What does a square mean in a chemical formula?
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 [].