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We find in the book Minerals and their Localities by Jan H Bernard and Jaroslav Hyrsl several chemical formulas containing a square. What does this square means? An example: Franciscanite $\ce{Mn3 (V,\square)(O,OH)3 (SiO4)}$

Thanks for your answer for I have been looking for answer for a long time.

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That square refers to a vacancy (or framework vacancy) in the crystal structure. Here's one reference from Advances in Electronic Ceramics. Ceramic Engineering and Science Proceedings. Volume 28, Issue 8. Ed. C. Randall ${\it et\;al.}$

In short, a framework vacancy is a feature of some inorganic materials in which a "guest atom" (or more than one) can fit into the structure or denotes a structure that is missing interstitial or packing atoms (as outlined here).

Here's the info on your mineral where you can see that the square is instead denoted by square brackets [].

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    $\begingroup$ Note that, according to IUPAC recommendations, a square box ($\square$) is indeed used for vacancies in certain contexts, but the use of italicized $V$ is preferred, the element vanadium being written with the upright letter $\ce{V}$. $\endgroup$
    – user7951
    Aug 30, 2015 at 10:07

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