FMF - Friends of Minerals Forum, discussion and message board

[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.

[Roger Warin]

Hi,
Two other SEM pics of epitaxy of hematite on rutile, from Niveligsberg (Eifel).
Take a look at the general site of our club.
https://www.agab.be
Roger.

[Roger Warin]

Hi,
Another classic epitaxial growth of rutile on hematite.
Roger.

[Jesse Fisher]

Epitaxial albite on microcline

[Pete Richards]

More epitaxy...

[Jesse Fisher]

Is it, or isn't it? I thought it quite interesting, at any rate. If not, what does one call it?

[Pete Richards]

The second generation crystals appear to have cube faces in their middles, where they should be if this generation is oriented parallel to the previous one. This could be referred to as "autoepitaxy" or "homoepitaxy" (they mean the same thing), or less technically as an oriented overgrowth or simply an overgrowth.

Octahedral fluorite seems to be prone to overgrowths on the corners. I know of examples from at least three other localities.

[Bob Morgan]

The other faces on the second growth are dodecahedral. These are cube-dodecahedral crystals on the octahedron tips. A lovely specimen. Several years ago I got a specimen from there with simple cubes on the tips.

[Bob Morgan]

I've collected for epitaxy, and here are a few, beginning with another albite on microcline from New Mexico. This one has albite crystals above in different orientation from those below. That is because they are on two sides of an underlying Carlsbad twin.

[Bob Morgan]

Another albite epitactic on microcline

[Bob Morgan]

Epitaxy of tetrahedrite on pyrite