The relationship between the rate at which material cools and mineral's crystals is:
The slower the cooling, the larger the crystals.
Explanation:
The reason behind this is the very nature of magma and its solidification. First of all, magma is supersaturated with minerals making it a very complex substance. Therefore if the magma cools quickly the different elements won't have enough time to establish enough cohesion to form large crystals. While if it has a larger amount of time making it cool slowly it will be able to form larger crystals.
