Feynman mentions in his lectures:
...the concept of a molecule of a substance is only approximate and exists only for a certain class of substances. It is clear in the case of water that the three atoms are actually stuck together. It is not so clear in the case of sodium chloride in the solid. There is just an arrangement of sodium and chlorine ions in a cubic pattern. There is no natural way to group them as "molecules of salt".
For me that's not clear. A molecule of water also is stuck in a pattern (at least when it's solid). I'd think of any molecule as 'atoms stuck in a pattern'. Thus why a crystal is not a special case of a molecule?