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When should one use (or not) parentheses in writing the formula for non stoichiometric compounds.

I mean, the mineral sphalerite is often represented by the formula $\ce{(Zn,Fe)S}$ and, all though I know that this really means $\ce{Zn_xFe_{1-x}S}$, I would like to know when the use of the first formula is recommended over the second.

That said, I have done a brief search on the IUPAC's red book about the use of parentheses and found nothing.

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Rule IR-2.2.3 / Parentheses, sub rule IR-2.2.3.1 / Use in formulae illustrates the use of parenthesis :

(d) In solid-state chemistry, to enclose symbols of atoms occupying the same type of site in a random fashion. The symbols themselves are separated by a comma, with no space.

Example: 9. $\ce{K(Br,Cl)}$

Note there is a subsequent, contrasting entry stating

(f) To indicate the composition of a non-stoichiometric compound.

Examples: 11. $\ce{Fe_{3x}Li_{4-x}Ti_{2(1-x)}O6}$ ($x = 0.35$)

  1. $\ce{LaNi5H_x}$ ($0 < x < 66.7$)

source: p. 21 of IUPAC's Red Book by 2005 the resources page (section nomenclature) links as freely available online release (pdf).

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    $\begingroup$ Certainly the example 11 is not one that could be denoted by parenthesis, since it does not represent atoms on the same site(s). $\endgroup$
    – Jon Custer
    Commented Oct 4, 2023 at 18:00
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    $\begingroup$ @JonCuster My interpretation of the OP's question was as the notations of $\ce{(Zn,Fe)S}$ and $\ce{Zn_xFe_{1 - x}S}$ were mutual synonyms of and fully equivalent to each other. So far, I speculate this is not the case and hence included example f). $\endgroup$
    – Buttonwood
    Commented Oct 4, 2023 at 18:55
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    $\begingroup$ I indeed agree, just pointing out that parenthesis could not be used for example 11. Coming from materials science, the implication of (Zn, Fe)S is you don't care what the ratio is, while for Zn$_{x}$Fe${1-x}$S you do care and there will be plots of something vs $x$. $\endgroup$
    – Jon Custer
    Commented Oct 4, 2023 at 20:12
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    $\begingroup$ (Zn,Fe)S and ZnxFe1−xS have different meanings, as stated by Jon Custer. In the 1st case Zn and Fe form a solid solution (Fe and Zn are located at the same crystallographic site). In the 2nd case, the formula implies a not-stoichiometric compound with Fe and Zn at different sites. (Zn,Fe)S and (ZnxFe1−x)S are equivalent; in the 1st case, the relative amounts of Fe and Zn are not specified (even though the global amount corresponds to a stoichiometric coefficient equal to 1). In the 2nd case, you can specify the specific amounts of Fe and Zn by putting, for example, x=0.1, 0.25, and so on $\endgroup$
    – gryphys
    Commented Oct 4, 2023 at 21:00

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