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EducationGreek letters as prefixes in chemical formulas

2nd Apr 2024 04:45 UTCHerwig Pelckmans

Some of the formulas on mindat start with a Greek letter. For example:

goethite: https://www.mindat.org/min-1719.html : α-Fe³⁺O(OH)

lepidocrocite: https://www.mindat.org/min-2379.html : γ-Fe³⁺O(OH)

The IMA-list does not have these prefixes for these minerals. Both their formulas are simply

FeO(OH)

If I remember correctly, for these minerals, the prefixes are used to indicate the different polymorphs of FeO(OH). As such, they provide structural information, not chemical information.

Since the prefixes don't provide any chemical information, should they be part of the formula?

Or should "α-Fe³⁺O(OH)" be regarded as a synonym of goethite, and the mindat formula be reduced to Fe³⁺O(OH) ?

2nd Apr 2024 07:13 UTCFrank K. Mazdab 🌟 Manager

One could argue the ( ) around the (OH) provides structural information rather than chemical information... that is, the formula could simply be written as FeOOH, or even FeO₂H, if we only cared about the chemistry, purely. So I don't see a problem with the Greek prefixes if they provide useful structural information, just as I don't see a problem with putting brackets around anionic groups [SiO₄][Si₂O₇] in epidote, adding coordination numbers or charges where's there ambiguity (Ag⁶⁺ for the six separate silver in argentotetrahedrite -vs- [Ag₆]⁴⁺ for metal-bonded Ag-cluster in kenoargentotetrahedrite), or separating one element into different structural sites (as above), and as even our original FeO(OH) does. Note the "3+" in "Fe³⁺" is redundant and thus unnecessary (one could easily derive this, if needed), but it's still nice to include for clarity.

2nd Apr 2024 08:18 UTCHerwig Pelckmans

Thanks for the reply, Frank.

I can see your POV, and I'm in favor of clarity (that's why I held on to the "Fe³⁺" in my last sentence, even though I knew it is redundant since easily calculable).

Since I assume the IMA at one time had α-FeO(OH) in their list, part of the question is: why did they remove the Greek letter prefixes? Does anyone know?

2nd Apr 2024 08:59 UTCUwe Kolitsch Manager

why did they remove the Greek letter prefixes? Does anyone know?

The argument was: Greek symbols are not easy to show in databases.

2nd Apr 2024 09:44 UTCRik Dillen Expert

Which is BTW also the case for some special characters in mineral NAMES in other languages than English, such as in Bouškaite (š), Bukovskýite (ý), Kaňkite (ň).

2nd Apr 2024 10:12 UTCJolyon Ralph Founder

This is why we have separate fields in the database for our own formula and the "official" IMA formula.

> Since I assume the IMA at one time had α-FeO(OH) in their list

The problem here is that the entire concept of an "official IMA list" is a bit misleading - different parts of the IMA maintain different lists, mostly as volunteer non-technical efforts by a single individual. In the past it was even less coherent than it is now, with IMA-related individuals publishing their own lists in their own style. It's just good fortune when any of these lists have the same data.

2nd Apr 2024 21:17 UTCRalph S Bottrill 🌟 Manager

I guess it was thought useful to indicate there are other polymorphs, but we rarely use it elsewhere, eg silica polymorphs, CaCO3, etc, so to be consistent perhaps best dropped?

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