FMF - Friends of Minerals Forum, discussion and message board

[Tobi]

[rweaver]

When one of your mentors was Josie Scripps, calling them anything other then Tourmaline was very very wrong.

[basti]

I would really stick to the label "tourmaline" if we want to be scientifically correct, because:

- dravite can be bright green or red
- schorl can be dark green or brown
- pink tourmaline might be elbaite, fluorelbaite, rossmanite, darrellhenryite, liddicoatite, tsilaisite...
- black tourmaline can be dravite, schorl, fluorschorl, foitite...
- one single crystal of "water melon" tourmaline can be in fact more then 5 IMA valid tourmalines...
...

You get the idea, just color is not the good indicator. Especially Li-rich tourmalines can be very variable and tricky.

Do not rely on scientific literature too much as many published papers - especially older then decade - are often incorrect. Previously, the oxy- and fluor- tourmalines were not recognized and even many modern studies do not properly analyze this. You can not be 100 % sure about the formula based solely on EMPA analyses, you also need Raman/IR or LA-ICP-MS or X-ray diffraction...

Tourmaline nomenclature (as well as micas, amphiboles and some other groups) is pretty useless for collectors. Nature can not be split into exactly defined separate "boxes" and tourmaline structure is a "trash can" accommodating very wide range of elements => resulting in dozens of tourmaline group minerals with good immiscibility.

The problem is that just simple black "schorl" crystal from primitive pegmatite might be internally zoned with foitite/magnesiofoitite core, schorl/fluor-schorl mid-zone and dravite rim. These zones can also repeat several times. LCT pegmatites often contain very complex and variable tourmalines where one tourmaline crystal consists of several IMA approved minerals. How do you label this? Like "crystal of dravite-schorl-elbaite-liddicoatite-tsilaisite"? How can you identify these tourmalines without extensive, expensive and often destructive analytical work?

[lluis]

Hi, Basti

And dravite could be also achroic and blue.... as for the black/amber dravites with a colorless portion topped by a blue cap (very nice, besides....)
Rarities in tourmalines are beyond limits....

Personally, I only follow the chemistry(and names associated with it)
I have foitites, analyzed, that could be said schorl....

With best wishes

Lluís

[basti]

Sure, it is not definite list, I just used common examples. I have analyzed hundreds of pegmatite tourmalines and I barely scratched the surface of their complexity. Btw. Raman is quite good indicator for tricky V and W positions but nobody did a proper research on that yet.

The other problem is the inconsistency of nomenclature. Some specimens are fluor- or oxy- but some are not named that way. The evolution will probably lead to some kind of semi-systematic nomenclature as was introduced in chemistry for organic compounds - when the growing list of trivial names became too huge and not possible to maintain anymore...

[Carles Millan]

Since not all black color tourmalines are schorl, and not all schorls are black, it is nonsense to automatically label all the black tourmalines as schorl. Since not all red/green color tourmalines are elbaite, and not all elbaites are red/green, it is nonsense to automatically label all the red/green tourmalines as elbaite.

Just to be sure, you should have the results of a chemical/physical analysis, which is not the case in most specimens on sale.

Perhaps the best solution would be naming them just tourmaline. It is enough most of the times and is never wrong.

[Peter Lemkin]

[basti]

I also personally use the "scientifically incorrect" labels, I simply do it best I can. The science is evolving all the time so the labels are never really "finished". if you visit any older museum or collection, you will notice many obsolete or incorrect labels. Some minerals are better analysed, some names are not IMA valid anymore etc.

As an example on tourmalines: All pink Li-tourmalines were previously labelled as rubellite, then it became elbaite (very variable color). Later, we found out that "pink elbaite" is in fact elbaite, fluor-elbaite, rossmanite or darrelhenryite... I suspect it will be even more complicated in 10-15 years :D


To the (portable) Raman spectroscopy: The problem is: 1) the measuring itself, which is not really stupid proof; 2) fitting the measured spectra; 3) referrence DB with verified and high quality spectra. To obtain relevant tourmaline Raman data for referrencing, you have to measure several specimens of each IMA valid tourmaline and you have to do the EMPA or EBSD + LA-ICP-MS + Mössbauer + IR as a minimum for each of these. The situation is even worse because of solid solutions between tourmalines = there are often no pure end members. This would be extremely expensive and time consuming.

I spent lot of time just getting Raman and EMPA data and it is viable way - once you have it confirmed you can identify many tourmalines (maybe all) based solely on Raman spectra. But I have no budget and time to finish such project myself. The other portable-preventing problem is the time - measurement of one good quality spectra easily takes 20-30 minutes as a minimum.