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]