FMF - Friends of Minerals Forum, discussion and message board

[Josele]

Is generally accepted that rutile forming 60º angle orientation between crystals is due to epitaxial growth on hematite. But very often hematite is not visible in the specimen, as in this case.

An infinitesimal (not visible) quantity of hematite is enough to start epitaxy?

Can rutile grow like this without hematite at all?

Thanks for your comment.

[John S. White]

Very nice close-up in the third photo. This is not epitaxy. What you have is twinned rutile in a very common form, often referred to as reticulated or geniculated, and rutile of this habit is known as sagenite. Twinned rutile like this is typical on quartz but can also be found on other substrates, such as siderite. Here are two examples from my collection of such twinned rutile in quartz from Lavro do Cascalho, near Conselheiro Pena, Minas Gerais, Brazil. The smaller one is 4 cm across and the larger elongated one is nearly 11 cm. In the larger piece some of the rutile is entirely enclosed in quartz while much of it is not.

My guess is that this rutile is black because of a relatively high iron content, while your sample probably contains very little iron.

[Josele]

Thank you John, then I was confused about sagenite growth.
In accord with this, I suppose that angles between crystals and twin plane is 65º (as is in rutile sixlings), not 60º as in epitaxial rutile on hematite. Given that there is two twinning operation affecting the same crystal, can this be considered as a particular case of cyclic twinning?

[John S. White]

In my earlier post I used the term geniculated incorrectly, for which I apologize. Geniculated twins are those that consist of just two components joined together like arm or leg bones at an angle.

[Martin Rich]

An other possibility is a pseudomorph of Rutile after preexistent Ilmenite. Only a thought of myself.

[Pete Richards]

[Jordi Fabre]

More info: Ilmenite pseudo

[Josele]

Epitaxy, twinning, pseudomorphism... all together... Wow! this becomes more and more interesting!
From my naive ignorance I can think only about angles to determine if a sagenite is due to epitaxy or twinning. There is not much difference between 60 and 65º having in mind that nature tend to create divergences in theoretically parallel growth crystals.
Let me add some fuel to the fire with two simple observations:

[marco campos-venuti]

Just some pictures of the famous star rutile quartz. In these samples crystallization starts with hematite crystal (iron oxide) and when iron finished, then start to crystallize the rutile (titanium oxide). The rutile is red to yellow because most of the iron was used in the hematite crystal.

[marco campos-venuti]

In this sample the rutile star grows along an elongated hematite.

[marco campos-venuti]

This is sagenite. It is called green rutile. Just a little bit green, I don't know why. It is from an old deposit.

[marco campos-venuti]

An other sagenite, but a bit different. It is yellow and is composed of individual stars.

[Mark Ost]

Stunning examples Marco

[stef4412]

[Josele]

More pics: