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I agree that at the right temperature and pressure diamond (or almost anything) can be transformed into a liquid, but I'm not sure if I would call it melting. Certainly the liquid would not be diamond, just some form of liquid carbon, even though it could return to diamond form if cooled at the right rate.Phase changes of lattice materials are also chemical changes. Molecular substances can melt without breaking any covalent bonds, so the chemical properties of the solid and liquid are generally close enough we can call it the same substance. I would not call water ice a lattice material--it is still a molecular solid, although it is about as close to a lattice material as can be achieved by a molecular solid. O-H bonds within water molecules are ~0.10 nm while the H--O distances between water molecules in ice are ~0.18 nm, which is consistent with a "hydrogen bond" which is somewhere between a covalent bond and a very strong dipole-dipole interaction.