What makes minerals brittle

While watching the horror movies of mineral destruction at Research at a snail’s pace I noticed that some minerals almost seem to explode. That is of course perfectly normal because these poor crystals got a serious blow from the hammer but there are more issues worth explaining that are deeply connected to the way minerals are built.

The first observation one can make while watching these videos is that minerals are brittle. This is hardly surprising, we all knew that before. But what makes them brittle? Is it universal among all minerals?

Minerals are naturally occurring inorganic crystalline solids. What is important for us here is that they are crystalline. It means that every atom or ion has a specific place in relation to neighboring ions. Everyone of them has neighbors and these neighbors are loyal to each other — they are not allowed to move freely in the lattice. The whole structure has to be electrically neutral (the sum of positive and negative charges or valence numbers is zero) and the distances and angles between the ions must form a stable three-dimensional network where the repulsive and attractive forces between the neighboring ions balance each other. There is usually not too many ways how a mineral with a given chemical composition can be put together to make such a structure. There is definitely nothing random here. People who like order should study crystallography. They would love it!

What happens to the crystal when we hit it with a hammer is that we deform the crystal structure and force the ions with a positive charge to be next to another ion with a positive charge and the same with anions (ions with a negative charge). Crystals can not take it because there are repulsive forces between the ions with the same sign. When that happens crystals simply break into pieces and often more violently than we would expect, leaving us an impression that they explode.

All right, that would explain why minerals with an ionic bonds can not tolerate hammer blows but what about the covalent bonds? These are bonds between atoms or atom groups that share electrons. Let’s take a look at diamond for example. It is composed of only one chemical element — carbon. There are no valence differences and hence the chemical bonds are 100% covalent. Is it safe to try the hardness of diamond with a hammer? No, I definitely do not recommend it. Diamond is hard but it is brittle as well. These are different things. Diamond is crystalline, every one of its countless atoms knows its neighbors and they still do not tolerate the idea that someone comes and tries to force things around. Remember, these atoms share electrons but they know with whom they share and never agree to change their mind.

But some minerals are not brittle, gold for example. What is the deal here? It is a special case. Gold atoms are bonded by a special type of covalent bonds which are called metallic bonds. Here again electrons are shared but this time atoms do not care much with whom they are sharing their electrons. Electrons are free to move around, forming an electron cloud which is among other things capable of carrying electric current. Gold is an excellent conductor of electricity. Gold atoms have specific locations in the lattice where they can be but they can change their locations because their free electrons allow them to move around. That’s why gold is malleable and we shouldn’t be afraid to break our gold rings when they happen to fall.

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