I had a student ask me this in class the other day; I thought about it, but I still can't work it out.
Many mineral and rock hydrates (take basalt, for instance) have much lower melting points than their anhydrous counterparts. I could not craft a plausible explanation for this.
I realize that hydrates have different crystal structures due to the presence of water, but I am unsure how this will affect the melting points. If anything, the possibility of hydrogen bonding would make the melting point higher, no?