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Improving Mindat.orgCobaltian Mansfieldite - fishy formula

3rd Mar 2019 09:57 UTCFranz Bernhard Expert

The formula of cobaltian mansfieldite

https://www.mindat.org/min-11204.html

is fishy:

(Al,Co)AsO₄ · 2H₂O

There must be some charge balance, if Co is Co²⁺!

However, if Co is Co³⁺, then this should be explicitely stated:

(Al, Co³⁺)AsO₄ · 2H₂O

Franz Bernhard

3rd Mar 2019 10:20 UTCClosed Account 🌟

Also, why is it cobaltoan adamite but cobaltian mansfieldite?

Cheers,

Branko

3rd Mar 2019 12:02 UTCFrank K. Mazdab 🌟 Manager

The answer to Branko's question is the easier of the two: -oan is the suffix used when an element's lower oxidation state is involved, and -ian is the suffix used when element's higher oxidation state is involved. The "o" and "i" here are akin to the "o" and "i" in the prefixes "ferro-" and "ferri-", or "mangano-" and "mangani-" (among others). Use of the oan and ian are actually discouraged, because for elements with more than two oxidation states, the usage is ambiguous.

Franz's observation is more problematic. "Cobaltian mansfieldite" implies Co³⁺, but the mineral almost certainly contains Co²⁺ instead, because Co²⁺ in octahedral coordination with O is typically pink. That means that the name is probably more properly "cobaltoan mansfieldite". Co³⁺, although seemingly easier to justify as the substitution based on charge balance considerations, is probably less likely because I'm fairly sure Co³⁺ in octahedral coordination with O is supposed to be blue, as would be Co²⁺ in tetrahedral coordination.

If one assumes it really is Co²⁺ present, then that leaves the question of charge balance, and I can envision several possibilities, but I don't know which, if any, may be correct. One possibility is that some of the tetrahedral As⁵⁺ is replaced by a small amount of something with a higher charge like S⁶⁺ or W⁶⁺. Alternatively, a small amount of something like Ti⁴⁺ could balance out the charge deficiency of nearby Co²⁺. Another possibility is that while some of the O atoms are already protonated (hence the 2H₂O in the formula), additional O could be protonated as well. Given that the substitution of Co for Al is only about 5% (a couple of papers online given the composition for the Mt. Cobalt material), the disruption to the basic structure from any of these possibilities doesn't seem too problematic.

Finally, the possibility exists that maybe Co³⁺ in this particular coordination environment actually would give a pink color. Then the substitution is easily accomplished, but it would also make one wonder if more Co-rich compositions, or even a Co³⁺-end-member, should exist?

3rd Mar 2019 12:05 UTCFranz Bernhard Expert

>>why is it cobaltoan adamite but cobaltian mansfieldite?<<

Good point and thanks for the hint!

Maybe it has something to do with the valence:

cobaltoan = Co²⁺; cobaltian = Co³⁺

Like: ferroan = Fe²⁺ and ferrian = Fe³⁺.

(But can not find anything definitive at the moment)

If this is all true, formula should be:

(Al, Co³⁺)AsO₄ · 2H₂O

Franz Bernhard

3rd Mar 2019 13:58 UTCClosed Account 🌟

… I should have elaborated more, but in essence, as Frank said: "...the name is probably more properly "cobaltoan mansfieldite"...".

I have some spectroscopic evidence that scorodite (from the 3rd level of the Hilarion Area, Lavrion) has divalent iron balanced by OH in small amounts. Interestingly that work lead to the discovery of mansfieldite from Lavrion.

Branko

3rd Mar 2019 14:04 UTCAlan Pribula

The -oan/-ian designation ties to the officially-outdated-but-still-sometimes-used-by-oldtimers chemical nomenclature system in which the ion of lower charge was given the ending -ous and the one of higher charge was given the ending -ic. Thus, we had ferrous/ferric, stannous/stannic, cuprous/cupric, etc. These have now been replaced with names in which the charge is shown in Roman numerals: iron(II)/iron(III), tin(II)/tin(IV), etc. (These are pronounced as "iron-two," "iron-three." etc.)

