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Improving Mindat.orgTennantite Formula Error?

3rd Oct 2019 19:45 BSTSteve Hardinger Expert

The formula given for tennatite is: Cu6Cu4(Fe2+,Zn)2As4S12S (note sulfur listed twice), whereas the formula given in Fleischer's 2014 is Cu6[Cu4(Fe,Zn)2]As4S13.

3rd Oct 2019 20:28 BSTErik Vercammen Expert

Both formulas have 13 atoms of S, so they are the same, but I suppose the Fleischer gives some additional structural information.

3rd Oct 2019 21:12 BSTRichard Gunter Expert

There seems to be some variability in the final S within the tennantite-tetrahedrite series. That may be why it is isolated in the tennantite formula.

3rd Oct 2019 21:15 BSTRalph Bottrill Manager

All other formulae for the members of the tetrahedrite group give S12S, though the last S can be vacant ( sorry I forget how to do subscripts here). It follows the review by
IMA proposal 18-K.

 It seems somebody forgot to change the formula for tetrahedrite, which come from the IMA website.

3rd Oct 2019 21:48 BSTJolyon Ralph Founder

Actually, it's worse than all of that.

Firstly, tennantite is no longer a valid mineral name.  It's now redefined and there are at least two new species, tennantite-(Zn) and tennantite-(Fe).

Have fixed the formula and the IMA status.

4th Oct 2019 20:54 BSTFrank K. Mazdab Manager

S is listed twice because it occurs in two structurally distinct sites (in the same way that the "Cu" is listed twice too, and in the same way that there are structurally different (OH) in tourmaline minerals).  The 12 S are in tetrahedral coordination and the lone S (that can be variably vacant) is in a special octahedral coordination (so please change any S13 back to S12S ).

We had this discussion about changing tetrahedrite group mineral formulas a few months ago (primarily about the Zn and Fe suffixes), and all the formulas were changed to conform to the new 18-K nomenclature rules.  But then it was pointed out that a lot of tetrahedrites and tennantites (both considered sensu lato) were not identified by collectors as being Fe-dominant or Zn-dominant, and so in addition to the newest 18-K nomenclature we re-added the old "tetrahedrite" and "tennantite" as generic names to accommodate collectors' typical lack of knowledge of their samples precise composition (as we have "hornblende" to serve the same purpose for uncharacterized calcic amphiboles).

So the formulas were changed (less than 2 years ago!), then partially changed back, and now it sounds like they were changed again.  Although I haven't looked at them recently, they were correct the last time I looked at them, and they should be changed back to reflect at least basic (at least the separate distinct sites) correct structural information. Whatever happens in the short term, however, I'm guessing this same discussion will happen again in 6 months, and that probably whatever is the status quo then will get changed again... grrr.

4th Oct 2019 22:20 BSTRalph Bottrill Manager

I fixed a couple Mindat formulae but note argentotennantite looks like it needs a suffix or two, but perhaps is still under review?

4th Oct 2019 22:21 BSTJolyon Ralph Founder

I'm not really that fussed whether we have S13  or S12S  - but the formula listed by the IMA (at least according to the site) is with S13

4th Oct 2019 22:33 BSTRalph Bottrill Manager

It would be nice if we were consistent across the group. There are a lot of groups with totally different formulae, really obscuring their relationships, eg vesuvianite group.

4th Oct 2019 23:11 BSTFrank K. Mazdab Manager

After we had our discussion a few months ago on tetrahedrite group formulas in light of then new 18-K IMA report, I *did* go and change all the formulas to conform to the report, but also including the additional structural information, because, well, we're supposed to be a "scientific" site and such information would be useful to people pondering substitutions in the group.  And then many of the formulas were subsequently largely changed back (now twice... lol), and I simply gave up and walked away from worrying about it after that; if I need good formulas for minerals, I simply now know that mindat is not necessarily the place to look for that, and look to recent publications instead.

