FMF - Friends of Minerals Forum, discussion and message board

[Roger Warin]

Hello,
The island of Elba is of course the type-locality of elbaite.
It still often happens that its top is black or very dark brown in color.
In France, this ending is called "Tête de Maure", ie. moor's head.
These are impurities which slip into the lattice at the end of crystallization.
When the elbaite is completely colorless (achroite variety) it is amazing.
What are they? Iron oxides?

[Bob Carnein]

This question brings up another, which is why do tourmaline crystals often vary in color along their lengths? One normally thinks that crystals grow outward from a "seed", which explains phantoms (e.g. phantom quartz or fluorite). I would guess that this has been studied and answered, but I don't remember seeing the answer anywhere.

[John S. White]

A quick answer is that a tourmaline crystal, as it is growing, selectively chooses different elements from what is available to incorporate as it grows. An easy example are the common tourmalines with black at their bases only. This is because they favored iron early on as they began to grow, so they used up all of the iron that is available, causing them to be black at first. I don't know why so many elbaites from Elba have black terminations.

[Bob Carnein]

John, I get that, but it doesn't explain why growth seems to be upward (or downward) toward the termination rather than outward, as in a phantom quartz xl. If growth was outward, one would expect to see a particular colored core with a different color on the outside, as in the tourmaline slices one often sees offered by dealers. Most elbaite xls. that I've seen have different colored bases and terminations. It may have something to do with the hemimorphism of tourmaline. (?)

[Daniel Garcia]

[Johan Kjellman]

Federico Pezzotta has at least touched upon similar questions.
Two titles of his:
Color anomalies at the termination of pegmatitic gem-tourmaline crystals from Elba Island (Tyrrhenian Sea, Italy): a genetic model. conference paper 2021
Internal structures, parageneses and classification of the miarolitic (Li-bearing) complex pegmatites of Elba Island (Italy) 2000

His work is available at researchgate.

cheers

[Roger Warin]

Thanks a lot Johan,
The reference looks good.
I tried to create a ResearchGate account.
The interrogation is military. I had to prove that I was the author of my publications, the first of which dates from 1960.
I have about 150 publications in radiochemistry, chemistry of natural substances, organometallic chemistry and macromolecular chemistry, with also the specialty in Nuclear Magnetic Resonance.
It is the result of a professional life at ULiege. I am currently a mineralogy consultant at ULiege.

I couldn't open the Researchgate account because I don't know any colleagues who have this account.
Is it forbidden for a young 88-year-old researcher to understand the black termination of the elbaites which appear at the base but also at the top of the crystal, being then well defined. In the case of the achroites, the contrast is remarkable.
Roger.
PS: As a password, I place the photo of a goniometer, which Johan will recognize.

[Johan Kjellman]

Roger,
I hate this, especially those sites where one registered say 5 years ago in order to see or get something, and then come back say 5 years later and try to register (cause one's completely forgotten), and the site responds "sorry there is already a person here by that name...;-)
Concerning the black ends on tourmaline if I recall rightly, initially they are black when the forming pegmatite is close to the Mg-Fe-bearing host-rock, then it wanes as the crystal grows. Then for pressure reasons the pocket erupts and the fluids, because of this, equilibrates a second time with Mg-F-bearing host-rock, hence the dark heads.
cheers

[Roger Warin]

John,
I dont share this point of view.
The Moor's head is typically at the top of the crystal.
In addition, the origin of the cations which compensate for the negative charges of the stacking of the anionic cyclosilicate rings come from the hydrothermal environment and not from the support rock.
It is true that the Elba deposit is rich in iron. Specimens of pyrite and hematite are famous there.
Thanks for your help.
Roger.

[Johan Kjellman]

Roger,
I think we can agree that the Moor's head signifies an abrupt change in fluid chemistry. A proposed explanation for that abrupt change has been that the pegmatite/pegmatite pocket cracks open and that the fluids then are in contact with host rocks and able to take up elements responsible for the darker phase.
I am not at all referring to Elba iron mines which are not associated with the pegmatites.

cheers