What does a square mean in a chemical formula?

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.




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 ${\it et\;al.}$](https://books.google.com/books?id=NlaxmzJobLYC&pg=PA233&lpg=PA233&dq=mineral+formula+denote+vacancy&source=bl&ots=2cL8prSB-b&sig=tBw3neAD_eLbfH9VZZQx5rE9rKk&hl=en&sa=X&ved=0CCoQ6AEwAmoVChMI-uSr6o3PxwIVwY8NCh0A1wLE#v=onepage&q=mineral%20formula%20denote%20vacancy&f=false)


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](https://books.google.com/books?id=9gbZBAAAQBAJ&pg=PT18&lpg=PT18&dq=square+brackets+in+chemical+formula+of+a+mineral&source=bl&ots=IilLvtLF6e&sig=4nRmSFW5XDpIBJyi9OaINOoZJ3U&hl=en&sa=X&ved=0CDMQ6AEwBGoVChMImcmDqrTPxwIVBdGACh3o4AGz#v=onepage&q=vacancy&f=false)).


[Here's the info on your mineral](http://webmineral.com/data/Franciscanite-VIII.shtml) where you can see that the square is instead denoted by square brackets [].


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