Yes, the vesuvianite group, the eudialyte group, and a number of others are internally inconsistent.  For the vesuvianite example, if vesuvianite (old sensu stricto... now I believe this is alumovesuvianite) were simply written as:


then all the vesuvianite group members could be easily conformed to variations of that formula.  The problem with the IMA formula here and elsewhere is that the "end-member" formula is presented with parentheses (although I believe they finally got their act together and recently updated a lot of formulas to eliminate parentheses and express true end-members).

I'm loathe to make the changes myself, because in a few months someone will post "why does the vesuvianite formula look strange?" and then everything will be changed back.

4th Oct 2019 23:26 BSTJolyon Ralph Founder

I have no problem with taking initiative on formula entries such as these, but as you've seen the problem is that if we all take initiatives at the same time it goes bad.  So we need to figure out a procedure for knowing who is taking responsibility for these and let them deal with it. I'm happy if you do! As long as we are all in agreement here about the changes being done (and I am) then let's do it

4th Oct 2019 23:39 BSTFrank K. Mazdab Manager

OK, well then if there aren't any objections to my proposed format for the vesuvianite group formulas, this weekend I'll take a couple of hours to standardize them.  If there are later objections, or if new data come out indicating some further change is necessary, the group is still small enough to easily re-adjust.

I'm more reluctant to tackle eudialytes without more in-depth discussion, as that group is much more complex and decisions would have to be made as to the level of detail to include in the formulas.

What about the tetrahedrite group? Where do we stand on that?

EDIT: also, I'll only be able to make standardization edits to the mindat versions of formulas... if the IMA versions are coming from RRUFF, these not editable.

5th Oct 2019 01:27 BSTKeith Compton Manager

I'm all infavour of Frank updating formulae.

We probably should have some notes in the formula section stating/providing the IMA approved formula and the reasons for alternative formulae. It may also be helpful to show all the reasons for any changes over time (even if just a minor correction due to our input error) so that we know why a change was made.

Alternatively perhaps formulae are "locked in". When anyone proposes a change - enter the change and before it goes live that info goes direct to Jolyon/Frank whoever, etc for consideration and only then updated.

5th Oct 2019 02:10 BSTFrank K. Mazdab Manager

Having a formula change log, akin to the more general change log we already have for any changes on a page (which unfortunately many people don't seem to use), isn't a bad idea, especially if we can add the extra functionality that perhaps with the click of a button, we can revert to an earlier or later incarnation as we get new data or discover that an earlier/later version holds more utility or simplicity.  A log as to *why* changes were made may be less necessary, since I suspect in most cases it should be fairly obvious if one compares the new to the old.

As for a formula "lockdown", that may be a bit hard core.  I don't think most people on the site that otherwise feel comfortable adding photos or locality data would necessarily feel comfortable changing formulas, and most would probably prefer to just inquire about anything odd or unfamiliar here in a discussion (as Steve did).  And the few that would likely feel comfortable going in and tinkering with a formula probably have sufficient chemical/structural knowledge to do so reasonably.  There may be rare occasions where even a "correct" change may not be a desirable change (for example, the Ca19 in vesuvianite is really Ca2Ca8Ca8Ca, but is this degree of detail necessary? perhaps, as new species are defined, but maybe not now???), and I imagine eventually questions about items like that would come to light in a discussion.

Speaking of the vesuvianite group, coming up with end-member formulas for some the "weirder" members turns out won't be quite as straightforward as I had initially assumed, because charge-balanced end-members have never been defined for several of the members. Since in complex minerals there may be multiple ways to charge balance an otherwise simple substitution, I may not be able to entirely escape the dreaded parentheses, that is, until someone re-evaluates the group end-member formulas and proposes proper formulas to the IMA.  Hmmm... maybe a new project to think about...

6th Oct 2019 21:51 BSTRalph Bottrill Manager

I think I would trust anything you do here Frank!

Keith is also correct that when we amend formulae we should really leave a comment under “Formulae notes”. When I make changes I’m guilty, like most people, of usually leaving only a very minimal note in the changes log, but will rethink that.

