and understand what textural features can distinguish between Epitaxial Overgrowth

and Crystallographically Controlled Dissolution

Like on the garnet pictured here for example

19th Aug 2014 20:36 UTCReiner Mielke Expert

An epitaxial overgrowth is not a textural feature so texture is not the basis on which the distinction can be made.

19th Aug 2014 20:57 UTCOlivier L.

Reiner Mielke Wrote:

-------------------------------------------------------

> An epitaxial overgrowth is not a textural feature

> so texture is not the basis on which the

> distinction can be made.

Please enlighten me! Tell me more :-)

19th Aug 2014 21:57 UTCAlfredo Petrov Manager

I wouldn't call something an epitaxic overgrowth unless it were a different mineral species. So the substrate and overgrowth would be two different species, with the younger one's orientation controlled by the crystal structure of the older mineral. You would often see parallel reflections coming from some of the faces of the older and younger mineral. If there's only garnet, and nothing else, there's no epitaxy.

19th Aug 2014 23:19 UTCRick Dalrymple Expert

Alfredo,

Does it have to be two different species? I thought, for example, a quartz crystal with multiple terminations would be called epitaxial. At least that is what I learned in college--a long, long time ago, in a galaxy far, far away:-)

20th Aug 2014 00:18 UTCOlivier L.

I really thought this sculpted surface was the result of crystallographically controlled dissolution

of the garnet in kimberlitic magma because I read about it in the publication cited below.

DIAMONDS AND ASSOCIATED HEAVY MINERALS IN

KIMBERLITE: A REVIEW OF KEY CONCEPTS AND APPLICATIONS

TOM E. NOWICKIa, RORY O. MOOREa, JOHN J. GURNEY

AND MIKE C. BAUMGARTNERa

20th Aug 2014 16:20 UTCAlfred L. Ostrander

A basic definition for a dissolution surface is the material is coming apart. It is not stable in the environment it is in. So the garnet was unstable and coming apart. In crystal lattices certain bond directions are weaker than others. The crystal comes apart first on a molecular level along the weaker bonds. That is what left the textural surface you see on the pictured garnet.

Epitaxy involves a mineral forming on another mineral aligning itself to crystallographic similarities of the original. The big boys of crystallography (mineralogy) say epitaxy has to involve two different minerals. In nanotechnology, if a single layer of a mineral is layed down on a single layer of the same mineral it is homoepitaxy. John Sampson White proposed homoepitaxy be used when later crystals of a different crystallographic form are found aligned with the original crystallography. I believe he published this in Rocks and Minerals as the result of a discussion on the Friends of Mineralogy Forum.

I am not sure how you interpreted what you read in the article you brought up as I haven't read it myself. However, I think the comments made by here by all contributors are on the right track. Any questions or comments might help us sort out what you are getting at.

Best Regards,

Al O

20th Aug 2014 17:15 UTCReiner Mielke Expert

Hello Rick,

I consider the example of quartz you gave parallel growth since the orientation of the crystal structure of the multiple terminations is the same as the main crystal. As far as the term homoepitaxy goes where do you draw the line? How different in form does it have to be to qualify? According to that definition a phantom could be homoepitaxial if the underlying crystal had different modifications than the outer layer. I would be more supportive of such a term if the new growth had a different crystallographic orientation but wouldn't that then just be a twin?:-S

Hello Olivier,

Like Alfred says etching involves the removal of material whereas epitaxial growth involves the addition of material. Epitaxial growth always results in new crystals ( with faces) that stick out from the underlying crystal. Etching on the other hand may produce faces but these are set back into the crystal. If the etching is extreme you may have a situation in which the original crystal faces are no longer evident but then the result is a structure with very few crystal faces unlike an epitaxial situation in which all the crystals have faces.

20th Aug 2014 17:28 UTCRick Dalrymple Expert

Rainer,

Here is an example I called epitaxial and something you would call homoepitaxail.

ON this specimen, the main quartz crystal is difficult to see as it is broken on the back. The epitaxial quartz crystals are different in that they are smoky amethyst and the main crystal is white (without smoky or amethyst colors).

When I was in college a long time ago there was no distinction made for epitaxial having two or more species. It was used to describe two or more species as Alfredo said above but it was also used to describe multiple terminations on the same end of a crystal.

I hope I have not been using this term incorrectly all these years:-(

20th Aug 2014 17:52 UTCReiner Mielke Expert

Hello Rick,

My understanding is that epitaxial overgrowth involves two different species.

20th Aug 2014 17:53 UTCEverett Harrington Expert

Hi Guys,

I have calcite specimens from KY that show epitaxial growth. There is a brown calcite scalenohedral substrate with rhombs of clear calcite orientated to the growth of the scalenohedral crystals on the very tips of the crystals. If anybody is going to be at the fall Detroit show in Oct I can show you some of them :) Or homoepitaxail since it is the same mineral I guess....

thanks

E

20th Aug 2014 17:57 UTCReiner Mielke Expert

Would a scepter quartz crystal be homoepitaxial?

20th Aug 2014 18:23 UTCRob Woodside 🌟 Manager

Parallel growth of even two habits is still parallel growth. Let's leave homoepitaxi to those who defined it as the deposition of a monomolecular layer.:-)

20th Aug 2014 18:41 UTCRick Dalrymple Expert

So to clarify for me, epitaxial is two species and not parallel growths?

If that is the accepted definition I will be glad to go back and edit some of my photos to reflect that.

Thanks

Rick

20th Aug 2014 19:10 UTCRob Woodside 🌟 Manager

Yes Rick, please do.

20th Aug 2014 19:36 UTCVolker Betz 🌟 Expert

Hi all,

Acta Cryst. (1977). A33, 681-684

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.

Volker

22nd Aug 2014 16:57 UTCOlivier L.

This picture right here taken from the publication I've cited in this thread is the

source of my confusion. The author states that sculpted surfaces on Cr-Diopside

and Garnet are typical of crystallographically controlled dissolution of these grains

in Kimberlite

Being curious I've showed this and my sample's pictures to a friend who is very

knowledgeable and he told me it is a expitaxial overgrowth and that furthermore

he has never seen such features on kimberlite indicating minerals

I wanted a third opinion :-)

22nd Aug 2014 17:05 UTCRob Woodside 🌟 Manager

Your book is right.:-) Your friend is wrong.:-(

22nd Aug 2014 18:57 UTCAlfredo Petrov Manager

What Rob said.

22nd Aug 2014 19:25 UTCFerdinando Giovine

Olivier this is a classic epitaxy: albite on microcline:

http://www.mindat.org/photo-462574.html

22nd Aug 2014 19:33 UTCOlivier L.

Rob Woodside Wrote:

-------------------------------------------------------

> Your book is right.:-) Your friend is wrong.:-(

Excellent!!!!!!!! But, is this feature really closely related to kimberlitic magma or

can this type of dissolution happen in other conditions as well? If so, could you

name one example that would be appreciated and will help me focus my readings

25th Aug 2014 18:22 UTCRob Woodside 🌟 Manager

Kimberlites must ascend very rapidly so the diamonds don't all dissolve out as the pressure drops. The rounded shapes of phenocrysts of other minerals in the kimberlite are also showing dissolution features. Similarly acsending carbonatites have rounded phenocrysts. If the growth hillocks or pits on xls from any geologic environment are smooth, I would suggest that this indicates dissolution. I think it is a pretty common phenomenon, but others will know better and might chime in.

25th Aug 2014 19:36 UTCJohannes Swarts

Interesting thread...

One of my favorite examples from Mont Ste-Hilaire - labuntsovite epitaxial on elpdidite...

photo-209225.html

Hans

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