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cobaltian calcite - and its fluorescence
  
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Pete Modreski
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PostPosted: Jul 30, 2008 08:32    Post subject: cobaltian calcite - and its fluorescence  

Hi to the Minerals Forum,

I'd like to pass on some questions about cobaltian calcite that have arisen on another internet minerals discussion group ("rockhounds@lists.drizzle.com") to see if some here on Jordi's forum can help provide some answers to these questions--some from that listserv, and some of my own, as I realized that when I tried to answer these myself, I did not understand this mineral as well as I thought I did.

Bright pink cobaltian calcite occurs at two notable localities worldwide that I know of; the Shaba copper-cobalt district, Democratic Republic of Congo (I must here also admit that I am a little unclear as to whether this and Zaire are the same, different, or adjacent localities), and Bou-Azzer, Morocco.

Someone had written to the Rockhounds listserv, asking about the fluorescence of cobaltian calcite from Morocco. This gives rise to several questions:

To my knowledge, Congo/Zaire cobaltian calcite is not fluorescent, but that from Morocco is; is this in fact correct?

"Conventional wisdom" is that cobalt is a "quencher" of fluorescence, so cobalt-bearing calcite would not be expected to fluoresce; yet the Moroccan calcite does. What gives? Does it really contain cobalt, not manganese? Does it contain both cobalt and manganese, with the latter causing its fluorescence?

I realize that I'm also a bit confused about the terminology of this calcite. Classically, "cobaltoan" would be used for a mineral containing cobalt in its lower oxidation state (Co+2) and "cobaltian" would be used for cobalt in a higher oxidation state (Co+3). Does the pink "cobaltian" calcite truly contain Co+3, or is "cobaltian" just being used for any cobalt-bearing calcite? I have seen it referred to both as "cobaltian" and "cobaltoan" calcite.

I feel like I should be able to answer all these questions myself, but I find that my experience and knowledge here are just not adequate. Perhaps someone else can help shed light on some or all of these questions! Thank you, and I'll look forward to what some of you may be able to share about this topic,

Pete
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Jordi Fabre
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PostPosted: Jul 30, 2008 09:24    Post subject: Re: cobaltian calcite - and its fluorescence  

I think that I can help on this topic. We usually checked our specimens to UV light and, as we have frequently cobaltian Calcites from Morocco, I always observed a very interesting phenomena with them. The paler ones shows an intense fluorescence under both lights, the medium colored ones shows just a medium fluorescence (low under long UV, medium under short UV) and the really colorful ones shows a pale or inexistent fluorescence. So, I consider that the amount of cobalt is that could give the ratio between color and fluorescence and that would be consistent with the fact that more or less cobalt (so, more or less color) could generate more or less fluorescence. I add photos of three specimens of Moroccan cobaltian Calcites to show the difference.

About the cobaltian Calcites from Congo D.R. (actual name of the country after the last political changes on the former Zaire) there is more complicate because some times the specimens from there, are not cobaltian Calcites, but cobaltian Dolomites or a mix of Calcite-Dolomite. So, honestly I can't say so much about the reason of the frequent intense fluorescence of the Congo specimens, nor if it is due to a major amount of Cobalt or not.

Hopefully it helps,

Jordi
PS: To see more cobaltian Calcites and their fluorescence please use:
https://www.fabreminerals.com/search_results.php?LANG=EN&SearchTerms=&submit=Search&MineralSpeciment=Calcite+%28cobaltian%29&Country=&Locality=&PriceRange=&Currency=USD%28%24%29



Cobaltian Calcite intense fluorescence.jpg
 Description:
Pale cobaltian Calcite from Morocco giving intense fluorescence under short UV and fluorescent under long UV.
 Viewed:  26370 Time(s)

Cobaltian Calcite intense fluorescence.jpg



Cobaltian Calcite medium fluorescence.jpg
 Description:
Regular color on this specimen, which is fluorescent with short UV and with minor fluorescence under long UV.
 Viewed:  26359 Time(s)

Cobaltian Calcite medium fluorescence.jpg



Cobaltian Calcite minor fluorescence.jpg
 Description:
Colorful moroccan cobaltian Calcite with just minor fluorescence.
 Viewed:  26364 Time(s)

Cobaltian Calcite minor fluorescence.jpg


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Pete Modreski
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PostPosted: Jul 30, 2008 13:41    Post subject: Re: cobaltian calcite - and its fluorescence  

Thank you for the help and for the photos, Jordi. I knew you would be familiar with this, because I knew you had many specimens of this mineral from Morocco; when I'd searched for images online, several had come up from your website. And thank you for straightening me out about Ziare = now, Democratic Republic of Congo.

What you say and your pictures seem to confirm what I'd concluded from what I'd read before; that there is an inverse relationship between cobalt content (depth of pink color) and fluorescence; which does make it seem likely that manganese is responsible for the fluorescence, whereas cobalt accounts for the intense pink color.

I suspect that the very bright pink, Congo cobalt-bearing calcites, have much cobalt and little or no manganese. I guess I will have to try some searching of the published literature about the mineralogy of the deposits in the two countries, to see if anyone has published actual chemical analyses of the respective pink calcites.

Answering more of my own question, I just reviewed the matter of "chemical adjectivial modifiers", and I see that the use of such terms as "cobaltian" is NOT recommended by the I.M.A.--because it is ambiguous, usually inaccurate, and causes confusion. A 2005 paper by Bayliss, Kaesz, and Nickel (Can. Min., v. 43, pp. 1429-1433), posted online at
https://www.geo.vu.nl/users/ima-cnmmn/modifiers.pdf
says, to quote excerpts from that article,

"Schaller-type adjectival modifiers, which have the endings -oan or -ian, formerly recommended by the CNMMN of the IMA, in many cases give erroneous information about the valence of an ion, and are therefore inappropriate. Instead of such modifi ers, the CNMMN has now approved a proposal that chemical-element adjectival modifiers employing the chemical-element symbol or the name of the chemical element together with a descriptive term should be used.

"Authors should therefore feel free to use chemical-element adjectival modifiers that are chemically correct and that meet their particular requirements. For example, “Mg-rich”,
“Mg–Fe-rich”, “Mn-enriched”, “Fe2+-poor”, “iron(2+)-enriched, “iron(II)-bearing”,
“alkali-deficient”, “sodium-exchanged”, “Cr-doped”, or “H2O-saturated” may be used.

"The Commission on the Nomenclature of Inorganic Chemistry of the International Union of Pure and Applied Chemistry (CNIC of IUPAC) very strongly disapproves of the “-oan” and “-ian” chemical-element adjectival modifiers (Leigh 1990)."


So, our use of "cobaltian calcite", still obviously in common use for the describing mineral specimens, is really not what is recommended by the I.M.A. For the record, I checked about the chemistry of cobalt, too. Most known cobalt compounds contain cobalt in the +2 oxidation state, and just a few contain Co+3. Cobalt-bearing calcites, dolomites, etc., are most likely to contain cobalt as Co+2, therefore, "cobaltoan" would in fact be the most appropriate adjective to use--but it is still frowned upon by the IMA, so their "official" advice would be to call these specimens, cobalt-bearing calcite. (I doubt whether we are going to change dealers' and collectors' labelling practices in this regard, any time soon.) I looked to see what MINDAT uses; I see there that it describes the localities for these pink calcites under Cobaltoan Calcite (a variety of Calcite), and lists cobaltian calcite and cobaltocalcite, as synonyms. (It's generally now agreed, of course, that "cobaltocalcite" is not a desirable term to ever use, because it implies that this is a separate mineral species, which it is not.)

Pete
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