monotropic transition

The irreversible transition from a metastable polymorphic form to the stable polymorph. Example: The transition of metastableCaCO3 (aragonite-type) to the stableCaCO3 (calcite-type).
Note:
For liquid crystals the term refers to a liquid crystal to liquid-crystal transition that occurs below the melting point and is revealed by supercooling of the crystal.
Source:
PAC, 1994, 66, 577 (Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)) on page 586