I have been thinking about the notion of a bench top decompression melting demo. That would be wayyy cool. But I think we are just outta luck.

First of all, finding a substance that melts at classroom conditions is a challenge unto itself. Several references mention gallium but, as we know, it is backwards and melts with compression, not decompression.

But, even if we could track down the appropriate material, a benchtop demo would be challenging (if not impossible) because solid-liquid boundaries are generally VERY steep in PT space. Most melting diagrams have Kbar scales on them for this reason. This means you would have to change the pressure A LOT to see a distinct phase change, and that pressure change is probably more than you could safely accomplish on the benchtop.

And finally, I think I am correct about this: solid to liquid phase changes are endothermic (absorb heat) - - - so thermodynamics will work against us. The endothermic process will cause T to drop unless we design some fancy apparatus to control it. Without that control, conditions would just follow the solid-liquid boundary (both P and T decrease) and you would never see a distinct phase change.

But speaking of melting: A student wants to know why there is such a wide range in melting temperatures of minerals. And, it occurs to me that I do not really know. At some point we all learned that silicic magmas melt at lower T than mafic ones, so it is tempting to blame it on silica content. But this cannot be the complete explanation because SiO2 melts at higher T than CaMgSi2O6, I also learned at some point that ionic crystals melt at lower temps than covalent crystals - so are bond type and strength the controlling factors? Or are there other considerations that I am overlooking?

I hope you all are staying vertical and positive while testing negative.

Hi Dex-

Yeah, I was hoping that there would some new organic substance, but it looks grim. I guess I'm just going to have to shell out for a diamond cell ;)

On the subject of mineral melting points, there is an interesting plot of mp v hardness by Bruce Railsback available at his site on mineralogy and geochemistry (http://railsback.org/Fundamentals/HardnessMeltingPlot06.jpg). Of course, as we all know, the melting points of the minerals that compose a rock and its solidus are not well correlated - even under dry conditions.

Cheers,
Mike

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From: Perkins, Dexter via MSA-talk msa-talk@minlists.org
Sent: Thursday, February 25, 2021 9:42 AM
To: MSA list (msa-talk@minlists.org)
Subject: [MSA-talk] SOL re benchtop demo

I have been thinking about the notion of a bench top decompression melting demo. That would be wayyy cool. But I think we are just outta luck.

First of all, finding a substance that melts at classroom conditions is a challenge unto itself. Several references mention gallium but, as we know, it is backwards and melts with compression, not decompression.

But, even if we could track down the appropriate material, a benchtop demo would be challenging (if not impossible) because solid-liquid boundaries are generally VERY steep in PT space. Most melting diagrams have Kbar scales on them for this reason. This means you would have to change the pressure A LOT to see a distinct phase change, and that pressure change is probably more than you could safely accomplish on the benchtop.

And finally, I think I am correct about this: solid to liquid phase changes are endothermic (absorb heat) - - - so thermodynamics will work against us. The endothermic process will cause T to drop unless we design some fancy apparatus to control it. Without that control, conditions would just follow the solid-liquid boundary (both P and T decrease) and you would never see a distinct phase change.

But speaking of melting: A student wants to know why there is such a wide range in melting temperatures of minerals. And, it occurs to me that I do not really know. At some point we all learned that silicic magmas melt at lower T than mafic ones, so it is tempting to blame it on silica content. But this cannot be the complete explanation because SiO2 melts at higher T than CaMgSi2O6, I also learned at some point that ionic crystals melt at lower temps than covalent crystals – so are bond type and strength the controlling factors? Or are there other considerations that I am overlooking?

I hope you all are staying vertical and positive while testing negative.