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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!

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If you can get your hands on a copy of Fegley's book Practical Chemical Thermodynamics for Geoscientists, he gives a good description in Chapter 07, which I'll summarize here.

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.

A monotrope is a polymorph that does not have a reversible transformation into another polymorph at atmospheric pressure. One polymorph is always stable and the other is metastable. Monotropy exists in systems where the transition temperature between solid polymorphs is above the melting point of the lower-temperature polymorph. In many cases, upon heating the metastable monotrope melts, then resolidifies to the stable monotrope, which melts at a higher temperature.

Metastability refers to a nonequilibrium state that is approaching equilibrium slowly enough that we can say it's stable over a given period of time.

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    $\begingroup$ Just to be sure I get it: enantiotropes are able to become polymorphs while on solid state, whereas monotropes need to melt and recrystallize to become a polymorph? If so, then monotropes only have "one direction" to go without needing to melt and recrystallize, which is to go from the metastable polymorph to the stable one? And, if I am to turn a stable polymorph into a metastable one, I'd have to melt it, whereas an enantiotrope would only require T/pressure change? $\endgroup$
    – Matheus
    Commented Jun 30, 2018 at 23:05
  • $\begingroup$ It sounds like you have a solid understanding. In order to further convince yourself, and because it sounds like you're interested in mineralogy, it might be worthwhile to relate this to some real minerals and convince yourself that you understand it. Also, I highly recommend Fegley's book. If you're interested in mineralogy and thermodynamics (it sounds like you are) you will learn A LOT from it. If you want more info on the crystallography business, I recommend Bloss' book "Crystallography and Crystal Chemistry." $\endgroup$
    – g.z.
    Commented Jul 2, 2018 at 6:12
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    $\begingroup$ I'm a Geology student, hence my interest in mineralogy, haha. I actually do have examples, but I just couldn't really interpret them. I suppose now things are clearer. I'll check those books, as they are available at my university's library. Thanks for the input! $\endgroup$
    – Matheus
    Commented Jul 3, 2018 at 12:37
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A solid to solid transition can occur between two enantiotropically related polymorphs without going through a melt. Whereas for monotropes conversion can only take place once the material has melted. In other words the transition temperature between enantiotropically related polymorphs is below the melt of the lower temperature melting polymorph whereas for monotropically related polymorphs the transition temperature is above it.

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