What is a covalent crystal?
A covalent crystal is one in which the lattice structure is held together by powerful covalent bonds. These crystals tend not to be soluble in water and have a much higher bond strength than ionic crystals. For example, silica, SiO2, does not dissolve in water and has a high melting point. Silica is what glass is made out of. Imagine trying to make a glass out of salt crystals; it would dissolve before you could drink a glass of water out of it! Solubility is a major difference between covalent and ionic crystals.
Diamonds are a great example of how strong the bonds in a covalent crystal can be. The lattice structure of diamonds produces the hardest known substance in the universe.
To complicate things, the vast majority of complex minerals are a combination of ionic and covalent bonds. These crystals will not be discussed in depth in this lesson, but it is important to know that this third category exists, and it is the largest category of minerals and crystals. If you observe the complex formulas of many minerals, like muscovite, which is KAl3Si3O10(OH)2, you will see that it is not so cut and dry what type of bond is holding these atoms together. Is this a covalent or ionic mineral? The answer is: it is both.