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On the other hand, clearly, after the shock wave passes, pressure falls faster relative to temperature than it rose during the shockwave (because the Hugeniot curve converts some of the kinetic energy of the impact into heat), otherwise we wouldn't have the low pressure high temperature conditions necessary for the rock vapor we've been talking about. Thus, MAYBE we could also have some gas-like supercritical forms of rock or metal, and we certainly could have higher pressure gasses than we've considered, but my intuition from my vague memory of all the phase diagrams I looked at a few years ago is that the rock and metal will mostly be gas, liquid, and liquid-like super critical fluid (and mostly those last two by mass, though in at least some kinds of impacts, the gas would fill a larger VOLUME, much much larger than our current atmosphere, as you can see (or guess) by looking simulations of giant planet collisions like these ones by Durham University https://icc.dur.ac.uk/giant_impacts/ . Also, technically all of space is filled with gas, so you'd need to define a minimum density or use a von-Kármán-like line or something to define the volume filled with gas.)

On the other hand, clearly, after the shock wave passes, pressure falls faster relative to temperature than it rose during the shockwave (because the Hugeniot curve converts some of the kinetic energy of the impact into heat), otherwise we wouldn't have the low pressure high temperature conditions necessary for the rock vapor we've been talking about. Thus, MAYBE we could also have some gas-like supercritical forms of rock or metal, and we certainly could have higher pressure gasses than we've considered, but my intuition from my vague memory of all the phase diagrams I looked at a few years ago is that the rock and metal will mostly be gas, liquid, and liquid-like super critical fluid (and mostly those last two by mass, though in at least some kinds of impacts, the gas would fill a larger VOLUME, much much larger than our current atmosphere, as you can guess by looking simulations of giant planet collisions like these ones by Durham University https://icc.dur.ac.uk/giant_impacts/ . Also, technically all of space is filled with gas, so you'd need to define a minimum density or use a von-Kármán-like line or something to define the volume filled with gas.)

On the other hand, clearly, after shock wave passes, pressure falls faster relative to temperature than it rose during the shockwave (because the Hugeniot curve converts some of the kinetic energy of the impact into heat), otherwise we wouldn't have the low pressure high temperature conditions necessary for the rock vapor we've been talking about. Thus, MAYBE we could also have some gas-like supercritical forms of rock or metal, and we certainly could have higher pressure gasses than we've considered, but my intuition from my vague memory of all the phase diagrams I looked at a few years ago is that the rock and metal will mostly be gas, liquid, and liquid-like super critical fluid (and mostly those last two by mass, though in at least SOME kinds of impacts, the gas would fill a larger volume, much much larger than our current atmosphere, as you can see (or guess) by looking simulations of giant planet collisions like these ones by Durham University https://icc.dur.ac.uk/giant_impacts/ .)

On the other hand, clearly, after the shock wave passes, pressure falls faster relative to temperature than it rose during the shockwave (because the Hugeniot curve converts some of the kinetic energy of the impact into heat), otherwise we wouldn't have the low pressure high temperature conditions necessary for the rock vapor we've been talking about. Thus, MAYBE we could also have some gas-like supercritical forms of rock or metal, and we certainly could have higher pressure gasses than we've considered, but my intuition from my vague memory of all the phase diagrams I looked at a few years ago is that the rock and metal will mostly be gas, liquid, and liquid-like super critical fluid (and mostly those last two by mass, though in at least some kinds of impacts, the gas would fill a larger VOLUME, much much larger than our current atmosphere, as you can see (or guess) by looking simulations of giant planet collisions like these ones by Durham University https://icc.dur.ac.uk/giant_impacts/ . Also, technically all of space is filled with gas, so you'd need to define a minimum density or use a von-Kármán-like line or something to define the volume filled with gas.)

You should check out this paper called "The Vaporization of The Earth": https://iopscience.iop.org/article/10.1088/0004-637X/755/1/41 That mostly talks about composition. As for "nature", I believe the compounds it gives are all "molecules" if not clearly lone atoms or ions, though I don't know if the bonds are covalent. There are weird things at such temperatures like free radicals (e.g., neutral OH) and "molecular" forms of what are normally salts (e.g. NaCl and Na2Cl2 molecules: See the question: https://chemistry.stackexchange.com/questions/14174/what-is-sodium-chloride-like-in-gas-state )

I originally typed this as a comment, but I just realized that it's really kind of an answer. I think it's better for you to check out the paper yourself than for me to try and tell you what it says, though.

