X[Y|Z] formulas

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? 




[The IUPAC rules for punctuation marks](http://old.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap2-3.04.pdf) 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](http://www.academia.edu/7540967/Brochantite_Cu4SO4_OH_6_OD_character_polytypism_and_crystal_structures) 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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