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I often find formulas of minerals where some element symbols are grouped within square brackets and separated by a vertical bar. An example can be brochantite, whose formula I find expressed as $\ce{Cu_4[(OH)_6|SO_4]}$.

I wonder what does such a notation means, and I have made some conjectures, although I find no reference on line or in books. I suppose that the square brackets are used to enclose the set of the anions present in the compound and the vertical bar is used to separates different anions, while the cations remain outside the square brackets. That is just an intuitive supposition. Is my supposition correct? If it is not, what does the described notation mean?

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    $\begingroup$ I'm not sure but wikipedia has just written a normal formula for Brochantite: $\ce{Cu4SO4(OH)6}$. Could you give me some reference of where you saw that formula? $\endgroup$ – M.A.R. Jan 18 '15 at 13:32
  • $\begingroup$ @MARamezani Thank you very much for the comment! I've found this particular formula in an Italian language book: A. Mottana, R. Crespi, G. Liborio, "Minerali e rocce" ('Minerals and rocks'), but I've found similar notations in several other books of earth sciences. $\endgroup$ – Self-teaching worker Jan 18 '15 at 14:16
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    $\begingroup$ Maybe, and I say maybe, they wanted to demonstrate something like, for example, brochantite has $\ce{OH-}, \ce{SO4^{2-}}$ and copper. $\endgroup$ – M.A.R. Jan 18 '15 at 15:24
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    $\begingroup$ Now that I think about it, I see that two brackets like this ][, if not awkward, would've looked way messier than a | . Maybe it was the author's or the publisher's attempt to make chemical formulas look a little bit neater. $\endgroup$ – M.A.R. Jan 18 '15 at 16:25
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    $\begingroup$ @Self-teachingDavide Since $UO$ exists as a compound, then $UO_2^{2-}$ can also exist. webelements.com/compounds/uranium/uranium_oxide.html $\endgroup$ – LDC3 Jan 18 '15 at 17:39
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The IUPAC rules for punctuation marks in chemical formulas don't mention [|], so that notation must be peculiar to geology. They need different notation; chemists write formulas for molecules, while geologists focus more on complicated superstructures built from smaller structures that can be arranged in different ways, even within the same superstructure.

This paper describes the structure of brochantite as layers and chains of copper(II) ions octahedrally coordinated with hydroxides. The octahedra in different layers and chains are crosslinked by sulfate ions.

Perhaps the notation $\rm [(OH)_6|SO_4]$ is an attempt to indicate this arrangement. The brackets denote the electronegative part of the structure, with what comes before the vertical bar representing directly coordinated ions, with what comes after representing ions that aren't directly coordinated.

I'm speculating, though. Sorry. I would have put this into a comment, but it's too long and I wanted to give you the links to think about.

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