You should check out this paper called "The Vaporization of The Earth": https://iopscience.iop.org/article/10.1088/0004-637X/755/1/41 That mostly talks about composition. There are weird things at such temperatures like free radicals (e.g., neutral OH) and "molecular" forms of what are normally salts (e.g. NaCl and Na2Cl2 molecules: See the question: https://chemistry.stackexchange.com/questions/14174/what-is-sodium-chloride-like-in-gas-state ). There are also more familiar gasses non-metal like CO2 and HF.

The fact that they're molecules doesn't mean the bonds are any more covalent, and I don't see why they would be. Thus, you could imagine them as just the smallest possible crystals necessary to be electrically neutral, which IS necessary at temperatures lower than those capable of producing highly ionized plasmas. (It's important to note that the SURFACE of the Sun is actually still on the cold side for thermally ionized plasma, and I think it's MOSTLY electrically neutral, though you may want to check some other sources on that. I'm not saying temperatures in giant impacts are in any way related to the surface of the Sun, but the paper I found only goes up to 4000K, which is less than the surface of the Sun, and in the range of Red Dwarf Star surfaces.) That being said, the paper does indicate that some Na$^+$ appears above 3600K with Bulk Silicate Earth elementary composition, and some K$^+$ appears above 3400K in with Continental Crust elementary composition.

What's more interesting (or at least less obvious to me) than the fact that they're electrically neutral is the fact that unionized metals are not very common. Even at these high temperatures, MOST of the metal forming rock seems to prefer to bond to oxygen or halogens. Again, though, at the highest temperatures it shows, we do see some monotomic metals: For some reason K and Na show up first as monotomic gasses. This is strange since they're the most electropositive and so generally most likely to be bonded metals, though they might just generally be overrepresented in the gas phase in lower temperatures because they're also the most volatile (lowest boiling point) metals considered. Then, Fe and, in the Bulk Silicate Earth case, Magnesium show up as monotomic gasses at the highest temperatures considered.

We also see Oxygen and Hydrogen appear as monomic gasses, which goes along with OH and NO to indicate non-metal free radicals can be stable (or at least generated quickly enough to keep up with their destruction) at high temperatures and low pressures, though NO is stable enough to be encountered at normal (human) PT conditions as well. A slightly harder to spot free radical that appears is NaO. There are also some molecules which are not free radicals, but just less stable under normal conditions, like SiO, SO, and CO.

Again, this paper only goes up to 4000K, so it's possible you would need to consider higher temperatures, where you might start to see some plasma.

It also doesn't address high pressures, which of course are necessary to model chemistry in large parts of the volume of a planetary collision, since they already have high pressures inside them, and crashing into each other temporarily makes even higher pressures, but after a certain point (which is definitely reached in planets, whether stationary or colliding) you're really dealing with supercritical fluid, not vapor.

It should be noted that you can get gas-like supercritical fluid if the temperature is much more above the critical temperature than the pressure is above the critical pressure (I think this happens in the cores of stars), but this kind of situation doesn't happen for rocky or metallic materials in the interior of Earth-like planets, whose critical temperatures I think are mostly not even reached. (For example, the critical point of iron is estimated as 9250K$\pm$1000K in this paper I just found https://www.nature.com/articles/srep07194). Moreover, the temperatures and pressures created by sudden compression as in shock waves (or planetary collisions) follow what called "Hugeniot curves" starting at the the original pressure-temperature (PT) conditions (or follow "Rankine-Hugeniot relations"), and these curves increase density as they increase temperature. In order for a supercritical fluid to be gas-like, it has to gas-like density, so compressing rocky planets, where the rock and metal starts out denser than a gas, will not create gas-like supercritical fluids on the wave front of the impact shock wave.

On the other hand, clearly, after shock wave passes, pressure falls faster relative to temperature than it rose during the shockwave (because the Hugeniot curve converts some of the kinetic energy of the impact into heat), otherwise we wouldn't have the low pressure high temperature conditions necessary for the rock vapor we've been talking about. Thus, MAYBE we could also have some gas-like supercritical forms of rock or metal, and we certainly could have higher pressure gasses than we've considered, but my intuition from my vague memory of all the phase diagrams I looked at a few years ago is that the rock and metal will mostly be gas, liquid, and liquid-like super critical fluid (and mostly those last two by mass, though in at least SOME kinds of impacts, the gas would fill a larger volume, much much larger than our current atmosphere, as you can see (or guess) by looking simulations of giant planet collisions like these ones by Durham University https://icc.dur.ac.uk/giant_impacts/ .)

I'm starting this answer after having written a lot of comments, and I may edit it later after looking at those comments again. (By "starting", I mean the edit I made on March 6, 2024, when most of it was written. I'd only made a few comments when I first started and then deleted this answer on October 15 2020.)