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Mineralogical ClassificationPriscillagrewite-(Y)

17th Jan 2022 23:13 UTCMartin Stark

Hi,

Priscillagrewite-(Y), a garnet of the form

(Ca₂Y)Zr₂(AlO₄)₃

is put under "Nesosilicates", see below. Why?

The cubic garnet structure has no intrinsic relation to silicates. As far as I understand, the term nesosilicates refers to [SiO₄]⁴⁻ blocks with no Si-Si bonds.

Here I'd rather see an aluminate with

[AlO₄]⁵⁻ building blocks.

To my eyes the term "nesosilicate" is definitely misused here.

Cheers.

Strunz-mindat (2022):

9.AD.

9 : SILICATES (Germanates)
A : Nesosilicates
D : Nesosilicates without additional anions; cations in [6] and/or greater coordination

18th Jan 2022 02:53 UTCFrank K. Mazdab 🌟 Manager

While of course technically not a silicate, it seems Strunz is OK with it being included with silicates. Other related species, including several other garnets with [AlO₄], [Fe³⁺O₄] and even [◻H₄O₄] are also included among the silicates. On the other hand, yafsoanite, with [ZnO₄], isn't included with the silicates, but different classification schemes can't seem to agree on whether it should be an oxide or a IV-tellurate (and no one seems to want to consider it as a zincate, despite that being the best analogy with silicate).

An alternative classification scheme by Hölzel (see mineralienatlas.de entries for the garnets) places several of the IV-aluminate and IV-ferriate garnets into the oxide category instead of silicates, but oddly he has them reside under "dioxides" (which they also are not); priscillagrewite-(Y) is not included in his classification... perhaps it's too new. He also includes katoite with the silicates, although the end-member is both Si-free and tetrahedral cation-free. So there are some inconsistencies here as well.

Everyone else (Lapis, Dana) all seem OK with these Si-free analogs to be quietly lumped in with the silicates. Although perhaps Hölzel was on to something with his "garnet structure oxides" category, it probably needs to be moved to under the M₂O₃ hierarchy. As for us, while adding new minerals to pre-existing categories as part of our Struntz-mindat update is an ongoing process, I don't know if revamping Strunz's classification with brand new categories is something we intend to undertake.

18th Jan 2022 04:20 UTCAlfredo Petrov Manager

My pet peeve is those classifications that lump Quartz in with the silicates ;((

18th Jan 2022 05:05 UTCKevin Conroy Manager

Yes, I hate it when that happens. It's kinda like someone saying that they're a vegetarian while eating a steak.

18th Jan 2022 06:29 UTCMartin Stark

Hi Frank

Thanks for that. However, Strunz 9 is 2009 but Priscillagrewite-(Y) is 2020 - who maintains it? And still, even if there are more examples of

[SiO₄]⁴⁻ free minerals put into that classs - why so? What is the reason to put them there? Why not rename the whole tree accordingly (assuming, the 2022 version is kind of unapproved anyway, this step could be done without causing more damage...),

My understanding was, that the term "nesosilicates" originates rather from silicate chemistry than from mineralogy. Maybe I am wrong?

Cheers, Martin

EDIT: Just from a quick look through the other Silicate-classes I got the impression, that only nesosilicates are treated that way? But as I said, this was a quick shot.

18th Jan 2022 11:03 UTCFrank K. Mazdab 🌟 Manager

You might have hit the nail on the head. It seems from a quick glance that all the IV-aluminate and IV-ferriate garnets were approved in 2009 or later. Katoite is older, but the original formula wasn't the end-member and still included some Si (with x subscripts).

Officially, I don't think anyone is maintaining Strunz anymore. But informally, here at mindat, we've taken on the mantle of adding post-2009 species to a new "Strunz-mindat" classification. But we've only added new numbers under already-existing categories... we haven't created any new categories.

I was apparently looking at an old cache earlier when I looked at some of these other garnets, because now all of them (except priscillagrewite-(Y) and katoite) are classified as oxides. So I've changed the Strunz-mindat (2022) classification for priscillagrewite-(Y) to 04.CC (and katoite to 04.FF).

As for you observation about this only being an issue with nesosilicates, it's because aluminates essentially don't polymerize. There are polymerized borates and a few polymerized phosphates, but since borates and phosphates are already Strunz groups (like silicates), the polymerized members just fall under those pre-existing headings. There is, however, no aluminate category (minerals like diaoyudaoite, NaAl₁₁O₁₇, aren't aluminates but are really multiple oxides) because all these minerals are either simple oxides or multiple oxides. I suppose the non-silicate Ca-Ti-Tschermaks pyroxene, CaTi[Al₂O₆], would be a polymerized "ino-aluminate", if it existed as a mineral.

18th Jan 2022 19:37 UTCMartin Stark

it's because aluminates essentially don't polymerize.

Hi Frank

that was my initial thought that brought in the question. Thank you for shifting priscillagrewite and katoite - looks far better now! Yet, as remnant one mineral still appears under nesosilicates that should move:

Xuite Ca₃Fe₂[(AlO₃(OH)]₃ (approved Dec 2020). May I ask you to shift this one as well?

Again many thanks!

All the best,

Martin

18th Jan 2022 20:12 UTCFrank K. Mazdab 🌟 Manager

will do...

edit: xuite is now 04.CC.

18th Jan 2022 20:15 UTCMartin Stark

Perfect, Thanks!

18th Jan 2022 10:22 UTCAndreas Karlsson

This is also at odds when it comes to the other minerals with garnet structure, i.e bitikleite (AlO4)3 , dzhuluite (Fe3+O4)3, elbrusite(Fe3+O4)3 and usturite(Fe3+O4)3, currently these are all classified as oxides 04.CC which makes alot more sense. So either you move all the non-silicates garnets to the nesosilicates or you stick them with the oxides, splitting them makes the least sense.

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