Re the vesuvianite group, the existence of Manaevite-(Ce) as a member suggests that there may be a number of possible substitutions of other elements for Ca, as in the epidote group. This will require detailed single crystal XRD work to look at ordering, before we can provide consistent formulae, but the general vesuvianite formula may need to be at the very least something like  Ca11(Ca,H2O,REE)8...

Sorry Steve for hijacking your thread!

7th Oct 2019 02:39 BSTFrank K. Mazdab Manager

Hi Ralph,

thanks for the vote of confidence.

This weekend I did update the vesuvianite group mineral formulas to a consistent format (I'll start a separate thread on that here shortly).  It was not as easy as I expected, as five of the members (half the group) can be arrived at by various substitutions.  For manaevite-(Ce), I used the formula subscripts out of the reference for milanriederite (this "species" probably shouldn't even be a mineral)... the only liberty I took was in was in how I distributed cations into the Y2 and Y3 sites (not provided in the reference)... more on that in my new thread.

9th Oct 2019 21:25 BSTSteve Hardinger Expert

Then perhaps I should rephrase my initial question: What information is the formula trying to convey? In organic chemistry there are essentially three formula types. I'll use acetic acid as an example. The empirical formula gives the lowest whole number ratio of atoms without attempting to provide any structural information. For acetic acid the empirical formula is CH2O. Next we have the molecular formula which gives the true number of each atom within one molecule, but again without trying to convey any structural information. The molecular formula for acetic acid is C2H4O2. (In organic chemistry we list first carbon, then hydrogen, then all remaining elements in alphabetical order). Finally we have an abbreviated structural formula (often called a condensed formula because its written on a single line of text). For acetic acid the condensed formula is CH3CO2H or CH3COOH. To the slightly trained organic chemistry (first week or so of college level organic chemistry) this representation tells us one carbon is attached to three hydrogens (CH3; a methyl group) and a carbon, and the other carbon is bonded to an oxygen and a hydroxyl (OH). More complex structures or attempts to convey higher levels of structure generally require abbreviations referring to entire groups of atoms, or just most complex representations such as bond-line or stick formulas.

So what precisely is meant to be conveyed in the tennantite formula, and why is this level of structural information no consist across all Mindat formulas?

9th Oct 2019 22:07 BSTJolyon Ralph Founder

The structural formula shows that with S12S  you have  12 sulfurs in one position and 1 sulfur in a different position. This can be very important. Imagine we had a variant where selenium substitutes for one sulfur atom.  If that selenium is in the first position we have

[S,Se]12S   and we have a variant of the mineral with some selenium replacing sulfur.  But the same mineral as before.

but, if that selenium is in that second position we have

S12Se   which ends up being an entirely new species based on current rules.

9th Oct 2019 22:08 BSTJolyon Ralph Founder

And why no-one has updated all the other 5000+ formula?   Because no-one's had the time to do it.

10th Oct 2019 01:19 BSTSteve Hardinger Expert

At what point to the formula which attempt to convey some aspect of structure become cumbersome and unwieldy?

10th Oct 2019 10:07 BSTFrank K. Mazdab Manager

I don't understand the reasoning behind your comment, especially if you call yourself a chemist and purport to understand the value and utility of structural formulas in organic chemistry?  That same value and utility applies to mineralogy too.  If the structural formula for tetrahedrite is too complicated or unwieldy for you, certainly feel free to just write Cu-Sb-S on your labels... that list of elements conveys the most basic composition of the mineral (and several others too).  There's no requirement here or anywhere that you use the more complex formula for your own purposes.

For others here with a more scientific interest in minerals, the structural formula provides a fascinating wealth of information on how a mineral relates to others in its family, how compositional vectors can be used to model substitutions (this can be very interesting from a paragenetic standpoint, since these vectors can often relate directly to compositional changes in the environment), and even simply from an aesthetic prospective, since structural formulas show there's a certain elegance of how minerals are put together.  While the more complex formulas might seem, well, more complex, if you actually take some time to study them, you might come to appreciate them... and perhaps learn something new as well.

