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wwjgd27 t1_jdewitu wrote

Reply to comment by CrustalTrudger in Does the metal in the solid core of a rocky planet have any special properties? by VillagerNo4

Just a minor addition that pressure has a minor effect on the thermodynamic state function when compared to temperature so the crystal phase of the inner core shouldn’t be much different than that of what we see at the surface. We need to start talking about neutron star densities to see a pronounced effect!

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Jon_Beveryman t1_je7i8q0 wrote

Except we do know, both from seismic wave studies and from high pressure experiments above ground, that iron has a high pressure phase transformation around 13 GPa (body centered cubic to hexagonal close packed).

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wwjgd27 t1_je7jaw0 wrote

13 GPa of pressure is ridiculous compared to the 600 or so degrees Celsius required to get the same phase transformation. Temperature will always have a more pronounced effect than pressure in thermodynamics.

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Jon_Beveryman t1_je7kkey wrote

(A) there is no thermally induced HCP phase in iron at atmospheric pressure, in pure iron the HCP epsilon phase is solely a high pressure phase, (B) I don't see what the temperature vs pressure effect size has to do with any of this - the assertion was that in the core you'd have the same crystal structure as you would on the surface and it is observably not true.

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wwjgd27 t1_je7njji wrote

FCC in the (111) planar direction is the same as HCP. We just call it something else but effectively it’s the same right?

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Jon_Beveryman t1_je7oxqk wrote

No, they're not the same. The (111) plane in FCC and the (1000) plane in HCP are equivalent but if you look down the [111] and [1000] directions you will see that the stacking sequence is different. This is usually described as ABCABC (FCC) vs ABAB (HCP). This is, for instance, why you can have FCC <--> HCP phase transformations produced solely by stacking faults.

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wwjgd27 t1_je8nghy wrote

I thought ABAB stacking was for graphite and other graphitic structures since each stack of graphene is missing a carbon atom at the center of the hexagonal rings which would give it the symmetry allowing for ABCABC stacking in both HCP (0001) planes and FCC (111) planes? Interesting conversation by the way!

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