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Elbaite, ending moor's head
  
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Roger Warin




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PostPosted: Apr 19, 2022 04:28    Post subject: Elbaite, ending moor's head  

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?



Elbaite_50008-b_R.jpg
 Mineral: Elbaite
 Locality:
Elba Island, Livorno Province, Tuscany, Italy
 Description:
with black ending
 Viewed:  615 Time(s)

Elbaite_50008-b_R.jpg



Elbaite-achroite.jpg
 Mineral: Elbaite
 Locality:
Elba Island, Livorno Province, Tuscany, Italy
 Description:
 Viewed:  612 Time(s)

Elbaite-achroite.jpg


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Bob Carnein




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PostPosted: Apr 19, 2022 09:33    Post subject: Re: Elbaite, ending moor's head  

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.
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John S. White
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PostPosted: Apr 19, 2022 10:49    Post subject: Re: Elbaite, ending moor's head  

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.
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Bob Carnein




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PostPosted: Apr 19, 2022 16:22    Post subject: Re: Elbaite, ending moor's head  

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. (?)
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Daniel Garcia




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PostPosted: May 13, 2022 13:18    Post subject: Re: Elbaite, ending moor's head  

Bob Carnein wrote:
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.


Thats a good question, indeed.
I don't know any published paper on this issue, but I have a comment, and a suggestion.

It is quite common to observe a radial zoning in tourmaline, at least in thin section, hence the longitudal one is not the rule.

Concerning a possible mechanism to get a longitudinal zoning in macrocrysts, it seems difficult to advocate unidirectional growth along the c axis, because this would require the crystal to grow first on the transverse direction.
One suggestion is that the crystal may grow normally in 3D with a primary zoning, then diffusion works in the crystal and the impurities (or the vacancies, or whaterver gives the color) diffuse. Diffusion needs time and is severely limited by distances, but If for any crystallographic reason diffusion is allowed to work in the transverse direction, where the distances are shorter, the apparent zoning will change from 3D (normal) to 1D (longitudinal).
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