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.


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