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Basalt melting point: wet vs dry

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?

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    $\begingroup$ Basalt is not a hydrate. :-( $\endgroup$ – MaxW Jan 25 '17 at 2:37
  • $\begingroup$ Thanks for the correction, but that still doesn't answer my question. See the hyperlink I edited in for the image. It refers to basalt as "wet" or "dry." $\endgroup$ – Jesuspowder Jan 25 '17 at 3:10
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    $\begingroup$ Melting point depends on symmetry of the crystal structure. So you need data on structures. If the structure is symmetric the melting point will be higher. This many a times results in data which can't be explained on the basis of things like hydrogen bonding. It also depends on the surface area of the molecule $\endgroup$ – Raghav Jan 25 '17 at 3:23
  • $\begingroup$ masterorganicchemistry.com/2010/07/09/chemical-tetris Though this is about organic chemistry these concepts can safely be applied to inorganic. $\endgroup$ – Raghav Jan 25 '17 at 3:33
  • $\begingroup$ Melting of crystal hydrates is more like a dissolution in their own water. Then again, basalt (even if wet) is not a hydrate and may be a totally different story. $\endgroup$ – Ivan Neretin Jan 25 '17 at 6:42

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