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.