How can ionic substances have an infinitely-repeating lattice structure? The title's pretty much it. At school, we're taught that ionic substances are composed of an infinitely-repeating lattice, with atoms at fixed angles from other atoms. How is this possible when I can get an individual grain of salt? Clearly if there's an individual grain, it can't be infinitely repeating.
To be technically accurate, it's "practically" infinite.

It's not infinite since as you mention a grain of salt (or even a large crystal) has some finite size.

You have to remember the actual atomic dimensions $10^{-10} \:\mathrm{m}$. So while it's not truly infinite, there are an incredibly large number of atoms in a crystal.