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Mineralogical ClassificationIMA 2017-118 = jahnsite-(MnMnMg)

22nd May 2019 09:01 UTCMarco E. Ciriotti Manager

Reference:

▪ Vignola, P., Hatert, F., Baijot, M., Rotiroti, N., Risplendente, A., Varvello, S. (2019): Jahnsite-(MnMnMg), Mn2+Mn2+Mg2+2Fe3+2(PO4)4(OH)2·8H2O, a New Phosphate Mineral Species from Sapucaia Pegmatite, Sapucaia Do Norte, GalilÉia, Minas Gerais, Brazil. Canadian Mineralogist, 57, 363-370.

Abstract:

Jahnsite-(MnMnMg), Mn2+Mn2+Mg2+2Fe3+2(PO4)4(OH)2·8H2O, is a new phosphate mineral belonging to the jahnsite subgroup contained in the jahnsite group of minerals. It was found in the dumps of the Sapucaia pegmatite, Sapucaia do Norte, Galiléia, Minas Gerais, Brazil by one of the authors (SV). The mineral occurs on the surfaces of hydrothermal cavities and is associated with frondelite, mangangordonite, and leucophosphite. It forms prisms weakly elongated along a, flattened on [010], and twinned by reflection on {001} up to 200 μm in length. The new species is a late stage alteration product of primary phosphates such as triphylite. Its color is yellow to honey-colored or greenish-yellow and it is partially transparent with a vitreous luster. The streak is very pale yellow. Jahnsite-(MnMnMg) is brittle with good cleavage on {001} and irregular fracture. Its inferred Mohs hardness is 4 and the calculated density is 2.625 g/cm3. The mineral is optically biaxial (–) with α = 1.616, β = 1.619, γ = 1.656 and 2V (calc.) = 74° and non-fluorescent under 254 nm (short wave) and 366 nm (long wave). The empirical formula is: (Mn0.50Ca0.25Na0.20)Σ0.95Mn(Mg1.26Mn0.52Fe2+0.21Zn0.01)Σ2(Fe3+1.63Al0.37)Σ2(PO4)4(OH)1.70·8H2O. The endmember formula is Mn2+Mn2+Mg2+2Fe3+2(PO4)4(OH)2·8H2O. Jahnsite-(MnMnMg) is monoclinic, with space group P2/a and unit-cell parameters a 15.177(2) Å, b 7.176(1) Å, c 10.006(3) Å, and β 110.01(2)° with V 1017.3(2) Å3 for Z = 2. Its crystal structure was refined and the mineral is isostructural with other members of the jahnsite group.

22nd May 2019 10:45 UTCKeith Compton 🌟 Manager

Why does the IMA insist on creating mineral names with chemical elements following the same group name?

We now have 11 members of the Jahnsite group - all with the same first name followed by combinations of elements in brackets.... Why?

The IMA's own guidelines re naming new minerals include:

• Dedication to a personality prominent in the field of mineralogy or a related field;

• Allusion to the morphology or a physical property of the mineral. In such a case, Greek and/or Latin roots are most often juxtaposed;

• Allusion to the chemical composition of the mineral, often by juxtaposing chemical symbol (eg tantalite);

• Name of the geographical locality of the occurrence: country, province, locality, mine, river.

There is no suggested "guideline" regarding using the same name with elements following.

In fact the IMA guidelines indicate that the "name should be sufficiently different from the already existing minerals". Ten other minerals with Jahnsite as a precursor is hardly "sufficiently different".

Come on IMA - I really suggest that you get with your own program.

Ok, I'll probably get some flak for this outburst, but we only have a few thousand minerals - so why can't we have more individualised names?

In the current case we could have had Sapucaite - a perfectly reasonable and unique name that would have had historic relevance in future - being the type locality name.

Maybe we should have a committee decide on replacement names for the rest of the Jahnsite group member names.

There is nothing to stop these minerals being renamed - afterall, various other minerals have been renamed over the years (e.g.: fluorapophyllite).

22nd May 2019 11:38 UTCFrank K. Mazdab 🌟 Manager

Nomenclature-wise, what really is kind of strange is that in the related whiteite subgroup, there are a half dozen whiteites (akin to the dozen or so jahnsites), but there's one outlier with a unique non-whiteite name... rittmannite. What's that all about? It seems like it ought to be whiteite-(MnMnFe), if one follows the pattern?

As for the chemical suffixes in general, jahnsites overall seem obscure enough that probably not a lot of people will even be familiar enough with them to be concerned about 11 group members names that differ only in suffix. Now, when they did that silliness to apatite and renamed (temporarily... lol) fluorapatite as apatite-(CaF)... that was a scandal!

I have to admit I can see an advantage to the present scheme... if someone says jahnsite-(MnMnMg), I might be able to deduce something about the mineral... maybe its geologic environment, and certainly a bit of its chemistry. If someone were to say "sapucaite", that would mean nothing to me, unless I was specifically familiar with that Brazilian pegmatite. But perhaps I should be careful what I wish for... once this suffix scheme gets imposed on the eudialyte group, for example, and turning a mineral like kentbrooksite into "eudialyte-(NaNaNaNaNaCaMnNbSiF)", then we'll really be in trouble! LOL

22nd May 2019 11:56 UTCKeith Compton 🌟 Manager

Frank

I feel like saying Nah Nah Nah n.. Nah .. it won't happen - but who knows !! ((-:) Poor old kentbrooksite - I think it needs more loving, just in case.

22nd May 2019 12:18 UTCJolyon Ralph Founder

There's a ton of Eudialyte group minerals which are, to all extents, the same damn thing with tiny tiny % difference between Fe, Mn and Mg in some minor position.

I wish we could just say 'this position really isn't that important, it can be Mn, Fe or Mg and it's still the same stuff. Because in reality it IS the same stuff

22nd May 2019 19:54 UTCFrank K. Mazdab 🌟 Manager

Hi Jolyon,

Maybe... but for better or worse, as mineral structure analysis gets even more precise and the shapes, sizes and C.N. of crystallographic sites are characterized with increasingly better precision, my suspicion is that we're going to be moving in a direction perhaps opposite than you'd prefer... a direction where groups of "new" minerals will just be single old minerals re-defined by more precise site understandings.

Down the road I may end up being guilty of one or two of those new minerals myself, as a couple of otherwise already "simple" minerals I've been looking at on the microprobe have compositions that seem oddly suggestive of having more apparent order than I would have expected. And I'm seeing this as from the chemistry side, not the crystallography side where these observations are typically made, so that makes these examples even more compelling.

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