My personal opinion on this, and others here may disagree, is that since we generally have three places on a mineral page where we can express a formula (the "IMA formula", the "mindat formula", and the "idealized formula"), at least one of those should represent the most detailed and descriptive "state-of-modern-knowledge" version available for that species.  My suggestion is that one be the "mindat formula", because adding detail (or alternatively, making simplifications) to the "IMA formula" is somewhat out of our control, and the "idealized formula" is by definition an already simplified version.  Alternatively, we can add a place for a fourth formula version, the "structural formula" placeholder.

In any case, some here will be greatly interested in studying the "state-of-modern-knowledge" formulas for what hidden scientific clues they may hold... this is a website for scientists as well as collectors, after all.  But of course, for those here who aren't interested or comfortable with the more complex formulas, one of the other simplified versions are still there and can easily be consulted and utilized.

10th Oct 2019 15:47 BSTSteve Hardinger Expert

I don't 'call myself a chemist'. I **am** a chemistry with 30 years academic experience and some time in industry as well. Recently retired as a University Distinguished Lecturer (in Organic Chemistry) from UCLA. I agree that to the untrained novice as perhaps some appear to be the structural information inherent in various methods of representing the formula of an organic molecule may be a mystery. I don't have a formal training in mineralogy (and never claimed to have it) so what I was asking is what information do various polymorphs of a mineral formula attempt to imply? Or perhaps you can just point me to a reference that answers the question.

10th Oct 2019 19:42 BSTFrank K. Mazdab Manager


both Jolyon and I have already noted a couple of times in our respective replies what information structural formulas convey. But to offer an organic chemistry analogy:

You may be familiar with trifluoroacetic acid (its Na salt is a rat poison controversially used in NZ and Australia).  Three H in acetic acid are replaced by F, but if acetic acid is written C2H4O2, which three H are replaced?  It's not clear.  Of course, you could just string everything all together, C, H, F and O, but that becomes progressively unhelpful as eventually there become multiple compounds with those elements and even subscripts. But if acetic acid is written as CH3COOH, then it's clear that the three H associated with the methyl group C (and not the lone H associated with the OH) are the ones to be replaced: hence CF3COOH.  But to make the Na salt, we then do replace that last readily removable H.  Now with our [CF3COO]Na, we can go kill some rats.

So if you were running an "O-chemdat" type website akin to mindat, you might find it advantageous to write the more complex condensed formulas (so they stay on one line of text) than just the empirical formulas, for the benefit of your readers to think about reactions/substitutions like these.

It's true that in mineralogy, as opposed to organic chemistry, there are less possible combinations of elements, so structural formulas may seem like overkill.  But one simple example might be MgSiO3. We would look at that and probably just call it enstatite (a pyroxene). But if written as Mg3(MgSi)[SiO4]3 , it's actually majorite garnet (count up the Mg, Si, and O and observe the ratios are still 1:1:3, just as in enstatite). Not only that, but because one of the Si is in an atypical VI-coordination (which we know from the garnet formula, but we could also add roman numeral coordination number [C.N.] superscripts to the formula to emphasize that), it tells us that the mineral is likely a high pressure mineral, and indeed it is a deep mantle equivalent of magnesium silicate (among other structural types as well... there are "ilmenite" and "perovskite" versions of that formula too). If I added all the C.N. superscripts to these various versions, a clever mineralogist could even order them into increasing depth stability... useful, huh?

11th Oct 2019 22:23 BSTRalph Bottrill Manager

Well put Frank.

I’m always very envious of organic chemists, having an easy ability to show their structural formulae in an elegant 2D manner on paper, with topology, coordination, double bonds, etc., and you rarely have more than 3-4 elements in a molecule. Admittedly it’s amazing how many combinations you can make with just C and H! But with mineralogy you want to try to portray the essentials of a 3D ionic solid which may have a dozen or more distinct structural sites with commonly 6-8 essential elements, several of which can have different coordination polyhedra plus varying polymerisation of Si-O groups etc., and sometimes hundreds of atoms in a unit cell. Some people are happy with very simple formulae just showing elemental ratios but others of us really what some idea of what anion groups, ordering etc are happening and what are the relationships between different minerals and other inorganic compounds. There is a place for both extremes.
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