# 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 $\ce{Mn3 (V,\square)(O,OH)3 (SiO4)}$

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 ${\it et\;al.}$
• 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 $\ce{V}$.