As I understand it, a crystal cracks when an outside force causes the charged particles to become misaligned forming a repulsive force rather than an attractive force.
Why isn’t it possible to stick to two pieces back together? If they could be lined up again perfectly, wouldn’t the attractive forces hold them together?
I don’t know anything about the “charged particles” and I don’t know how alignment has anything to do with joining two pieces of crystal unless they’re magentic (which is not the force in question here)
The forces that bond (lattice structures) only form during the transition from liquid to solid.
Turn the middle liquid again and then solidify it while the two pieces are together and you will reform your crystal. Not as easy as it sounds, but thats the gist of it.
There are different types of solid crystals.
Ionic crystals (such as sodium chloride) are solids in which the lattice positions are occupied of ions of opposite charge held together by strong attractive forces between the oppositely charged ions. They are hard and brittle, which reflects the presence of these strong attractive forces, as well as repulsive forces that occur when ions of like charge are near each other. These forces are indeed electromagnetic in nature, pan1.
Covalent crystals (such as diamond) are solids in which lattice positions are occupied by atoms covalently bonded to other atoms at adjacent lattice sites. This results in a crystal that is essentially one giant molecule. These forces are also electromagnetic in nature.
You also have metallic crystals (solid metals) and molecular crystals (such as H[sub]2[/sub]O ice). The forces that hold the units together in these types of crystals are also electromagnetic in nature, though there are different theories to explain them, such as the band theory of solids for metals, and London forces and/or hydrogen bonding for molecular crystals.
To address the OP’s question, the basic problem is properly lining up the crystal units into the lowest-energy lattice structure at the atomic level. This is difficult to do for macroscopic solids. In the case of a liquid that is solidifying, the crystal units line up themselves. This is NOT to say that the crystalline bonds “only form during the transition from liquid to solid,” which is somewhat simplistic.
Isn’t that essentially what vacuum welding is? You put two same metal pieces in contact under pressure and place in a vacuum to remove gases that would get in the way, and some of the lattice structures bond together.
I think the main problem with putting cracked crystal back together is that molecules in the lattice rearrange themselves to accommodate the break. They’re not sitting there with their electrons hanging out waiting for you to put the crystal back in place - having lost their formerly stable state, they seek out a new stable state. Getting them out of that broken stable state and into the original (unbroken) stable state is not easy.
Heating it would be one way to accomplish that with at least some crystalline structures.
I would have guessed that when a crystal breaks, some material is lost, leaving faces that are far from complementary at molecular scales. In any case, the faces formed by the break will be irregular, so getting them lined up so they can re-form the crystal interactions would be well-nigh impossible.
You can repair a cracked crystal, but only when the three suns are in alignment. And you need a gelfling.
I’ve heard from people who polish optical surfsaces that if you have two perfectly polished flat surfaces and you put them together, you can get them to stick together by such inter-atomic forces, acting over a large area. I’ve never seen it myself, though.
So my guess to the answer to the OP’s question is that, after a crystal cracks, it’s *extremely[/i hard to get the faces sufficiently close together for this to happen – some mechanical force elsewhere is acting as a 'spring" and/or some crumbled bits got between the two pieces somewhere and/or the jaggedness of the break makes it difficult to align and/or some chemical tarnishing happens along the faces and/or dirt gets in and/or et cetera. Getting two surfaces close together is hard enough when they’re both perfectly flat. Doing it when they aren’t, and can only fit together in one precise three-dimensional orientation, accurate to the size of an atomic layer will be damned near impossible.
on top of which, having cleaved a lot of crystals, I can attest that the interfaces aren’t perfectly flat – if you look at the reflections of your freshly-cleaved face, you can see portions that tilt in slightly different directions, in what you know is a large single crystal.