But, the -oan/-ian endings aren't always used correctly or consistently. For example, copper almost always occurs as Cu(II), and only rarely as Cu(I), so should be designated as "cupri-," but is almost universally designated as "cupro-" in mineral names such as cupromolybdite, cuprosklodowskite, etc.

In the case of cobalt, cobalt can occur with either a 2+ or 3+ charge, but the Co(II) ion is much more stable. So, in this case, I'd speculate that "cobaltian" mansfieldite should really be called "cobaltoan" mansfieldite. (And, yes, Co(II) usually gives a pink color when octahedrally coordinated by oxygens.) If so, as Frank has suggested, some means of maintaining charge balance must be present along with the substitution of Co²⁺ for Al³⁺.

3rd Mar 2019 19:49 UTCUwe Kolitsch Manager

As a reminder:

Bayliss, P., Kaesz, H.D., Nickel, E.H. (2005): The use of chemical-element adjectival modifiers in mineral nomenclature. Canadian Mineralogist, 43, 1429-1433.

Chemical-element adjectival modifiers are not part of the name of a mineral species. 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 modifiers, 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. The valence (nominal numerical charge plus sign) or the numerical oxidation state may be added, if required. Authors should therefore feel free to use chemical-element adjectival modifiers that are chemically correct and that meet their particular requirements. For example, chemical-element adjectival modifiers such as “Mg-rich”, “Mg–Fe-rich”, “Fe2+-poor”, “iron(2+)-enriched”, “iron(II)-bearing”, “alkali-deficient”, “sodium-exchanged”, “Cr-doped”, or “H2O-saturated” may be used. Synthetic or hypothetical analogues of mineral species or natural analogues of mineral species unapproved by the CNMMN could be written with a chemical-element(s) suffix. The synthetic product “topaz-OH” is the OH-dominant analogue of topaz, Al2SiO4F2. The use of quotation marks around “topaz-OH” is essential to show that the name is not approved by the CNMMN and to avoid confusion with names of real mineral species, such as chabazite-Ba. A chemical-element symbol should not be used as a prefix to a name of a mineral species. However, if used in a diagram, table, or running heading owing to space limitations, then a correct version must be used in the text together with the short version in quotation marks to show that the name is not approved by the CNMMN. For example, “Al-goethite” used in a table owing to space limitations should be shown as Al-rich goethite in the text.

3rd Mar 2019 20:08 UTCAlfredo Petrov Manager

Has it been determined that the pink color of Co-bearing mansfieldite is really due to Co incorporated into the mansfieldite structure itself, and not just (sub)microscopic inclusions of some other Co species? If the latter happens to be the case, then Co should not be listed in the formula of "cobaltoan mansfieldite" at all.

3rd Mar 2019 20:17 UTCRalph S Bottrill 🌟 Manager

Good point Alfredo, I was thinking the same myself but I’m not aware of any analyses, I suspect it’s just based on colour. I will see about testing some if I can find a bit to sacrifice.

3rd Mar 2019 20:25 UTCFrank K. Mazdab 🌟 Manager

your're right, Uwe... use of "oan" and "ian" is more than discouraged... they're "officially" inappropriate as parts of optional modifiers in mineral names. But their use is also common in general chemistry outside of the IMA's kingdom of control (and indeed have been in use in general chemistry long before the IMA ever existed, and will likely persist long after the IMA is replaced). But nonetheless, the endings are ambiguous and the IMA is correct on this one. "Cobaltian" (or "cobaltoan") mansfieldite should ideally be replaced simply by Co-bearing mansfieldite... whether that should be better detailed as Co²⁺-bearing mansfieldite (my suspicion) or Co³⁺-bearing mansfieldite can await further work on the material (or, as Alfredo and Ralph note, perhaps dropped altogether if the coloring Co isn't even present as a chemical substitution). To that latter issue, however, someone actually did do a structure refinement on specifically cobalt-bearing mansfieldite from Mt. Cobalt, so one would hope that researcher would have noticed an admixed phase during their X-ray work.

4th Mar 2019 10:23 UTCDavid Von Bargen Manager

Seems to be replacement

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2968216/ (full article)

https://www.researchgate.net/publication/51136056_Rietveld_refinement_of_a_natural_cobaltian_mansfieldite_from_synchrotron_data

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