There should be F in Uvite...

The arrival of Fluorelbaite has reminded me of something...

Currently mindat's formula for Uvite contains a lone OH in the brackets, standing for the O(1) anion site on the z axis.

Methinks this should be either an F (for the ideal end member), or (F,OH).

References:

The Handbook unfortunately lumps the O(1) and O(3) sites together into a single (OH,F)₄ parenthesis, but the representative sample composition has more than 0.7 apfu F which are generally believed to reside all at O(1).

(Several other sources follow this pattern.)

The American Mineralogist Crystal Structure Database cites a number of specimens from MacDonald and Hawthorne (1995), carefully distinguishing O(1) and O(3) sites; none of them has fewer than 0.737 apfu F (at O(1)).

Table 3 in Dyar et al. (1998) contains a few samples (No.5, 14, 16) which are Uvites by their Ca and Mg content. All of these have enough F to dominate the O(1) site - assuming they all reside at O(1), and the article has some discussion of this point. Interestingly sample number 14, which is near the midpoint between Dravite and Uvite going by cations but just a little on the Uvite side, also has just 0.56 apfu F. (Table 2 lists this specimen as a Dravite, presumably because that's how it had been labeled by the Canadian Museum of Nature. I would guess that must have happened before the Uvite species had been defined.)

Comments...?

(And so there would be theoretical room for an unnamed (OH)-analogue of Uvite to be discovered...?)

The IMA-CNMNC just approved (October 2009) a nomenclature of the tourmaline group minerals. We'll have to wait for publication of the final report.

You might also take a look at Hawthorne and Henry, "Classification of the minerals of the tourmaline group", European Journal of Mineralogy 11, 201-215(1999), which probably was an impetus to establish the tourmaline nomenclature subcommittee.

The IMA list has both uvite and feruvite as F-dominant. Both pages fixed.

Thanks a lot!

Chasing citations of Hawthorne and Henry has led me to some interesting further reading... will immerse myself for some long winter evenings whilst pondering the mutual preferences among the X and O(1) (and Y and Z and...) site occupants in my various Tourmaline specimens,

Cheers, Gerhard

With the new tourmaline defined (http://www.mindat.org/min-39672.html) based on ordering only observable with high resolution single crystal XRD, it could open the group up to many further divisions (we may never be able to label our tourmaline specimens again!)

Ralph

I have solved the problem of any future IMA nomenclature changes by labeling all of my specimens as 'Species: Mineral. Locality: Earth." Saves a lot of hair pulling and gnashing of teeth.

Hi
Just call 'em ROCKS !!!! or ROX !!!! to be different - no species, no type and no locality.

Then we could simply have MINDAT ROCKS !!!!


Hey..... it does already .... !!!!

Cheers

Here's a thought for discussion:

Does it really matter if it has an OH or an F in this position? Are we creating new mineral names just for the sake of it? Does it add to the science or is it just mineralogical bureaucracy in creating new names every time the 50% rule is violated?

Where we have long complex silicates with many sites with variable possible anions/cations, we can have a new mineral species defined on such a tiny overall % of chemical change that it would be a statistically insignificant impurity in a simpler mineral (Calcite, for example).

Where this change defines something measurable such as a change in crystallographical properties or a change in x-ray patterns then I can understand there's something of value from being able to differentiate it, or if it is a useful indicator to divide two different geological environments because of a difference in OH/F ratio or similar.

But I do wonder with things like the tourmalines, amphiboles and especially the eudialyte group that we're just creating new mineral names for the sake of it. Perhaps if we had a better set of guidelines for defining what a unique species really is would mean we need less IMA subcommittees to sort out the mess in particular groups every now and then!

My personal choice would be to allow minerals to violate the 50:50 rule and to say this mineral has this formula, but position Y, because it is of little significance, can contain any percentage mix of either F, OH, Cl . We'd still need different species for Apatite-(CaF) etc because that can hardly be described as insignificant.

I think the fundamental guideline has to be "what is the scientific merit of splitting this existing species into two based on variance in chemistry on position X?" If there isn't any, it shouldn't be split.

Jolyon

"Does it really matter if it has an OH or an F in this position?"

It does. The F- or OH-dominance is directly related to different environments and the observation of several F-dominant species/samples in the tourmaline group in the last two or three decades was also partly responsible for a more intense study of the role of F in rock and ore deposit genesis.


"I think the fundamental guideline has to be "what is the scientific merit of splitting this existing species into two based on variance in chemistry on position X?" If there isn't any, it shouldn't be split."

IMO it has not necessarily something to do with "merit" - science just observes and describes and if the occupancy of a X position is different, there is enough reason for use of a modifier or new name.

> It does. The F- or OH-dominance is directly
> related to different environments and the
> observation of several F-dominant species/samples
> in the tourmaline group in the last two or three
> decades was also partly responsible for a more
> intense study of the role of F in rock and ore
> deposit genesis.

I'm no expert on tourmaline mineralogy and would be happy to accept the importance of this variation, but by this logic we would have kept the different feldspar species as distinct minerals rather than lumping them all into Albite or Anorthite by the 50:50 rule.

Jolyon

The various feldspar names are so entrenched in centuries of petrology that it was decided to keep them.
There are other examples where the 50:50 rule was not strictly applied for similar reasons.

Ah. but Andesine, etc are not valid mineral names according to the IMA, just 'varieties' (my term not theirs) of Albite or Anorthite.

Jolyon

The reason that the plagioclase series got broken up into a number of different minerals was that it was an important rock forming mineral that gave a fair amount of useful information on the conditions of formation. But probably the most important point was that it was fairly easy to get a good chemical analysis by optical methods. You don't see similar breakdowns for olivines, pyroxenes or amphiboles.

Uwe, I think you are talking about Microcline and Orthoclase and Jolyon is talking about Albite and Anorthite. Oligoclase, Andesine, Labradorite and Bytownite are very important petrologically and well entrnched in history and yet they are mere varieties!!! I think the weak kneed approach to Microcline was caused by the uproar over the good effect of the 50/50 rule on the albite series. The trouble is that mineralogy has allowed itself too many masters. What is good for clay mineralogists, ore mineralogists, and petrologists is a nightmare for museum curators. Humans are visual animals and our eyes give our first line of attack. That is where the species level ought to be set. Instead visual discrimination seems to be set more or less at the group level. By all means descibe all the differences we can find with advancing microtechnology, but we should do so at the variety level, not the species level. My solution is to collect at the group level, but I get probes and x-rays to find out more about my specimens. The denegration of varieties and flat hierarchy of only species, that now is thankfully changing, has put social pressure on the pros to name more species. After all " Bravoite, A New Member of the Pyrite Group" looks much more impressive than "Notes on a Nickel Rich Pyrite", but the latter gives more information.