My original treatise focused on plain table salt, so that every reader could have something in hand to look at.
This topic will really get into the weeds, or down the rabbit hole, so I'm going to use the simplest models and visual images I can concoct.
It usually combines 400 and 500 level Chemistry, Mathematics, and Physics, depending on which thing you are looking at. I'm going to stay more between chemistry and physics, and I'll maybe toss in some math references in case there's someone that is more intune with that. This is high nerddom guys, so, I can't do a perfect job on this anymore. And as always, if someone comes along and says it's all crap, I'm OK with that.
This is really just an explanation for why we can rub off colors, and why they can disappear, or morph.
Table salt is NaCl Sodium Chloride. A simple molecule of 1 Sodium atom and 1 chlorine atom. They stick together because Chlorine is electron greedy, and wants an 8th electron in it's outermost orbital shell, and Sodium wants to be rid of it's single outermost electron so it too can find a more stable state. A match made in heaven.
I picked sodium, because it's simple, and, it's chrystalline form is a little cube. Dump out some salt. It's all little cubes, and no matter how finely you grind it, it makes ever smaller little cubes. Little scaled hay bales that neatly stack, with a bit of attraction for each other.
Notice how with the application of energy, salt molecules only cleave along the lines that make the cubes. It doesn't leave us with metallic sodium pieces and clouds of chlorine gas. Thank God! that's because the bonds between salt molecules ie. between those that are right next to each other, are weaker than the internal bonds keeping the NaCl molecule together. They break first.
So, we can establish that there are fault lines in stacks of these crystals, and they are easier to break along than the bonds of the molecules.
You can find spectrographs of salt molecules on the web.
They look like big stacks of hay bales.
Lets add color.
Color is light absorbed by molecules, and that which isn't absorbed, reflects back to us in certain wavelengths, we see as color. So, a banana is yellow, an orange orange, etc.
SO, Where does pink salt, red salt, blue salt come from? Pure NaCl is white. Well, From the addition of impurities. Potassium (K), Magnesium (Mg), Lithium (Li), Cobalt (Co), Sulphur (S) and many others. All absorbing different wavelengths into their frameworks/cages, and reflecting light back that we see as colored.
And, none of these impurities is the same size as the Na atom. So, their molecule formed with Cl is a bit different shaped than the NaCl molecule. In fact, MgCl2 is an Octohedral, linear molecule, with a great big Mg atom in the middle. Try stacking pipes sometime.
So, if you stack the salt hay bales, and some of the bales are a different size and shape, how stable is the stack of bales? They'll still attract somewhat to each other, but, sooner or later the pile will fall over. You could call that seeking a lower energy state. A pile of hay bales after a storm is a more natural state than a precise pre storm pile of mixed shape bales. But also notice, a precisely stacked pile of identical bales can withstand a pretty severe storm without toppling. Storm being addition of energy.
Doctor Mendeleev is groaning right now.
So, I'm pretty sure that we can see that at the molecular level, it isn't a huge stretch to imagine our globules of impure Iron Oxide compounds could be disrupted similarly to our simple salt hay bales that have some weird shaped impurities added.
I'll throw this in for a little support, instead of just 1 state possible, like the Na Sodium atom and it's simple desire to shed it's one measly electron for it to be happy, metallic Iron has 8 different ones possible. It's a very fickle atom. Some days it wants electrons to be happy, other days it wants to shed electrons to be happy. It depends on the relative desirability of the new neighbor, as to what it does.
Because of that, I started with simple salt, because cubes are easier to talk about than piles of dodecahedrons.
Before I move on, imagine big piles of the Dungeons and Dragons dice. We don't make hay bales shaped like them for a reason.
