# What exactly are enantiotropic and monotropic polymorphic transitions?

Recently, I've been studying the subject of polymorphism and met two important concepts, the monotropic and enantiotropic polymorphic transition types. However, I can't really seem to grasp the difference between them practically.

What I do understand is that enantiotropic polymorphism is reversible, but what does "reversible" mean practically?

Also, the definition for monotropic polymorphism says that it is irreversible due to metastability. But what exactly is metastability, what does it mean practically? Also, what does "irreversible" mean? Because, surely, I can transform diamond into graphite and graphite into diamond despite it being a monotropic polymorphism.

Thanks in advance for any insight given!

An enantiotrope is a polymorph that undergoes a reversible transformation into another polymorph at atmospheric pressure. If you're not familiar with the concept of reversibility you may want to read up on entropy, spontaneity, and the 2nd law of thermodynamics. Briefly, a reversible transformation is a process in which the system and surroundings are in equilibrium at all times. An important property of these processes is that $\Delta$ S$_{universe}$=0. Reversible processes don't actually occur in nature but they can make good approximations for some things we observe.