Hi, I'm not sure if it has been mentioned yet, but you could take an experimental approach if you still have crystals. You could soak them with EDTA/EGTA and if the density disappears that is good evidence it was Mg (or other divalent cation), if not then putatively water. You could also soak with Mn (assuming the Mn will bind the Mg site) and look for anomolous signal.
John
