FMF - Friends of Minerals Forum, discussion and message board

[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).

[Roger Warin]

Hi,

A case of epitaxy forgotten hematite on rutile.
Epitaxial assemblies are sometimes surprising. So, while such epitaxy of rutile on hematite is well known, it is not the same for the reverse assembly, i.e., hematite upon rutile. This is of course always the same epitaxy law.
The rarity of this latter possibility is related to the relative abundance of iron versus titanium. Iron is often more abundant than titanium.
This assembly of hematite on rutile has been discovered in 1997 by Alain Robert, for the 1st time in Niedermendig (Eifel, Germany). But only few micromounts were discovered on this occasion.
Afterwards, a fabulous find was made in Madagascar in 2005 and published in “Le Règne Minéral" - 2008.
Locality: Tetikana, area of Ambatofinandrahana-Mandrosonoro. It is in the southwest of Ambositra.
I had the opportunity to describe this fine epitaxy (with Alain Robert and Prof Frederic Hatert – Univ.Liege, B) in the French magazine “Le Règne Minéral", No. 124 (July-August) 2015.
Roger.

[Tobi]

I should mention my Chinese galena specimen from Rucheng Mine, already discussed in Strange kind of crystal growth of galena and sphalerite

[Roger Warin]

This small specimen is a new epitactic intergrowth of hematite on rutile, which has been discovered by Alain Robert in 1997 at Niveligsberg, Drees, Germany (Photo by Eddy Van Der Meersche). The sample consists in two rutile prisms twinned along (031), showing hematite lamellae on their terminations. These lamellae are oriented in two directions at 90° from each other, in agreement with the tetragonal symmetry of the rutile crystals. In a article published in Le Règne Minéral, Nr 124 (2015), Frederic Hatert and me describe this intergrowth. It is compared with the classical rutile-on-hematite epitaxy, illustrated by historical samples from the collections of the University of Liège, Belgium. The epitaxy phenomenon is described in its historical context, with a peculiar focus on the relationships between the crystal lattices of rutile and hematite.

The epitaxy phenomenon implies the existence of compatibility to some degree between different crystalline lattices. It is also essential that cohesion links are established between the interphase atoms. Since the epitaxial rutile on hematite is common and well known, why could not it be the opposite, namely a crystallization of rutile on hematite under the laws of the epitaxy? The reverse situation is of course possible, but it is unlikely because in most deposits, iron is dominant compared to titanium. The observation of this Niveligsberg epitaxy therefore indicates a geochemical context particularly enriched in Ti versus the iron content (volcanic rocks).
The same situation occurs in Madagascar (Tetikana, Ambatofinandrahana, Fianarantsoa Province), where exceptional and very rare specimens were discovered (around 2006) (see Thomas, in Le Règne Minéral 2008) in this region where the pegmatites alongside volcanics.

As shown in the figure when crystallographic axes "a" of rutile and hematite are parallel, there is a triple cell of rutile in which the nodes are in approximate agreement with hematite nodes (hematite a = 5.30 Å and c = 13.75; rutile: a = 4.59 Å and c = 2.96).

[Pete Richards]

Very nice, Roger! Great to see the epitaxy theme active again!

[Roger Warin]

Pete thank you. Your opinion honors us.
Roger.